introductiontofreshwateralgae · 2020. 1. 16. · 7.chrysophytes (chrysophyta a, c 1, c 2, c 3 α,...

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1Introduction to Freshwater Algae

1.1 General introduction

Algae are widely present in freshwater environments,such as lakes and rivers, where they are typicallypresent as microorganisms – visible only with the aidof a light microscope. Although relatively inconspic-uous, they have a major importance in the freshwaterenvironment, both in terms of fundamental ecologyand in relation to human use of natural resources.

This book considers the diversity of algae in fresh-water environments and gives a general overviewof the major groups of these organisms (Chapter1), methods of collection and enumeration (Chap-ter 2) and keys to algal groups and major genera(Chapter 4). Algae are considered as indicators ofenvironmental conditions (bioindicators) in terms ofindividual species (Chapter 1) and as communities(Chapter 3).

1.1.1 Algae – an overview

The word ‘algae’ originates from the Latin word forseaweed and is now applied to a broad assemblageof organisms that can be defined both in terms ofmorphology and general physiology. They are simpleorganisms, without differentiation into roots, stemsand leaves – and their sexual organs are not enclosedwithin protective coverings. In terms of physiol-ogy, they are fundamentally autotrophic (obtain-ing all their materials from inorganic sources) and

photosynthetic – generating complex carbon com-pounds from carbon dioxide and light energy. Somealgae have become secondarily heterotrophic, takingup complex organic molecules by organotrophy orheterotrophy (Tuchman, 1996), but still retaining fun-damental genetic affinities with their photosyntheticrelatives (Pfandl et al., 2009).

The term ‘algae’ (singular alga) is not strictly ataxonomic term but is used as an inclusive labelfor a number of different phyla that fit the broaddescription noted earlier. These organisms includeboth prokaryotes (cells lacking a membrane-boundnucleus; see Section 1.3) and eukaryotes (cells witha nucleus plus typical membrane-bound organelles).

Humans have long made use of algal species, bothliving and dead. Fossil algal diatomite deposits, forexample, in the form of light but strong rocks, havebeen used as building materials and filtration media inwater purification and swimming pools. Some fossilalgae, for example Botryococcus, can give rise to oil-rich deposits. Certain species of green algae are culti-vated for the purpose of extracting key biochemicalsfor use in medicine and cosmetics. Even blue-greenalgae have beneficial uses. Particularly, Spirulina,which was harvested by the Aztecs of Mexico, is stillused by the people around Lake Chad as a dietarysupplement. Spirulina tablets may still be obtained insome health food shops. Blue-green algae are, how-ever, better known in the freshwater environment asnuisance organisms, forming dense blooms. Thesecan have adverse effects in relation to toxin build-up

Freshwater Algae: Identification, Enumeration and Use as Bioindicators, Second Edition. Edward G. Bellinger and David C. Sigee.© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.

COPYRIG

HTED M

ATERIAL


2 1 INTRODUCTION TO FRESHWATER ALGAE

and clogging filters/water courses – affecting the pro-duction of drinking water and recreational activities.

1.1.2 Algae as primary producers

As fixers of carbon and generators of biomass, algaeare one of three major groups of photosyntheticorganism within the freshwater environment. Theyare distinguished from higher plants (macrophytes) interms of size and taxonomy and from photosyntheticbacteria in terms of their biochemistry. Unlike algae(eukaryotic algae and cyanophyta), photosyntheticbacteria are strict anaerobes and do not evolve oxygenas part of the photosynthetic process (Sigee, 2004).

The level of primary production by algae in fresh-water bodies can be measured as fixed carbon per unitarea with time (mg C m−3 h−1) and varies greatlyfrom one environment to another. This is seen, forexample, in different lakes – where primary produc-tion varies with trophic status and with depth in thewater column (Fig. 1.1). Eutrophic lakes, contain-ing high levels of available nitrogen and phospho-rus, have very high levels of productivity in surfacewaters, decreasing rapidly with depth due to lightabsorption by algal biomass. In contrast, mesotrophicand oligotrophic lakes have lower overall productiv-ity – but this extends deep into the water column dueto greater light penetration.

Although algae are fundamentally autotrophic(photosynthetic), some species have become sec-ondarily heterotrophic – obtaining complex organiccompounds by absorption over their outer surface orby active ingestion of particulate material. Althoughsuch organisms often superficially resemble proto-zoa in terms of their lack of chlorophyll, vigorousmotility and active ingestion of organic material,they may still be regarded as algae due to theirphylogenetic affinities.

1.1.3 Freshwater environments

Aquatic biology can be divided into two major dis-ciplines – limnology (water bodies within conti-nental boundaries) and oceanography (dealing withoceans and seas, occurring between continents). This

book focuses on aquatic algae present within con-tinental boundaries, where water is typically fresh(non-saline), and where water bodies are of two maintypes:

� Standing (lentic) waters – particularly lakes andwetlands.

� Running (lotic) waters - including streams andrivers.

The distinction between lentic and lotic systemsis not absolute, since many ‘standing waters’ suchas lakes have a small but continuous flow-through ofwater, and many large rivers have a relatively lowrate of flow at certain times of year. Although thedifference between standing and running waters isnot absolute, it is an important distinction in relationto the algae present, since lentic systems are typicallydominated by planktonic algae and lotic systems bybenthic organisms.

Although this volume deals primarily with algaepresent within ‘conventional freshwater systems’such as lakes and rivers, it also considers algae presentwithin more extreme freshwater environments suchas hot springs, algae present in semi-saline (brackish)and saline conditions (e.g. estuaries and saline lakes)and algae present within snow (where the water isin a frozen state for most of the year).

1.1.4 Planktonic and benthic algae

Within freshwater ecosystems, algae occur as eitherfree-floating (planktonic) or substrate-associated(largely benthic) organisms. Planktonic algae driftfreely within the main body of water (with somespecies able to regulate their position within thewater column), while substrate-associated organismsare either fixed in position (attached) or have lim-ited movement in relation to their substrate. Thesesubstrate-associated algae are in dynamic equilib-rium with planktonic organisms (Fig. 2.1), with thebalance depending on two main factors – the depthof water and the rate of water flow. Build-up of phy-toplankton populations requires a low rate of flow


1.1 GENERAL INTRODUCTION 3

10

20

50

Depth

(m

)

(a) Highly eutrophic

(b) Eutrophic

(d) Oligotrophic

(c) Mesotrophic

Approximate areal values

mg C m–2 h–1

George 380

Clear 116

Castle 70

Baikal 60

Tahoe 0.6

Carbon fixed (mg C m–3 h–1)

10–1 102 1031 10

Figure 1.1 Examples of algal pri-mary production in lakes of differ-ent trophic status, showing how ratesof production typically change withdepth. Examples of each lake typeinclude (a) highly eutrophic; LakeGeorge (Uganda). (b) eutrophic; Blel-ham Tarn (English Lake District),Clear Lake (USA), Erken (Sweden).(c) mesotrophic; Grasmere (EnglishLake District), Castle Lake (USA).(d) oligotrophic; Lake Tahoe (USA),Lake Baikal in part (Russia), Wastwa-ter (English Lake District). Adaptedfrom Horne and Goldman (1994)

(otherwise they flush out of the system) and ade-quate light levels, so they tend to predominate atthe surface of lakes and slow-moving rivers. Ben-thic algae require adequate light (shallow waters)and can tolerate high rates of water flow, so pre-dominate over phytoplankton in fast-flowing riversand streams. Benthic algae also require adequateattachment sites – which include inorganic sub-strate, submerged water plants and emergent waterplants at the edge of the water body. The distinc-tion between planktonic and non-planktonic algaeis ecologically important and is also relevant toalgal sampling and enumeration procedures (seeChapter 2).

Planktonic algae

Planktonic algae dominate the main water body ofstanding waters, occurring as a defined seasonal suc-cession of species in temperate lakes. The tempo-ral sequence depends on lake trophic status (seeSection 3.2.3; Table 3.3) with algae forming denseblooms in eutrophic lakes of diatoms (Fig. 1.16),colonial blue-green algae (Fig. 1.5) and late popula-tions of dinoflagellates (Fig. 1.10). During the annualcycle, phytoplankton blooms correspond to peaks inalgal biovolume and chlorophyll-a concentration andtroughs in ‘Secchi depth’ – the inverse of turbidity(Fig. 2.8).



Benthic algae

Benthic algae occur at the bottom of the water col-umn in lakes and rivers and are directly associ-ated with sediments – including rocks, mud andorganic debris. These algae (usually attached) mayform major growths on inorganic surfaces or onorganic debris, where they are frequently presentin mixed biofilms (bacteria, fungi and invertebratesalso present). Under high light conditions, the biofilmmay become dominated by extensive growths of fil-amentous algae – forming a periphyton community(Fig. 2.23). Attached algae may also be fixed to liv-ing organisms as epiphytes – including higher plants(Fig. 2.29), larger attached algae (Fig. 2.28) and largeplanktonic colonial algae (Fig. 4.35). Some substrate-associated algae are not attached, but are able to moveacross substrate surfaces (e.g. pennate diatoms), areloosely retained with gelatinous biofilms or are heldwithin the tangled filamentous threads of mature peri-phyton biofilms. (Fig. 2.29).

Many algal species have both planktonic andbenthic stages in their life cycle. In some cases,they develop as actively photosynthetic benthicorganisms, which subsequently detach and becomeplanktonic. In other cases, the alga spends most of its

actively photosynthetic growth phase in the plank-tonic environment, but overwinters as a dormantmetabolically inactive phase. Light micrographs ofthe distinctive overwintering phases of two majorbloom-forming algae (Ceratium and Anabaena) areshown in Fig. 2.7.

1.1.5 Size and shape

Size range

The microscopic nature of freshwater algae tendsto give the impression that they all occur within abroadly similar size range. This is not the case witheither free floating or attached algae.

In the planktonic environment (Table 1.1), algaerange from small prokaryotic unicells (diameter <

1 μm) to large globular colonies of blue-green algaesuch as Microcystis (diameter reaching 2000 μm) –just visible to the naked eye. This enormous sizerange represents four orders of magnitude on a linearbasis (×12 as volume) and is similar to that seenfor higher plants in terrestrial ecosystems such astropical rainforest.

Table 1.1 Size Range of Phytoplankton.

CategoryLinear Size (Cell or

Colony Diameter) (μm) Biovolumea (μm3) Unicellular Organisms Colonial Organisms

Picoplankton 0.2–2 4.2 × 10−3–4.2 Photosynthetic bacteriaBlue-green algae

SynechococcusSynechocystis

-

Nanoplankton 2–20 4.2–4.2 × 103 Blue-green algaeCryptophytes –

CryptomonasRhodomonas

Microplankton 20–200 4.2 × 103–4.2 × 106 DinoflagellatesCeratiumPeridinium

DiatomsAsterionella

Macroplankton >200 >4.2 × 106 - Blue-green algaeAnabaenaMicrocystis

aBiovolume values are based on a sphere (volume 4/3Πr3).


1.2 TAXONOMIC VARIATION – THE MAJOR GROUPS OF ALGAE 5

Planktonic algae are frequently characterised inrelation to discrete size bands – picoplankton (<2μm), nanoplankton (2–20 μm), microplankton(20–200 μm) and macroplankton (>200 μm). Eachsize band is characterised by particular groups ofalgae (Table 1.1).

In the benthic environment, the size range ofattached algae is even greater – ranging from smallunicells (which colonise freshly exposed surfaces) toextended filamentous algae of the mature periphy-ton community. Filaments of attached algae such asCladophora, for example, can extend several cen-timetres into the surrounding aquatic medium. Thesemacroscopic algae frequently have small colonialalgae and unicells attached as epiphytes (Fig. 2.28), sothere is a wide spectrum of sizes within the localisedmicroenvironment.

Diversity of shape

The shape of algal cells ranges from simple singlenon-motile spheres to large multicellular structures(Fig. 1.2). The simplest structure is a unicellularnon-motile sphere (Fig. 1.2b), which may becomeelaborated by the acquisition of flagella (Fig. 1.2c),by a change of body shape (Fig. 1.2a) or by thedevelopment of elongate spines and processes(Fig. 1.2d). Cells may come together in smallgroups or large aggregates but with no definite shape(Figs. 1.2d and 1.2e), or may form globular coloniesthat have a characteristic shape (Figs. 1.2f and 1.2g).Cells may also join together to form linear colonies(filaments), which may be unbranched or branched(Figs. 1.2h and 1.2i).

Although motility is normally associated with thepossession of flagella, some algae (e.g. the diatomNavicula and the blue-green alga Oscillatoria) canmove without the aid of flagella by the secretion ofsurface mucilage. In many algae, the presence of sur-face mucilage is also important in increasing overallcell/colony size and influencing shape.

Size and shape, along with other major phenotypiccharacteristics, are clearly important in the classifica-tion and identification of algal species. At a functionaland ecological level, size and shape are also impor-tant in terms of solute and gas exchange, absorption oflight, rates of growth and cell division, sedimentation

a

c

b

d

f

e

h

g

i

Figure 1.2 General shapes of algae. Non-motile uni-cells: (a) Selenastrum; (b) Chlorella. Motile unicells: (c)Chlamydomonas. Non-motile colony: (d) Scenedesmus;(e) Asterionella. Motile colony: (f) Pandorina; (g)Volvox. Unbranched filament: (h) Spirogyra. Branchedfilament: (i) Cladophora.

in the water column, cell/colony motility and grazingby zooplankton (Sigee, 2004).

1.2 Taxonomic variation – themajor groups of algae

Freshwater algae can be grouped into 10 majordivisions (phyla) in relation to microscopicalappearance (Table 1.2) and biochemical/cytologicalcharacteristics (Table 1.3). Some indication of


Tabl

e1.2

Maj

orD

ivis

ions

ofFr

eshw

ater

Alg

ae:M

icro

scop

ical

App

eara

nce.

Alg

alD

ivis

ion

(Phy

lum

)In

dex

ofB

iodi

vers

itya

Typi

calC

olou

rTy

pica

lMor

phol

ogy

ofFr

eshw

ater

Spec

ies

Mot

ility

(Veg

etat

ive

Cel

ls/C

olon

ies)

Typi

calE

xam

ples

1.B

lue-

gree

nal

gae

(Cya

noph

yta)

297

Blu

e-gr

een

Mic

rosc

opic

orvi

sibl

e–

usua

llyco

loni

alB

uoya

ncy

regu

latio

nSo

me

can

glid

eSy

nech

ocys

tis

Mic

rocy

stis

2.G

reen

alga

e(C

hlor

ophy

ta)

992

Gra

ss-g

reen

Mic

rosc

opic

orvi

sibl

e–

unic

ellu

lar

orfil

amen

tous

colo

nial

Som

eun

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loni

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gella

Chl

amyd

omon

asC

lado

phor

a3.

Eug

leno

ids

(Eug

leno

phyt

a)12

4V

ario

usco

lour

sM

icro

scop

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unic

ellu

lar

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tlyw

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gella

Eug

lena

Col

aciu

m4.

Yel

low

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enal

gae

(Xan

thop

hyta

)73

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lar

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(Din

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ta)

54R

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All

with

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um6.

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rypt

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15V

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lour

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icro

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Uni

cellu

lar

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tlyw

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gella

Rho

dom

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Cry

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7.C

hrys

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tes

(Chr

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5G

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rosc

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–un

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usA

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Red

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phyt

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Mic

rosc

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orvi

sibl

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on-m

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rach

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Ban

gia

10.B

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2B

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with

inth

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6


Tabl

e1.3

Maj

orD

ivis

ions

ofFr

eshw

ater

Alg

ae:B

ioch

emic

alan

dC

ytol

ogic

alC

hara

cter

istic

s.

Pigm

enta

tion#

Chl

orop

last

Fine

-Str

uctu

re

Alg

alD

ivis

ion

(Phy

lum

)C

hlor

ophy

llsC

arot

enes

Dia

g.*

caro

teno

ids

Star

ch-L

ike

Res

erve

Ext

erna

lCov

erin

gO

uter

mem

bran

esT

hyla

koid

grou

ps

Flag

ella

(Veg

etat

ive

Cel

lsan

dG

amet

es)

1.B

lue-

gree

nal

gae

(Cya

noph

yta)

aβ

zea-

Cya

noph

ycea

nst

arch

αPe

ptid

ogly

can

mat

rice

sor

wal

ls

00

0

2.G

reen

alga

e(C

hlor

ophy

taa,

bα,

β,γ

viol

a-T

rue

star

chα

Cel

lulo

sew

alls

,sc

ales

22–

60–

man

y.Si

mila

r(i

soko

nt)

3.E

ugle

noid

s(E

ugle

noph

yta)

a,b

β,γ

Para

myl

onβ

Prot

ein

pelli

cle

33

1–2

emer

gent

4.Y

ello

w-g

reen

alga

e:(X

anth

ophy

ta)

a,c 1

,c2

α,β

Chr

ysol

amin

arin

βPe

ctin

orpe

ctic

acid

wal

l4

32

uneq

ual

(het

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5.D

inofl

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late

s(D

inop

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)a,

c 2β

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rue

star

chα

Cel

lulo

seth

eca

(or

nake

d)3

32

uneq

ual

(het

erok

ont)

6.C

rypt

omon

ads

(Cry

ptop

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)a,

c 2α,

βal

lo-

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est

arch

αC

ellu

lose

peri

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equa

l(is

okon

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7.C

hrys

ophy

tes

(Chr

ysop

hyta

)a,

c 1,c

2,c

3α,

β,ε

Chr

ysol

amin

arin

βPe

ctin

,plu

sm

iner

als

and

silic

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43

2un

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l(h

eter

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(Bac

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a,c 1

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3β,ε

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line

silic

afr

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41

repr

oduc

tive

cell

only

9.R

edal

gae

(Rho

doph

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aα,

βFl

orid

ean

star

chα

Wal

lsw

ithga

lact

ose

poly

mer

mat

rix

20

0

10.B

row

nal

gae

(Pha

eoph

yta)

a,c 1

,c2

,c 3

β,ε

Lam

inar

inβ

Wal

lsw

ithal

gina

tem

atri

x4

32

uneq

ual

(het

erok

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only

#Maj

orpi

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tsar

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own

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ldty

pe.

*Dia

gnos

ticca

rote

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1):z

ea-

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7



the ecological and taxonomic diversity of thesegroups is given by the number of constituent species(Table 1.2) for freshwater and terrestrial algae in theBritish Isles (taken from John et al., 2002), with greenalgae and diatoms far outnumbering other groups –reflecting their widespread occurrence and abilityto live in diverse habitats. Diatoms in particular(over 1600 species) are ecologically successful,both as planktonic and benthic organisms. Inaddition to the above groups, John et al. (2002)also list other phyla – Raphidophyta (2 species),Haptophyta (5), Eustigmatophyta (3), Prasino-phyta (13) and Glaucophyta (2). Although theseminor phyla have taxonomic and phylogeneticinterest, they have less impact in the freshwaterenvironment.

In terms of diversity, freshwater algae also have amajor division into prokaryotes (blue-green algae)and eukaryotes (remaining groups) based on cellsize, ultrastructure, antibiotic resistance and gen-eral physiology. Even within the eukaryote groups,fundamental differences in phenotype and molec-ular characteristics indicate evolutionary deriva-tion from a range of ancestral types (polyphyleticorigins).

1.2.1 Microscopical appearance

The colour of freshwater algae is an important aspectof their classification (Table 1.2), and ranges fromblue-green (Cyanobacteria) to grass green (Chloro-phyta), golden brown (Chrysophyta, Bacillario-phyta), brown (Phaeophyta) and red (Rhodophyta).Variations in colour are shown in Fig. 1.3 and in thecolour photographs of Chapter 4. The use of colouras a taxonomic marker can be deceptive, however,since the normal balance of pigments may vary. Greenalgae living on snow, for example, may have a pre-ponderance of carotenoid pigments – forming a ‘redbloom’ (Hoham and Duval, 2001).

The presence of chlorophylls and associated pig-ments is also variable (Sigee, 2004). Obligate het-erotrophs, entirely dependent on uptake of organicmolecules by organotrophy or phagotrophy, may havecompletely lost their chloroplasts (e.g. Fig. 1.11) and

appear colourless. Facultative heterotrophs (photo-organotrophs, mixotrophs) retain plastids and showgreen pigmentation. Even within a ‘normal’ ecologi-cal situation, the colour of a particular alga can showconsiderable variation (see, for example, Anabaena,Fig. 4.24).

Apart from colour, the other obvious characteris-tics viewed under the light microscope are overallsize, whether the organism is unicellular or colo-nial and whether it is motile (actively moving) ornon-motile. Within different groups, algae may belargely unicellular (euglenoids, dinoflagellates, cryp-tophytes), multicellular (brown algae) or a mixture ofthe two (other groups). Motility (single cells or entirecolonies) is also an important feature, with somealgal groups being entirely flagellate (dinoflagellates,cryptophytes) while others are a mixture of flagellateand non-flagellate organisms (green algae, xantho-phytes). Other groups of algae are entirely withoutflagella, but are able to move by buoyancy regula-tion (blue-greens), gliding movements on substratum(blue-greens, diatoms) or are entirely non-motile (redand brown algae).

Resistant spores

While the above description of microscopical appear-ance relates particularly to actively growing vegeta-tive cells, it should be remembered that during wintermonths the algae of temperate water bodies typicallyoccur as thick-walled resistant spores. Colour is oftenobscured in these structures or entirely altered, mak-ing taxonomic identification difficult. Examples ofresistant spores are illustrated for blue-green algae(e.g. Anabaena akinetes – Figs. 2.7 and 4.24c), greenalgae (e.g. Haematococcus cysts – Fig. 4.54) anddinoflagellates (e.g. Ceratium cysts – Fig. 2.7).

1.2.2 Biochemistry and cell structure

Major biochemical features of freshwater algaeinclude pigmentation, food reserves and external cov-ering (Table 1.3). Different groups have distinctivecombinations of chlorophylls and carotenes, whileonly three groups (blue-greens, cryptomonads andred algae) have phycobilins. All pigmented algae have



20 μm 10 μm

100 μm

Figure 1.3 Colour characteristicsof different algal groups. Top: Freshlake phytoplankton sample show-ing colour differences between majoralgal phyla: Dinophyta (brown: C),Cyanobacteria (blue-green: An, Aph,M) and Chlorophyta (grass-green:P). Algal genera: An, Anabaena;Aph, Aphanothece; C, Ceratium; G,Gomphosphaeria; M, Microcystis;P, Pandorina. Bottom left: Synura(cultured alga, lightly fixed) showinggolden brown colour of Chrysophyta.Bottom right: End of filament ofAulacoseira granulata var. angustis-sima (with terminal spine) from lakephytoplankton showing olive-greenchloroplasts (Bacillariophyta).

chlorophyll-a, which can therefore be used for theestimation of total biomass (see Chapter 2). Diagnos-tic carotenoids have been particularly relevant in theapplication of high performance liquid chromatog-raphy (HPLC) for the identification and quantitationof major algal groups within mixed phytoplanktonsamples (see Section 2.3.3, Fig 2.11) – and havebeen used, for example, in the analysis of estuarineeutrophication (see Section 3.5.2).

Visualisation of key differences in cell struc-ture normally requires the higher resolution of oilimmersion (light microscopy), transmission electronmicroscopy (TEM) or scanning electron microscopy(SEM) and includes both internal (e.g. chloroplastfine structure) and external (e.g. location/number offlagella, cell surface ornamentation) features. Com-parisons of light and electron microscopic images areshown in Figs. 1.4 (light/TEM) and 4.56 (light/SEM).



1 μm3 μm

Figure 1.4 Prokaryote features ofblue-green algae. Top right: Phase-contrast (light microscope) image oflive filamentous colony of Anabaena,showing central patches of palechromatin (Ch). Top left: Transmis-sion electron micrograph of wholecell of Anabaena, showing centralpatch of granular chromatin. Bot-tom: Detail from top left, show-ing fine-structural features: Ch,central region of chromatin (no lim-iting membrane); Ca, carboxysome(polyhedral body); Gy, glycogengranule (cyanophycean starch); Th,peripheral thylakoid membranes; V,vacuole; P, thin peptidoglycan cellwall.

1.2.3 Molecular characterisation and identification

Although identification of algal taxa has traditionallybeen based on microscopical characteristics (mor-phology and colour), molecular techniques are beingincreasingly used to characterise algal communities.These are particularly useful where:

� No clear morphological characteristics are avail-able. This has particularly been the case for unicel-lular blue-green algae.

� Algae are relatively inaccessible and difficult tovisualise. This is the case for biofilms, wherealgae are enclosed in a gelatinous matrix, and inmany cases are a relatively small component of avery heterogeneous community of organisms. Thisproblem may also occur in highly radioactive envi-ronments, where handling and visual examinationof samples can present a health hazard.

� Diversity is being studied within species, wherestrains are often distinguished in biochemical andgenetic terms.



Ultimately, species definition and identification inboth prokaryote and eukaryote algae may depend onmolecular analysis, with determination of unique anddefining DNA sequences followed by developmentof species-specific nucleotide probes from these (seenext section).This approach would be particularly rel-evant in the case of blue-green algae, but there are anumber of problems in relation to species-specificityin this group (Castenholz, 1992).

� Polyploidy (multiple genomic copies per cell)may occur, with variation between the multiplegenomes. As many as 10 multiple genome copieshave been observed in some blue-green algae.

� Horizontal gene transfer means that some DNAfragments are dispersed over a range of species.

DNA/RNA sequence analysis

Analysis of DNA sequences has been widely usedfor the identification of both blue-green (16S rRNAgenes) and eukaryote algae (18S rRNA and chloro-plast DNA). This technique has been used by Droppoet al. (2007), for example, to determine the taxonomiccomposition of biofilms – identifying bacteria, blue-green algae and some eukaryote unicellular algae.The technique involves:

� Collection of a sample of biomass from the entiremicrobial community.

� Obtaining a DNA sample. This may involve extrac-tion from a mixed environmental sample such asbiofilm or soil (Miller et al., 1999).

� Polymerase chain reaction (PCR) amplification ofa specific nucleotide region – typically 16S or 18SrRNA genes, about 1500 bp in length.

� Separation of the amplified strands by denaturinggradient gel electrophoresis (DGGE), or purifica-tion of the PCR products using a rapid purificationkit.

� DNA sequence analysis, using a standard sequen-cer or by applying the relatively new technique ofpyrosequencing (Ronaghi, 2001). Comparison toknown 16S and 18S rRNA gene sequences in astandard database such as NCBI leads to tentativespecies identification, normally requiring a matchof at least 90%.

The pattern of bands in the DGGE gel gives anestimate of community complexity, while the inten-sity of individual bands (derived from particularspecies) provides a measure of species populationsize. In addition to providing taxonomic informationwhere classical morphological characteristics do notapply, molecular identification also has the advantagethat the whole of the microbial community (commu-nal DNA sample) is being analysed in an objectiveway. Non-photosynthetic bacteria, Archaea and pro-tozoa are also identified in addition to prokaryote andeukaryote algae (Droppo et al., 2007; Galand et al.,2008).

Potential limitations of molecular analysis are thatidentification may be tentative (often only to genuslevel) and taxonomic quantitation (relative numbersof different algae) is difficult. The technique hasbeen particularly useful in relation to the biodiversityof marine blue-green unicellular algae and is nowincreasingly used with freshwater systems (Table1.4). Molecular techniques can be used to probe par-ticular enzymes (e.g. nitrogenase) as well as moregeneral taxon-specific gene sequences.

Enzyme probes: Nitrogenase (nif) genesMolecular probes have been used to monitor genesencoding both the large (nifK) and small (nifH) nitro-genase subunits.

Stancheva et al. (2013) monitored nitrogenasegene expression in benthic river algae by develop-ing primers to the nifK gene, then using real-timereverse transcriptase PCR. The technique was usedto validate the potential role of N2-fixing blue-greenalgae as indicators of low ambient concentrations ofinorganic nitrogen.

Foster et al. (2007) used DNA quantitative PCR(QPCR) technology to amplify and detect the pres-ence of species-specific nifH genes in blue-green



Table 1.4 Molecular Identification of Algal Species in Aquatic Environmental Samples.

Environmental Samples Techniques References

Picoplankton in Lake Baikal Direct sequencing Semenova andKuznedelov (1998)

Flagellate nanoplankton SSU rRNA probes for Paraphysomonas (Chrysophyte) Caron et al. (1999)Mixed diatom populations

and laboratory culturesLarge subunit rRNA probe for Pseudo-nitzschia (Diatom) Scholin et al. (1997)

Estuarine river samples Use of ITS-specific polymerase chain reaction (PCR)assays for Pfiesteria (Dinoflagellate)

Litaker et al. (2003)

Laboratory biofilm samples:blue-greens and unicellulareukaryotes

Amplification of 16s RNA genes, with denaturinggradient gel electrophoresis (DGGE)

Droppo et al. (2007)

Diazotrophic blue-greenalgae within the AmazonRiver freshwater plume

Quantitative PCR (QPCR) analysis of the nifH gene(encodes part of the nitrogenise enzyme)

Foster et al. (2007)

Arctic ecosystems:Freshwater stamukhi lakeand inflow river

Sequencing of 18s rRNA genes to identify majorcryptophyte river populations, plus minor lakepopulations of diatoms

Galand et al. (2008)

Lake and river flagellatesamples

Population heterogeneity in Spumella (Chrysophyte) –SSU rRNA sequences

Pfandl et al. (2009)

Pond samples Population heterogeneity in Desmodesmus(Chlorophyte) – ITS2 rRNA sequences

Vanormelingen et al.(2009)

Stream N2-fixingcyanobacteria

Assessment of nitrogenase expression using real-timereverse transcriptase PCR

Stancheva et al. (2013)

SSU rRNA, small subunit ribosomal RNA; ITS, internal transcribed spacer.

algae. This gene encodes the iron-containing pro-tein nitrogenase (the key enzyme involved in nitrogenfixation) and provides a marker for nitrogen-fixing(diazotrophic) algae in the freshwater environment.The technique was used to demonstrate that the blue-green algal symbiont Richelia (associated with thediatom Hemiaulus hauckii) was specifically linked tothe Amazon freshwater outflow (river plume) in theWestern Tropical North Atlantic (WTNA) ocean, andthat the H. hauckii–Richelia complex could be usedas a bioindicator for pockets of freshwater within theWTNA ocean.

Taxon-specific DNA sequences The develop-ment of taxon-specific oligonucleotide probes fromDNA sequence data, followed by in situ hybridisa-tion, has considerable potential for the identificationand counting of algae in environmental samples.

As with direct sequencing, this technique hasparticular advantages with small unicellular algae,where there are often relatively few morphologicalfeatures available for identification. Caron et al.(1999) sequenced the small-subunit ribosomalgenes of four species of the colourless chrysophytegenus Paraphysomonas, leading to the developmentof oligonucleotide probes for Paraphysomonasimperforata and Paraphysomonas bandaiensis.

Molecular probes have major potential for thedetection of nuisance algae, particularly those thatproduce toxins. They have been used, for example,to distinguish toxic from non-toxic diatom species(Scholin et al., 1997), where differentiation wouldotherwise require the time-consuming application ofSEM and TEM. They have also been used for therapid identification of Pfiesteria piscicida, a poten-tially toxic dinoflagellate that has been the causeof extensive fish mortalities in coastal rivers of the


1.3 BLUE-GREEN ALGAE 13

eastern USA. Litaker et al. (2003) used uniquesequences in the internal transcribed spacer (ITS)regions ITS1 and ITS2 to develop PCR assays capableof detecting Pfiesteria in natural river assemblages.These have been successfully used to detect the poten-tially harmful organism in the St. Johns River system,Florida (USA).

1.3 Blue-green algae

Blue-green algae (Cyanobacteria) are widely occur-ring throughout freshwater environments, ranging insize from unicellular forms such as Synechococcus(barely visible under the light microscope; Figs. 2.16and 4.31) to large colonial algae such as Microcystis(Fig. 4.34) and Anabaena (Fig. 4.24a). Large coloniesof the latter can be readily seen with the naked eyeand show a simple globular or filamentous form withcopious mucilage. The balance of photosynthetic pig-ments present in blue-green algae (Table 1.3) varieswith light spectrum and intensity, resulting in a rangeof colours from brown to blue-green (Fig. 4.24a).Phycobilin pigments are particularly prominent, withphycocyanin tending to predominate over phycoery-thrin at low light levels, giving the cells the blue-greencolour typical of these algae. It is thus an advantageto observe material from shaded as well as betterilluminated situations wherever possible.

1.3.1 Cytology

Prokaryote status

The prokaryote nature of these algae is indicated bythe small size of the cells (typically<10 μm in diame-ter) and by the presence of central regions of nucleoidDNA (not enclosed by a nuclear membrane). Thesenucleoid regions (Fig. 1.4) can be observed both bylight microscopy (as pale central areas within livingcells) and by transmission electron microscopy (asgranular regions in chemically fixed cells) – where theabsence of a limiting membrane can be clearly seen.Although these algae lack the cytological complexityof eukaryotes (no membrane-bound organelles suchas mitochondria, plastids, microsomes and Golgibodies), they do have a range of simple granular

inclusions such as carboxysomes, cyanophycin gran-ules, polyphosphate bodies and glycogen particles(Fig. 1.4). Photosynthetic pigments are associatedwith thylakoid membranes, which are typically dis-persed throughout the peripheral protoplasm.

As with other prokaryotes, cell walls are made upof a peptidoglycan matrix with an inner lipopolysac-charide membrane and are generally quite thin.Blue-green algae resemble Gram-negative bacteriain having an additional outer cell membrane. Somespecies also have a mucilage layer outside the cellwall which may be dense or watery, structured orunstructured. The outside layers of the cell wallcan sometimes become stained straw coloured orbrownish from iron and other compounds in thesurrounding water as in Scytonema and Gloeocapsa.

The fundamental bacterial nature of these organ-isms distinguishes them from all other algae anddetermines a whole range of features – includingmolecular biology, physiology, cell size, cell struc-ture and general morphology.

Gas vacuoles

In some species, gas vacuoles may be formed, appear-ing under the light microscope as highly refractive orquite dark structures. Gas vacuoles aid buoyancy inplanktonic species, allowing the cells to control theirposition in the water column. Cells may then con-gregate at a depth of optimal illumination, nutrientconcentration or other factor for that species. This isnot necessarily at the surface - where the light inten-sity may be too great and cause photoinhibition orpermanent cell damage. Movement up and down inthe water column can enhance nutrient uptake as itallows the cells to migrate to depths where essentialnutrients, for example phosphates, are more abun-dant and then up towards the surface for light energyabsorption. When gas vacuole–containing species arepresent in a sample of water they often tend to float tothe surface (this can cause problems when preparingthe sample for cell counts; see Chapter 2).

In addition to gas vacuole–mediated movementsof planktonic blue-greens within the water column,other types of motility also occur – including gliding



movements of filamentous algae such as Oscillatoria)on solid substrata.

1.3.2 Morphological and taxonomic diversity

Blue-green algae are remarkable within the prokary-ote kingdom for showing the range of size andform noted earlier, with some organisms formingquite large, complex, three-dimensional colonies.Limited differentiation (Figs. 4.24a-c) can occurwithin colonies, with the formation of heterocysts(nitrogen-fixing cells) and akinetes (thick-walledresistant cells). In filamentous forms the cells maybe the same width along the filament or in somegenera they may narrow or taper towards the end,even forming a distinct hair-like structure in the caseof Gloeotrichia (Fig. 4.22). Clear branching occursin the most complex forms, with a fundamentaldistinction between ‘true branching’ as in Stigonema(Fig. 4.20) and ‘false branching’ as in Tolypothrix(Fig. 4.21). True branching involves division of asingle cell within the filament, giving rise to twoor more daughter cells which themselves formbranches. In contrast, false branching involveslateral extension of the filament without the singlecell division and daughter cell development asabove.

Freshwater blue-green algae can be divided intofour main groups (Table 1.5) in relation to generalmorphology, presence/absence of specialised cellsand the nature of branching in filamentous forms.These four groups form the basis for current taxon-omy of this phylum, as adopted by John et al. (2002)and Komarek et al. (2003a, b).

� Chroococcales. The simplest blue-green algae,occurring essentially as solitary cells (no filamen-tous forms), typically enclosed by a thin layer ofmucilage. The cells may remain as single cells orbe aggregated into plate-like or globular colonies.Typically planktonic with some colonial forms(e.g. species of Microcystis) forming massivesurface blooms containing individual colonies thatare recognisable with the naked eye. They typicallylack specialised cells, though one group (typifiedby Chamaesiphon) forms exospores.

� Oscillatoriales. Filamentous algae, lacking hetero-cysts and akinetes. These relatively simple algaeoccur as planktonic (some bloom forming) or ben-thic aggregations. In some cases, they form densemats on mud or rocky substrata which secondarilydetach as metaphyton into the main body of water(Fig. 2.1).

� Nostocales. A diverse group of filamentousalgae, planktonic or benthic, with heterocysts andakinetes but not showing true branching. Fila-ments may be unbranched or show false branch-ing. These algae are able to form large colonies bylateral association of filaments (bundles), 3D tan-gles or as radiating filaments from the centre of thesphere.

� Stigonematales. Filamentous algae, with hetero-cysts and akinetes and showing true branching.Structurally the most complex blue-green algae,with some thalli (e.g. Fischerella) differentiatedinto multiseriate/uniseriate basal filaments anduniseriate erect branches. Largely benthic algae,with genera such as Stigonema commonly attachedto substrata in standing and flowing waters, detach-ing to form planktonic masses.

The range of size and form noted in Table 1.5 indi-cates a wide morphological diversity, which is usefulin taxonomic identification. The relative importanceof molecular versus morphological characteristicsin relation to taxonomy reflects the debate as towhether these organisms should be treated as bac-teria (cyanobacteria – Stanier et al., 1978) or algae(blue-green algae – Lewin, 1976). Although theyfundamentally resemble bacteria in their prokaryotefeatures, they also differ from bacteria in carrying outphotosynthesis coupled to O2 evolution, and in thecomplexity of their morphology. Although currenttaxonomy is based primarily on phenotypic charac-ters (see above), ultrastructural and molecular dataprovide useful supplementary information (Komareket al., 2003a, b). Current molecular analyses supportthe separation of non-heterocystous and heterocys-tous genera (Rudi et al., 2000) and are particularlyrelevant in the case of unicellular blue-greens,where morphological features are not adequate. The


1.3 BLUE-GREEN ALGAE 15

Table 1.5 Taxonomic and Morphological Diversity in Freshwater Blue-Green Algae.

Order: Major Morphotype Colony Size and Form Attached or Planktonic Examples

Chroococcales(a) Unicellular to spheroid

colonies, lackingspecialised vegetative,resistant or reproductivecells

Single cells Planktonic or attached toplant and substrate surfaces

Synechococcus (Fig. 4.31)

Small colonies (4–32cells)

Free-floating, tangled-up withfilamentous algae orattached to substrates

Chroococcus (Fig. 4.33)Gleocapsa (Fig. 4.32)

Flat plate of cells Free-floating or sedentary MerismopediaLarge solid spherical

colonyPlanktonic Aphanocapsa (Fig. 4.35)

Microcystis (Fig. 4.34)Large hollow spherical

colonyPlanktonic Gomphosphaeria (Fig. 4.30)

Coelosphaerium(b) Unicellular, forming

exosporesCells remain single or

form multi-layeredcolonies

Attached to surfaces ofaquatic plants, algae andinorganic substrate

Chamaesiphon

OscillatorialesFilamentous algae, lacking

heterocysts and akinetesElongate straight

filamentsPlanktonic or benthicBenthic mat

Oscillatoria (Fig. 4.27)a

Phormidium (Fig. 4.29)Elongate spiral filaments Planktonic or on mud

surfacesSpirulina (Fig. 4.26)

NostocalesFilamentous algae forming

heterocysts and akinetes,but no true branching

Bundles of Elongatefilaments

Planktonic or benthic Aphanizomenon (Fig. 4.23)Nostoc (Fig. 4.25)

3-D tangle of filaments Planktonic Anabaena (Fig. 4.24)Spherical colony of

radiating filamentsPlanktonic Gloeotrichia (Fig. 4.22)

StigonematalesFilamentous algae forming

heterocysts and akinetes,with true branching

Branched mass offilaments

Benthic or planktonic Stigonema (Fig. 4.20)

Differentiation into basalfilaments and erectbranches

Benthic StauromatonemaFischerella

aPlanktonic species of Oscillatoria are placed in the genera Planktothrix, Pseudanabaena or Limnothrix by some authors.

classical (eukaryote) use of morphology to definespecies is also problematic in this group, since thespecies concept and definition of species are limitedby a complete absence of sexual reproduction.

In the absence of sexual processes, reproductionin this group is by vegetative or specialised asexualmeans. Asexual spores (akinetes) consist of vegeta-tive cells that are larger than normal. They gener-ally have thickened walls and, in filamentous forms,are often produced next to heterocysts (Fig. 4.24 c).Baeocysts (small spherical cells formed by division

of a mother cell) may be produced in some coc-coid species and are released into the environment.In some filamentous forms, deliberate fragmentationof the filament can occur – releasing the fragments(hormogonia) to produce new filaments.

1.3.3 Ecology

Blue-green algae are thought to have arisen approx-imately 3.5 billion years ago (Schopf, 1993) dur-ing which time they have been the dominant form



of life for about 1.5 billion years. As a result ofthis long evolutionary history, they have adaptedto (and frequently dominate) all types of fresh-water environment – including extreme conditions(thermal springs, desiccating conditions), brackish(semi-saline) conditions, high and low nutrient envi-ronments and planktonic/benthic habitats. The abilityof blue-green algae to dominate freshwater environ-ments is particularly important in standing waters,where algal blooms may result due to eutrophication(Section 3.2.3).

Algal blooms

In mid to late summer, eutrophic temperate lakesfrequently develop massive populations of colonialblue-green algae. These may rise to the surface of thelake, forming a thick layer of algal biomass at the topof the water column, out-competing other algae andhaving major impacts on zooplankton and fish popu-lations (Sigee, 2004). The ability of blue-greens toout-compete other freshwater algae has beenattributed (Shapiro et al., 1990) to a range of charac-teristics, including

� Optimum growth at high temperatures – summertemperatures.

� Tolerance to low light – important within the densealgal bloom, and enabling algae to survive lowerin the water column.

� Tolerance of low N/P ratios – allowing continuedgrowth when N becomes limiting.

� Depth regulation by buoyancy – avoiding pho-toinhibition during the early phase of populationincrease, and allowing algae to obtain inorganicnutrients from the hypolimnion when the epil-imnion becomes depleted in mid to late summer.

� Resistance to zooplankton grazing – limiting theimpact of herbivory on algal growth.

� Tolerance of high pH/low CO2 concentrations –allowing continued growth of blue-greens (but notother algae) at the lake surface during intensebloom formation.

� Symbiotic association with aerobic bacteria – bac-terial symbionts at the heterocyst surface main-tain the local reducing atmosphere required fornitrogen fixation and are also an important sourceof inorganic nutrients in surface-depleted waters(Sigee, 2004).

Species preferences

Many blue-green algae can tolerate a wide range ofenvironmental conditions, so strict ecological prefer-ences are unusual in this group. Microcystis aerug-inosa (Fig. 4.34), for example, can form massivegrowths in eutrophic lakes, but low numbers may alsooccur in oligotrophic waters (Reynolds, 1990). Thedistinction between planktonic and benthic organ-isms is also frequently not clear-cut. Microcystis isregarded as a typical planktonic alga, but may alsooccur as granular masses on lake bottoms, and (aswith the majority of temperate lake algae) over-winters on lake sediments rather than in the watercolumn. Many benthic algae become detached andeither remain loose on the substratum or rise in thewater column and become free-floating. Colonies ofGloeotrichia, for example, often detach from sub-strates and become planktonic (Fig. 4.22), where theygrow and may reach bloom proportions.

In spite of these cautions, many blue-green algaecan be characterised in relation to particular envi-ronments and whether they are typically attachedor planktonic. Some examples are shown in Table1.6, for both ‘normal’ and ‘extreme’ environments.As with other algae, ecological preferences typi-cally relate to multiple (rather than just single) envi-ronmental factors. Colonial blue-green algae, whichgrow particularly well in high-nutrient lakes, are alsosuited to hard waters (high Mg, Ca) and to the alkalineconditions typical of these environments. Conversely,oligotrophic lakes, which support unicellular ratherthan colonial blue-greens, tend to be soft, slightlyacid waters.

1.3.4 Blue-green algae as bioindicators

As with other algal groups, the presence or absenceof particular species can be a useful indicator of


1.4 GREEN ALGAE 17

Table 1.6 Ecological Diversity of Freshwater Blue-Green Algae.

Major Ecosystem Specific Conditions Benthic Algae Planktonic Algae

Standing waters –lakes and ponds

Meso- to eutrophic Gloeotrichia: Attached tosubstrates. Detachedcolonies planktonic

Colonial blue-greens:Microcystis, Anabaena

Oligotrophic Unicellular blue-greens:Synechococcus

Hard and soft waters AphanotheceSoft, acid lakes Cylindrospermum: often

forming dark patches onsubmerged vegetation

Tolypothrix: Planktonicor in submergedvegetation

Wetlands Soft waters – common inbogs

Chroococcus – Attached to substrates, mixed withtangles of filamentous algae or free-floatingMerismopedia – typical of bog-water communities

Open water in bogs AphanocapsaRunning waters –

streams and riversLow N concentrations, high

N:P ratioBenthic mats: Nostoc,

Calothrix(Stancheva et al., 2013)

Oscillatoria rubescens

Extreme environmentsPolar lakes, ponds,

soilsLow temperature, low light

(ice cover)Benthic mats: Calothrix,

Phormidium.Leptolyngbya(Martineau et al., 2013)

Thermal springs 45◦C or greater Mastigocladus laminosusOscillatoria(pigmented)(Darley, 1982)

Synechococcus lividus(Stal., 1995)

Brackish waters –saline lakes

Range of salinity Aphanothece halophytica(Yopp et al., 1978).

Spent nuclear fuelstorage ponds

High radioactivity, withrange of radionuclidesincluding 137Cs and 60Co.

Biofilm dominated byLeptolyngbya(Evans, 2013).

ecological status (Table 1.6). The dominant pres-ence of colonial blue-greens in lake phytoplankton(Fig 1.5.) provides a useful indicator of high nutrientstatus and these algae are a key component of varioustrophic indices (see Section 3.2.3). Conversely, pop-ulations of planktonic unicellular blue-green algaeare indicative of oligotrophic to mesotrophic con-ditions. In streams, the relative abundance of benthicN2-fixing (heterocystous) blue-greens can be used forrapid nutrient (N concentration) biomonitoring (seeSection 3.4.7).

Changes in the population of colonial blue-greensin lakes and estuaries may also act as an indicator

of climate change. An increase in the intensity ofMicrocystis blooms in San Francisco Bay (USA), forexample, has been predicted with expected increasesin water temperature and low stream flow duringincreasing incidence of droughts (Lehman et al.,2013).

1.4 Green algae

In the freshwater environment, green algae (Chloro-phyta) range in size from microscopic unicellular



50 μm

Figure 1.5 Mid-summer blue-green algal bloom in aeutrophic lake. SEM view of lake epilimnion phytoplank-ton sample (August), showing the dense population offilamentous Anabaena flos-aquae which totally domi-nated the algal bloom (chlorophyll-a concentration 140μg ml−1). Copious mucilage associated with this alga isseen as numerous fine strands, formed during the dehy-dration preparation process. See also Figs. 2.8 (seasonalcycle) and 4.24 (live Anabaena).

organisms to large globular colonies and extensive fil-amentous growths. They are characterised by a freshgreen coloration due to the presence of chlorophylls-a and -b, which are not obscured by accessory pig-ments such as β-carotene and other carotenoids. Incases where algae have excessive light exposure,the carotenoid pigments may be photoprotective andoccur at high levels, obscuring the chlorophylls andgiving the alga a bright red colour. This is seenwith the flagellate Haematococcus (Fig. 4.54), whichfrequently colours bird baths and other small poolsbright red, and with the snow alga Chlamydomonasnivalis where areas of frozen landscape are similarlypigmented.

1.4.1 Cytology

In addition to their characteristic pigmentationand other biochemical features (Table 1.3), green

algae also have a number of distinctive cytologicalaspects.

� Flagella, when present, occur in pairs. These areequal in length without tripartite tubular hairs andhave a similar structure (isokont). Some generahave four or even eight flagella, but this is unusual.

� Chloroplasts vary in shape, size and number. Inunicellular species, they tend to be cup-shaped(Fig. 4.55), but in filamentous forms may be annu-lar (Fig. 4.19), reticulate (Fig. 4.17), discoid orribbon-like (Fig. 4.13). They have a double outermembrane, with no enclosing periplastidal endo-plasmic reticulum.

� Production and storage of the photosyntheticreserve (starch) occur inside the plastid, with gran-ules frequently clustered around the pyrenoid. In allother eukaryote algae, the storage material occursmainly in the general cytoplasm.

In motile species, an eyespot is frequentlypresent, appearing red or orange (Fig. 4.38) in freshspecimens. The cell wall in green algae is made ofcellulose.

1.4.2 Morphological diversity

Green algae are the most diverse group of algae,with about 17,000 known species (Graham andWilcox, 2000). This diversity is reflected by thevariation in morphology, with organisms beinggrouped into unicellular, colonial or filamentousgrowth forms (Table 1.7). The level of greatestmorphological and reproductive complexity is rep-resented by Charalean algae (e.g. Chara, Nitella),which can reach lengths of over a metre, havewhorls of branches at nodes along the length ofthe thallus (Chapter 4, Plate 1) and have been fre-quently confused with aquatic higher plants such asCeratophyllum.

In the past, this morphological diversity providedthe taxonomic basis for green algal classification(Bold and Wyne, 1985) – with orders primarily beingdefined largely on a structural basis. These includedthe orders Volvocales (flagellate unicells and simple


1.4 GREEN ALGAE 19

Table 1.7 Range of Morphologies in Freshwater Green Algae.

Major Morphotype Attached or Planktonic Example

Flagellate Unicells Planktonic Chlamydomonas (Fig. 4.55)Non-flagellate Unicells Planktonic or benthic (often

associated with periphyton)Crescent shaped – Selenastrum (Fig. 4.78)

Equatorial division – Micrasterias (Fig. 4.81) Closterium(Fig. 4.80)

Colonial Planktonic Net – Hydrodictyon (Fig. 4.1)Hollow sphere – Volvox (Fig. 4.40), Coelastrum, Eudorina

(Fig. 4.39)Solid sphere – Pandorina (4.36) Plate of cells – Gonium

(Fig. 4.38), Pediastrum (Fig. 4.47)Small linear colonies – Scenedesmus (Fig. 4.50)Branching colonies – Dictyosphaerium (Fig. 4.45)

Disk-shaped plate, onecell thick

Attached to higher plants(epiphyte) or to stones

Coleochaete, Chaetosphaeridium

Unbranched filaments Typically Attached, but maydetach to becomeplanktonic

Uronema, Microspora, Oedogonium (Fig. 4.17), Zygnema(Fig. 4.15), Spirogyra (Fig. 4.13)

Branched filaments Chaetophora, Draparnaldia, Cladophora (Fig. 4.6)Large complex algae

with whorls ofbranches

Attached Chara, Nitella

colonial forms), Chlorococcales (unicells and non-motile coenobial colonies), Ulotrichales (unbranchedfilaments) and Chaetophorales (branched filaments).More recently, the application of cytological, com-parative biochemical and molecular sequencing tech-niques has demonstrated the occurrence of extensiveparallel evolution. On the basis that classificationshould reflect phylogeny, these original groupingsare thus no longer valid, and a new classificationis emerging where individual orders contain a mix-ture of unicellular, globular colonial and filamentousforms (Graham and Wilcox, 2000).

1.4.3 Ecology

Green algae are ecologically important as major pro-ducers of biomass in freshwater systems, either asplanktonic (standing waters) or attached (runningwaters) organisms – where they respectively mayform dense blooms and periphyton growths.

Algal Blooms

In mesotrophic and eutrophic lakes, green algaedo not normally produce the dense blooms seenwith diatoms and blue-green algae – but do becomedominant or co-dominant in early summer, betweenthe clear-water phase and mid-summer mixed algalbloom. In smaller ponds, filamentous Spirogyraand colonial Hydrodictyon frequently form surfaceblooms or scums, and in small garden ponds andbirdbaths Haematococcus may form dense popu-lations. When nutrient levels become very high, aswitch may occur from colonial blue-green to greenalgae as major bloom formers. This is can be seenin some managed fishponds, where organic and inor-ganic nutrients are applied to enhance fish produc-tion by increasing carbon flow through the wholefood chain. In the Trebon Basin Biosphere Reserve(Czech Republic), application of lime (supply of car-bonate and bicarbonate ions), organic fertilisers andmanure leads to high pH and hypertrophic nutrientlevels. In these conditions, a short diatom bloom



Table 1.8 Ecological Diversity of Freshwater Green Algae.

Major Ecosystem Specific Conditions Benthic Algae Planktonic Algae

Standing waters – Lakes andponds

Wide range of conditions Oedogonium Scenedesmus, Pediastrum,Tetraedron, Chlamydomonas

Eutrophic to Hypertrophic Cladophora Scenedesmus, Pediastrum.Oligotrophic DesmidsHigh (H) and low (L) pH Chara, Nitella (H)

Coleochaete (L)Mougeotia (L)

Hard water – high (Ca)concentration

Hydrodictyon

Salt lakes Small unicells, DunaliellaStanding waters – Wetlands Wide range of conditions Selenastrum Chlamydomonas

Low nutrient, low pH bogs Spirogyra, Mougeotia. Desmids.Running waters – Streams

and riversWide range of conditions Chlamydomonas

SpirogyraHardness (Ca) concentration Hydrodictyon

Running waters – Estuaries,brackish water

Wide range of conditions ScenedesmusHigh salinity Dunaliella

Specialisedmicroenvironments

Very high nutrients – e.g.high sewage level

Prototheca Euglena

Endosymbiont ininvertebrates

Chlorellaa

Associated with Calcareousdeposits

Oocardium stratum

aAlso widely occurring as a free-living organism.

(Stephanodiscus) is replaced in early summer by pop-ulations of rapidly growing unicellular and smallcolonial green algae (Scenedesmus and Pedias-trum), which continue to dominate for the rest ofthe annual cycle (Pechar et al., 2002). Extensivegrowth of attached algae (periphyton) may also occurunder high nutrient conditions. Branched thalli ofCladophora can form extended communities of shal-low water vegetation along the shores of eutrophiclakes and streams, breaking off in storms to formloose biomass which can degrade to generate nox-ious odours. Another filamentous alga, Mougeotia,can form large mucilaginous subsurface growths inlakes that have been affected by acid rain.

Species preferences

Individual species of green algae differ considerablyin their ecological preferences, ranging from broad

spectrum organisms (Spirogyra, Chlamydomonas) tospecies with very restricted habitats.

Spirogyra occurs in a wide range of habitats, whereit is typically attached to stable substratum (as peri-phyton) but also occurs as free-floating mats (Lembiet al., 1988 – derived by detachment either fromperiphyton (vegetative propagation) or from ben-thic zygotes (sexual derivation). In surveys of NorthAmerican sites, McCourt et al. (1986) recorded Spir-ogyra at nearly one-third of all locations, and Sheathand Cole (1992) detected Spirogyra in streams froma wide range of biomes – including desert chaparral,temperate and tropical rainforests, and tundra.

Other green algae show clear preferences for par-ticular environmental conditions (Table 1.8) thatinclude degree of water movement (lentic vs. loticconditions), inorganic nutrient status (oligotrophicto eutrophic), pH, hardness (Ca concentration)and salinity. These conditions frequently occur incombination, with many moorland and mountain


1.4 GREEN ALGAE 21

water bodies being low nutrient and neutral to acidic,while lowland sites in agricultural areas are typicallyeutrophic and alkaline.

Nutrient status Many desmids (e.g. Closterium)are particularly characteristic of oligotrophic (lownutrient) lakes and ponds – often in conditions thatare also slightly acidic (see above) and dystrophic(coloured water). Desmid diversity is particularlyhigh in these conditions, sometimes with hundreds ofspecies occurring together at the same site (Woelk-erling, 1976). Desmids are also typical of nutrient-poor streams, where they are permanent residentsof periphyton – making up to 10% of total com-munity biomass. These desmids are associated withplants such as the bryophyte Fontinalis, achievingconcentrations as high as 106 cells g−1 of substrate(Burkholder and Sheath, 1984). Desmids often con-stitute a significant proportion of algal biomass inpeat wetlands (see Section 3.3.2), where they are alsoa major aspect of species diversity.

Some species of desmids – such as Closteriumaciculare – are more typical of high-nutrient, slightlyalkaline lakes and are indicators of eutrophic con-ditions. Periphytic algae typical of eutrophic watersinclude Cladophora.

Acid lakes Although desmids are typical of neu-tral to slightly acid lakes, other green algae areadapted to more extreme conditions. These includethe filamentous green alga, Mougeotia, which canform substantial sub-surface growths in acidifiedwaters and is widely regarded as an early indicatorof early environmental change (Turner et al., 1991).Where acidification is the result of acid precipitation,experimentation (Webster et al., 1992) or industrialpollution, the acid conditions also tend to be associ-ated with

� Relatively clear waters, due to low levels ofphytoplankton

� Increases in the concentration of metals such as Aland Zn

� Reduced levels of dissolved organic carbon, withderivation largely from external lake (allochtho-nous) rather than internal (autochthonous) sources.

� Food web changes, including a reduction in thenumber of herbivores.

Laboratory studies (Graham et al., 1996a, b)showed that Mougeotia was physiologically adaptedto such conditions, with the ability to photosynthe-sise over a wide range of irradiances (300–2300 μmolquanta m−2 s−1) and a tolerance to a broad range ofboth pH (pH 3–9) conditions and metal concentra-tions. The physiological adaptations of this alga, cou-pled with reduced grazing from herbivores, probablyaccounts for the extensive growth and domination ofthe benthic environment in many acidic waters.

Alkaline and calcium-rich waters Someplanktonic (Closterium aciculare) and benthic algae(e.g. Nitella and Chara) are adapted to grow in alka-line habitats, many of which are also rich in cal-cium (hard waters). Nitella produces lush meadowson the bottoms of neutral to high pH lakes, andChara can form dense, lime encrusted lawns in shal-low alkaline waters. Hydrodictyon is also typical ofhard-water conditions, occasionally blanketing thesurface of ponds and small lakes, and occurringwidely in larger alkaline standing waters and hard-water streams. Extensive growths of this alga canalso be observed in agricultural situations such asirrigation ditches, rice paddy fields and fish farms –where there is some degree of eutrophication.

Some calcium-loving algae occur in very restrictedenvironments. The unusual, slow-growing desmidOocardium stratum, for example, occurs in calcare-ous streams and waterfalls at the top of branchedcalcareous tubes, in association with deposits of‘travertine’ and ‘tufa’.

Saline waters Various green algae are adapted tosaline conditions in salt lakes and estuaries, where saltlevels range from low (brackish waters) to high. Withsome widely occurring genera such as Scenedesmus,brackish conditions are at one end of the contin-uum of environmental tolerance. Other algae, such asDunaliella, are specifically adapted to a more extreme



situation. This organism occurs in the highly salinewaters of commercial salt pans (salterns) and naturalsalt lakes such as the Great Salt Lake of Utah (USA).Dunaliella is able to tolerate external salt concentra-tions of >5M, by balancing the high external osmoticpressure (OP) with a high internal OP generated byphotosynthetically produced glycerol.

Specialised environments In addition to thefree-living occurrence of particular algae in lakes andrivers that have a range of characteristics (see above),other green algae are adapted to more specialised con-ditions. Some of these involve a change in the modeof nutrition from autotrophic to heterotrophic andinclude various species of the non-flagellate unicel-lular algae, Chlorella – a widespread endosymbiontof freshwater invertebrates. Other green algae retaintheir free-living existence, but are able to supple-ment photosynthesis (mixotrophy) by absorption ofexogenous dissolved carbon (Tuchman, 1996) suchas amino acids and sugars through their cell surface(osmotrophy). The ability to use external organiccarbon may be important in two situations – whenphotosynthesis is limiting and when external solubleorganic is in excess.

� Limiting photosynthesis. This may occur, forexample, under conditions of chronic low lightintensity (e.g. arctic lakes). It is also typical ofmany acid lakes, where low pH reduces the level ofdissolved inorganic carbon to levels that are unableto saturate photosynthesis. Laboratory experimentson Coleochaete, an acidophilic alga, have demon-strated an ability to use dissolved organic carbonin the form of hexose sugars and sucrose (Grahamet al., 1994).

� High levels of external dissolved organic car-bon. Some osmotrophic green algae are photo-heterotrophic, only able to use organic carbonwhen light is present and when their photosyn-thesis is inhibited by the high availability of dis-solved organic carbon (Lewitus and Kana, 1994).The colourless unicellular alga Prototheca hascompletely lost the ability to photosynthesise andobtains all its carbon from external sources present

in soil and freshwater environments that are con-taminated by sewage. The evolutionary relation-ship of this obligate heterotroph to green algaeis indicated by the presence of starch-containingplastids.

1.4.4 Green algae as bioindicators

Contemporary algae

Habitat preferences of contemporary green algae(Table 1.8) are frequently useful in providing infor-mation on physicochemical characteristics of theaquatic environment. Filamentous green algae, forexample, often dominate environments stressed bycultural eutrophication, acidification and metal con-tamination (Cattaneo et al., 1995).

Fossil algae

Green algae do not typically produce resistant wallsthat persist in aquatic sediments and are much lessuseful than diatoms and chrysophytes as bioindica-tors in terms of the fossil record. There are someexceptions to this, where the cell wall contains fattyacid polymers known as algaenans that can with-stand millions of years of burial (Gelin et al., 1997).Scenedesmus and Pediastrum are among the mostcommon members of green algae that have fossilrecords, containing algaenans and also some silicon.The desmid Staurastrum is also frequently seen asfossil remains, due to the impregnation of cell wallmaterial with polyphenolic compounds which conferresistance to bacterial decay (Gunnison and Alexan-der, 1975).

1.5 Euglenoids

Euglenoid algae (Euglenophyta) are almost entirelyunicellular organisms, with a total of 40 genera world-wide (about 900 species) – most of which are presentin freshwater. Cells are typically motile, either viaflagella or (in non-flagellate cells) by the ability ofthe body to change shape (referred to as ‘metaboly’).


1.5 EUGLENOIDS 23

About one-third of the euglenoids are pho-tosynthetic and classed amongst the algae. Therest are colourless, being either heterotrophic orphagotrophic, and are usually placed in the Protozoa.In photosynthetic organisms, pigmentation is closelysimilar to that in green algae (Table 1.3), but thevariable presence of carotenoid pigments means thatthese organisms can routinely vary in colour fromfresh green (Fig. 4.51) to yellow-brown. In some sit-uations, the accumulation of the carotenoid astaxan-thin gives cells a bright red colouration. This is seenparticularly well in organisms such as Euglena san-guinea, which forms localised blooms in ponds andditches.

1.5.1 Cytology

Euglenoids are typically elongate, spindle-shapedorganisms (Figs. 1.6, 4.51) and usually contain

Eyespot

Paramylon

Nucleus

Plastid

Contractilevacuole

Pocket(reservoir)

Figure 1.6 Diagrammatic view of Euglena, showingmajor cytological features. Graham & Wilcox, 2000.Reproduced with permission from Prentice Hall. See alsoFig. 4.51 (live Euglena).

several chloroplasts per cell, which vary in appear-ance from discoid to star-, plate- or ribbon-shaped.Cytological features that distinguish euglenoids(Table 1.3) include the following:

� The presence of an anterior flask-shaped depress-ion or reservoir within which the flagella areinserted. Although two flagella are present onlyone emerges from the reservoir into the surround-ing medium, the second being reduced and con-tained entirely within the reservoir. An eyespot isoften present and is located close to the reservoir.

� The production of paramylon as storage reserve.This β-1,3-linked glucan does not stain blue-blackwith iodine solution and is found in both green andcolourless forms.

� The presence of a surface coat or pellicle that givesthe cell a striated appearance. This occurs justbelow the plasmalemma and is composed of inter-locking protein strips that wind helically around thecell. In some the pellicle is flexible (allowing thecell to change its shape), while in others the pellicleis completely rigid giving a permanent outline tothe cell. Genera such as Trachelomonas have cellssurrounded by a lorica, the anterior end of which isa narrow, flask-shaped opening through which theflagellum emerges.

Reproduction is asexual by longitudinal division,with sexual reproduction being completely unknownin this group of organisms.


Almost all euglenoids are unicellular, with colonialmorphology being restricted to just a few organ-isms where cells are interconnected by mucilaginousstrands. One of these is Colacium, a stalked euglenoidthat is widespread on aquatic substrates – but is par-ticularly commonly attached to zooplankton such asDaphnia. In the sessile state, this organism does nothave emergent flagella on the cells – which occurat the end of the branched mucilaginous attachment



stalks. Individual cells may produce flagella, how-ever, swim away and form a new colony elsewhere.

With the relative absence of colonial form, diver-sity in this group is based mainly on variation inultrastructural features – including feeding appara-tus, flagella and pellicle structure (Simpson, 1997).A further aspect of diversity is the distinction betweengreen and colourless (autotrophic/heterotrophic)cells, with approximately two-thirds of all speciesbeing heterotrophic. Some euglenoids (e.g. strainsand species of Euglena) are facultative heterotrophs,which are able to carry out heterotrophic nutritionwhen photosynthesis is limiting or when surround-ing concentrations of soluble organic materials arehigh. In the heterotrophic state, these organisms retaintheir plastids as colourless organelles and absorb sol-uble nutrients over their whole surface (osmotrophy).Other euglenoids (Petalomonas, Astasia, Peranema)are obligate heterotrophs and have lost their plastidscompletely. Many of these organisms consume partic-ulate organic material (phagocytic) and have evolveda complex feeding apparatus.

1.5.3 Ecology

Euglenoids are generally found in environmentswhere there is an abundance of decaying organicmaterial. This is in line with the heterotrophic natureof many of these organisms and the ability to take upcomplex organic material either in the soluble or par-ticulate state. Typical habitats include shallow lakes,farm ponds, wetlands, brackish sand and mudflats.Within these environments, euglenoids are particu-larly associated with interfaces such as sediment-water and air-water boundaries (Walne and Kivic,1990) and should probably not be regarded as openwater truly planktonic algae (Lackey, 1968).

Certain euglenoid algae are able to tolerate extremeenvironmental conditions. One of these, Euglenamutabilis, is able to grow in very low pH waters.This alga has an optimum pH of 3.0, can toleratevalues below pH 1.0 and is typical of acidic metal-contaminated ponds and streams draining mines.Other euglenoids, found in brackish habitats, are ableto tolerate a wide range of salinity (Walne and Kivic,1990).

1.5.4 Euglenoids as bioindicators

Euglenoid algae are not particularly useful asenvironmental bioindicators, in terms of either con-temporary populations or fossil records. Althoughpresent-day algae show some adaptations to specificenvironments (see previous section), there is no estab-lished environmental library and species may be dif-ficult to identify. Within lake sediments, the lack ofcalcified or silicified structures that are resistant todecay means that hardly any remains have survivedin the fossil record.

1.6 Yellow-green algae

Yellow-green algae (Xanthophyta) are non-motile,single-celled or colonial algae, with a distinctive pig-mentation that gives the cells a yellow or fresh greenappearance (e.g. Fig. 4.18 – Tribonema). Althoughthere is a wide range in morphology (Table 1.9), thisphylum contains relatively few species (compared tomajor groups such as green algae) and the algae tendto be ecologically restricted to small water bodies anddamp soils.

1.6.1 Cytology

Yellow-green algae are most likely to be confusedwith green algae when observed in the fresh condi-tion, but differ from them (Table 1.3) in a number ofkey features.

� Distinctive pigmentation, with chlorophylls (a, c1and c2 – but not b), carotenoids (especially β-carotene) and three xanthophylls (diatoxanthin,vaucheriaxanthin and herteroxanthin).

� Carbohydrate storage as oil droplets or chrysolam-inarin (usually referred to as leucosin) granules.

� Walls composed mainly of pectin or pectic acid(sometimes with associated cellulose or siliceousmaterial). The walls often occur as two splicedand overlapping sections, breaking into ‘H’-shapedpieces on dissociation of the filaments.


1.6 YELLOW-GREEN ALGAE 25

� When flagella are present they are of unequallength (heterokont) the longer one pointing for-wards and the shorter (which can be more difficultto see using a light microscope) sometimes point-ing forwards or backwards

� Chloroplast fine structure – four outer membranes(incorporating an endoplasmic reticulum) and thy-lakoids occurring in threes.

Cells contain two or more discoid and green toyellow-green chloroplasts, with associated pyrenoidsrarely seen. An eyespot may be present.


Yellow-green algae show a range of morphol-ogy, from unicellular to colonial and filamentous(Table 1.9). Unicellular forms are non-flagellate inthe mature (vegetative) state, with motility beingrestricted to biflagellate zoospores (asexual) ormotile gametes (sexual) at reproductive stages. Thezoospores have unequal and morphologically dissim-ilar flagella (heterokont algae – Fig. 1.7). Unicellularalgae include simple free-floating forms (Botrydiop-sis, Tetraedriella) or may have associated rhizoidal

systems or an attachment stalk. Some yellow-greenalgae form simple globular colonies, which may bemucilaginous and free floating (e.g. Gloeobotrys)or attached via a stalk (Ophiocytium). Filamentousforms may be unbranched (Tribonema – Fig. 4.18)or branched (Heterococcus) and reach their mostmassive state in the coenocytic siphonaceous generaBotrydium and Vaucheria (Fig. 4.3).

1.6.3 Ecology

Yellow-green algae are rather limited in theirexploitation of aquatic habitats, tending to occur ondamp mud (at the edge of ponds) and soil, but notoccurring extensively in either lentic or lotic systems.Where planktonic forms do occur, they tend to bein ditches or small ponds. Tribonema species areoften found as free-floating filaments in temporarywaters, and Botrydiopsis arrhiza is found at theedge of ponds and in patches of water in sphagnumbogs, where it may form yellowish water blooms.Even Gloeobotrys limneticus, which has widespreadoccurrence as mucilaginous colonies in lake plank-ton, never forms extensive or dominant populations(as seen, e.g. by blue-green algae). Various yellow-green algae are epiphytic, including Mischococcus

Table 1.9 Range of Morphologies in Yellow-Green Algae.

Major Morphotype Attached or Planktonic Examples

UnicellsCoccoid Planktonic or benthic BotrydiopsisPear-shaped, with rhizoidal system Present on mud surface BotrydiumOvoid cells on stalk Epiphytic on attached or planktonic algae CharaciopsisAmoeboid Benthic, often in hollow cells of Sphagnum. Chlamydomyxa

ColonialSmall to large globular colonies Planktonic GloeobotrysBranched colony on stalk Planktonic or attached Ophiocytium

FilamentsUnbranched Typically planktonic, occasionally attached

when youngTribonema (Fig. 4.18)

Branched Attached, damp soils HeterococcusCoenocytic, (siphonaceous) Attached Botrydium, Vaucheria

(Fig. 4.3).



FS

ESN

P

M

G

V

Figure 1.7 Line drawing of heterokont zoospore, typi-cal of xanthophyte algae, with a long anterior flagellumbearing two rows of stiff hairs plus a short posterior flag-ellum. This is smooth and often bears a swelling (FS)that is part of the light-sensing system. ES, eyespot; G,Golgi body; M, mitochondrion; N, nucleus; P, plastid.Graham & Wilcox, 2000. Reproduced with permissionfrom Prentice Hall.

and Ophiocytium (attached to filamentous algae) andChlorosaccus (attached to macrophytes).

As with other algal groups, some yellow-greenalgae have developed alternative modes of nutri-tion. Chlamydomyxa is an amoeboid, naked formthat has retained its photosynthetic capability (stillhas chloroplasts) but has also become holozoic,ingesting desmids, diatoms and other algae anddigesting them within internal food vacuoles.Chlamydomyxa is typical of low-nutrient acid bogs,and the holozoic mode of nutrition may be important

for supplementing nitrogen uptake in such adverseconditions.

1.6.4 Yellow-green algae as bioindicators

Yellow-green algae have not been widely used asbioindicator organisms, partly because they are not aprominent group in the aquatic environment. Differ-ent species do have clear environmental preferences,however, that could be used to provide informationon ambient conditions. These are discussed by Johnet al. (2002) and include algae that are prevalent inacid bogs (Botrydiopsis spp., Centritractus spp.),calcareous waters (Mischococcus spp., Ophiocytiumspp.), humic waters (Botrydiopsis spp., Tribonemaminus), organically rich conditions (Chlorosaccusspp.), inorganic nutrient-enriched waters (Goniochlo-ris fallax) and brackish (partly marine) environments(Vaucheria prolifera, Tetraedriella spp.).

1.7 Dinoflagellates

Dinoflagellates (Dinophyta) are mostly biflagellateunicellular algae, although some (e.g. Stylodinium)are without flagella and are attached. They are pre-dominantly found in the surface waters of marinesystems (about 90% are marine), with only about220 species present in freshwater environments.Dinoflagellates contain chlorophylls-a and -c, butare typically golden- or olive-brown (Fig. 1.3) incolour due to the major presence of carotene andthe accessory xanthophyll peridinin. The chloroplastscan be plate-like (Fig. 4.57) in some species or elon-gate in others. Pyrenoids are present, and the mainstorage product is starch. Lipid droplet reserves mayalso be found, and some dinoflagellates possess aneyespot.

1.7.1 Cytology

Dinoflagellates have a number of distinctive cytolog-ical features.

� A large central nucleus (usually visible underthe light microscope) containing chromosomes


1.7 DINOFLAGELLATES 27

Epitheca

Cingulum

Transverse

flagellum

Hypotheca

Longitudinal flagellumSulcus

(a) (b)

4′

3′

2′

1′ 1′′

1′′′

1′′′′

2a 3a

3′′

3′′′1′′′′

(d)(c)

2′′′

2′′′′

4′′′

4′′ 5′′

3′

5′′′

7′′

Figure 1.8 Dinoflagellate symme-try and plates. Top: Diagrammaticview of a typical dinoflagellatecell showing flagellar insertion andgrooves within the plate system.(a) Ventral view; (b) Dorsal view.Bottom: Pattern of numbered thecalplates characteristic of the dinoflag-ellate Peridinium. (c) Ventral view;(d) Dorsal view. See also Fig. 4.57.Wehr & Sheath, 2003. Reproducedwith permission from Elsevier.

that are typically condensed throughout the entirecell cycle. These have an unusual structure,with genetically active (transcriptional) DNAon the outside and genetically inactive (struc-tural) DNA forming a central core (Sigee,1984).

� Cell wall material lies beneath the cell mem-brane, in contrast to many other algae – wherethe cell wall, scales or extracellular matrix occurson the outside of the plasmalemma. The cellwall in dinoflagellates is composed of cellulose(within subsurface membrane-bound vesicles),forming discrete discs (thecal plates) which givethe dinoflagellate a distinctive armoured appear-ance. The presence of thecal plates is difficult tosee in light microscope images of living cells, butis clear when chemically fixed cells are viewed

by scanning electron microscopy (Fig. 4.57). Thenumber, shape and arrangement of thecal plates aretaxonomically diagnostic (compare Figs. 1.8 and1.9). A typical arrangement of plates can be seenin Peridinium (Fig. 1.8), with a division into twomajor groupings, forming an apical epitheca (orepicone) and a posterior hypotheca (or hypocone).Heavily armoured species tend to be more angu-lar in outline, and plates may be extended in somespecies into elaborate projections. The presence ofsuch projections or horns in Ceratium, for exam-ple (Fig. 1.9), increases the surface area to volumeratio of the cell and possibly reduces its sinkingrate by increasing frictional resistance. Projectionsmay also help in reducing predation. In contrast toarmoured dinoflagellates, some species have verythin plates or they may be absent altogether (nakeddinoflagellates).



25 μm

Figure 1.9 Morphology of Ceratium. Top: Diagram-matic views of a matured cell (left) and cyst formation(right). Bottom: Central region of a matured cell (SEMpreparation), showing details of equatorial groove andthecal plates with pores. See also Figs. 2.7 (cyst), 2.16(iodine-stained sample), 2.18 (cell death) and 4.56 (livecells).


These biflagellate unicellular organisms are of twomain types, depending on the point of insertion of theflagella.

� Desmokont dinoflagellates, where the two flag-ella emerge at the cell apex (e.g. Prorocentrum).

Relatively few species occur in this group, whichis also characterised by the presence of two largeplates (valves) covering a major portion of the cell.

� Dinokont cells, where the flagella emerge fromthe middle of the cell (e.g. Ceratium, Peri-dinium). Dinokont cells have a distinct symmetry(Fig. 1.8) with dorsoventral division into epithecaand hypotheca. These are separated by an equa-torial groove (cingulum), with a further smallgroove – the sulcus, extending posteriorly withinthe hypocone. The two are flagella inserted in theventral region of the cell. Beating of the flagella –one of which is extended while the other is con-tained in the equatorial groove – gives the cell adistinctive rotatory swimming motion. The term‘dinoflagellate’ is derived from the Greek worddineo, which means ‘to whirl’.

Dinoflagellates do not show the range of morphol-ogy, from unicellular to colonial forms, seen in otheralgal groups. There are some unusual non-flagellateamoeboid, coccoid and filamentous forms, however,which reveal their phylogenetic relationship to main-stream dinoflagellates by the characteristic biflagel-late structure of their reproductive cells – known asdinospores (zoospores).

1.7.3 Ecology

In freshwater environments, dinoflagellates are typi-cally large-celled organisms such as Ceratium, Peri-dinium and Peridiniopsis. Their large size reflects thehigh nuclear DNA levels (large amounts of geneti-cally inactive chromatin) and correlates with a longcell cycle and low rate of cell division. These featuresare typical of organisms that are K-strategists, domi-nating environments that contain high populations oforganisms living under intense competition (Sigee,2004).

Dinoflagellates are meroplanktonic algae, presentin the surface waters of lakes and ponds at certaintimes of year. The annual cycle is characterised bytwo main phases.


1.8 CRYPTOMONADS 29

200 μm

Figure 1.10 Autumn dinoflagellate bloom in aeutrophic lake. SEM view of epilimnion phytoplank-ton sample (September), showing almost complete dom-inance by Ceratium hirundinella. See also Figs. 2.8 (sea-sonal cycle) and 4.56 (live Ceratium).

� A midsummer to autumn bloom, when phyto-plankton populations are very high and the surfacewaters are dominated by either dinoflagellates (Fig.1.10) or colonial blue-green algae. At this point inthe seasonal cycle, the surface water concentrationof phosphorus is very low and dinoflagellates suchas Ceratium survive by diurnal migration into thelower part of the lake – where P levels are higher.Dinoflagellates are adapted to this daily migratoryactivity by their strong swimming motion – whichis coupled to an efficient phototactic capacity.

� An overwintering phase, where dinoflagellates sur-vive on the sediments as resistant cysts. These non-flagellate cells (Fig. 2.7) lack an equatorial grooveand initially form in surface waters at the end ofthe summer/autumn bloom before sinking to thebottom of the lake. Germination of cysts occursin early summer, either on sediments or recentlymixed waters, giving rise to vegetative cells whichtake up newly available phosphorus.

Ceratium and Peridinium are geographicallywidely dispersed, occurring particularly in waters thathave high calcium-ion concentrations (hard waters)and low levels of surface inorganic nutrients (see

above). Although these organisms are typically pho-totrophic, obtaining their nutrients in inorganic form,other dinoflagellates exhibit some degree of het-erotrophy. Some are able to ingest food particles byengulfment of whole cells, others by formation of afeeding veil (pallium) or use of a feeding tube (phago-pod). In some cases, these dinoflagellates are pig-mented and mixotrophic, combining heterotrophicnutrition with photosynthesis. Other dinoflagellatessuch as Peridiniopsis are obligate heterotrophs andcolourless. The common freshwater heterotrophicdinoflagellate Peridiniopsis berolinensis uses a finecytoplasmic filament to make contact with suitableprey such as insect larvae and then ingests the con-tents.

Heterotrophic dinoflagellates are particularlyadapted to conditions where photosynthesis is lim-iting. A number of colourless species, for example,are found under the ice of frozen lakes during thewinter season, where photosynthesis is severely lim-ited by low irradiance levels – which are about 1% ofsurface insolation.

1.8 Cryptomonads

Cryptomonads (Cryptophyta) are a group of rela-tively inconspicuous algae that are found in bothmarine and freshwater environments. They are gen-erally small- to medium-sized unicells, and in manystanding waters are a relatively minor part of the phy-toplankton assemblage – both in terms of cell num-bers and biomass.

1.8.1 Cytology

The fine structure of cryptomonads is illustrated bythe light microscope images of Rhodomonas (Fig.4.52) and Cryptomonas (Fig. 4.53), and by the linedrawing shown in Fig. 1.11. Each cell bears twounequal flagella, which are about the same lengthas the cell and slightly unequal. One of these flag-ella propels the cell, while the other is stiff and non-motile. The flagella are inserted near to an anteriorventral depression, the vestibulum. The cells tendto be more convex on one side than the other and



Contractilevacuole

Mitochondrion

Nucleus

Ejectisomes

Figure 1.11 Cryptomonad morphology. Diagram ofa non-photosynthetic cryptomonad with two flagellaemerging from a depression. Graham & Wilcox, 2000.Reproduced with permission from Prentice Hall.

the front end, where the flagella arise, tends to beobliquely truncated. On this side arises a gullet whichis not always clearly visible using a light microscope.Two chloroplasts are present each elongated in shapeand lying along the length of the cell (Fig. 4.53)and on either side of the mid-line. The colour of thechloroplasts may be blue, blue-green, reddish, olivegreen, brown or yellow-brown depending upon theaccessory pigments present. These may include Phy-cocyanin, phycoerythrin, carotene and xanthophylls.A pyrenoid is present and starch is stored. Furtherdistinctive cytological features include:

� A prominent contractile vacuole

� Large ejectile organelles (ejectisomes), which dis-charge their content into the vestibular cavity andprobably act as a defence response to herbivorouszooplankton

� The presence of an internal ‘cell wall’ (the perip-last) below the plasmalemma that is composed

of round, oval, rectangular or hexagonal organicplates

� A complex flagellar root apparatus

� distinctive plastids, with the associated presence ofa much reduced nucleus – referred to as a nucleo-morph. This phylogenetic survival is thought to bederived from the original (red algal) symbiont thatlead to the establishment of plastids in cryptomon-ads.

1.8.2 Comparison with euglenoid algae

Cryptomonads share a number of similarities witheuglenoid algae. In both cases:

� Algae are fundamentally biflagellate unicells, withflagella emerging from an apical depression.

� Cells are essentially naked, with a rigid coveringof the cell occurring inside the plasmalemma.

� Species able to produce thick-walled cysts that cansurvive adverse conditions.

� Different species occur in a variety of aquatic envi-ronments, both freshwater and marine.

� Heterotrophic (colourless) algae are well-represe-nted alongside autotrophic (green) species.

In spite of these similarities, there are a number ofmajor differences between the algal groups, indicat-ing a distinct phylogenetic origin. Key biochemicaldifferences include the presence of starch (stainsblue-black with iodine) in cryptomonads, withphotosynthetic pigments which include phycobilinsand show some similarity to red algae. Ultrastruc-tural differences also occur in terms of flagellarornamentation, and flagellar roots are differentfrom euglenoids. The environmental biology ofcryptomonads is also quite distinct from euglenoids,indicating a different ecological role for theseorganisms. Cryptomonads are genuinely planktonicalgae, widespread amongst the plankton of a range


1.8 CRYPTOMONADS 31

Table 1.10 Variation in Plastid Number and Colorationin Cryptomonads.

Trophic State Plastid Characteristics

Autotrophic Rhodomonas – a single boat-shaped,red-coloured plastid (Fig. 4.52)

Chroomonas – a single H-shapedblue-green plastid with a pyrenoid onthe bridge.

Cryptomonas – two plastids, each witha pyrenoid (Fig. 4.53).

Heterotropohic Chilomonas – a colourless cell thatcontains a plastid (leucoplast)lacking pigmentation. No pyrenoid.

of water bodies – in contrast to euglenoids, whichtend to occupy mud, sand and water interfaces.

1.8.3 Biodiversity

Cryptomonads are a relatively small group of algae,with a total of about 12 genera (Hoek et al., 1995) con-taining approximately 100 known freshwater speciesand 100 known marine species.

The low diversity in terms of number of speciesis reflected in the uniformity of morphology. Thereare no clear colonial forms, though some species areable to form irregular masses embedded in mucilage(palmelloid stage) under adverse conditions. Dif-ferences do occur in relation to plastids – includingpresence or absence, number and pigmentation (Table1.10). Variations also occur in other key cytologicalfeatures such as the presence or absence of nucleo-morphs, occurrence of ejectisomes and structure ofthe periplast – providing a basis for the classificationof these organisms (Novarino and Lucas, 1995).

1.8.4 Ecology

Cryptomonads are typical of temperate, high latitudestanding waters that are meso- to oligotrophic. Theyappear to be particularly prominent in colder waters,becoming abundant in many lakes in early spring,when they may commence growth under ice. Algae

such as Rhodomonas may form red blooms whenthe ice melts in early spring. In other (Antarctic)lakes, where ice persists, cryptomonad species suchas Cryptomonas sp. and Chroomonas lacustris maycontribute up to 70% of the phytoplankton biomass,dominating the algal flora for the whole of the limitedsummer period (Spaulding et al., 1994).

Cryptomonads are also typical of the surfacewaters of temperate lakes during the clear-waterphase of the seasonal cycle, between the diatomspring bloom and the beginning of the mixed summerbloom. During this period, when algal populationsare severely predated by zooplankton populations,the limited competition between algal cells favoursrapidly growing r-selected organisms such as cryp-tomonads (Sigee, 2004).

In oligotrophic freshwater lakes, cryptomonadsoften form large populations about 15–20 m deepwithin the lake, where oxygenated surface watersinterface with the anoxic lower part of the watercolumn. In this location, these algae form deep-wateraccumulations of photosynthetic organisms, referredto as ‘deep-chlorophyll maxima’. Studies on culturesof Cryptomonas phaseolus and Cryptomonas undu-lata isolated from deep chlorophyll maxima (Gervais,1997) showed that these organisms had optimalgrowth under light-limiting conditions, suggestingphotosynthetic adaptation to low light intensities.Other factors which may be important in the abilityof these organisms to form large depth populationsinclude the ability for heterotrophic nutrition,relative absence of predation by zooplankton,access to inorganic nutrients regenerated by benthicdecomposition and tolerance of sulphide generatedin the reducing conditions of the hypolimnion.

The ability of cryptomonads to carry out het-erotrophic nutrition, using ammonium and organicsources as a supply of carbon and nitrogen, isecologically important. Evidence for uptake of par-ticulate organic material comes from direct obser-vation of phagocytosis and has been documentedin both pigmented (e.g. Cryptomonas ovata) andvarious colourless species (Gillott, 1990). Ultra-structural studies also provide evidence that manypigmented cryptomonads are mixotrophic, capableof both autotrophic and heterotrophic nutrition. Theblue-green cryptomonad, Chroomonas pochmanni,



has a specialised vacuole for capturing and containingbacterial cells. Bacteria are drawn into this through asmall pore at the cell surface, and subsequently passinto digestive vesicles within the cytoplasm.

1.8.5 Cryptomonads as bioindicators

Although they are common in oligo- and mesotrophicconditions, cryptomonads also occur in eutrophiclakes (Fig. 2.16) and have limited use as bioindi-cators. Individual species do not provide the contem-porary diagnostic range for different habitats seen inother algal groups, and cryptomonads do not surviveas clearly identifiable remains within lake sediments.

1.9 Chrysophytes

Chrysophytes (Chrysophyta), commonly referred toas the ‘Golden Algae’, are a group of microscopicalgae that are most readily recognised by their golden-brown colour (Figs. 1.3 and 4.37). This is due tothe presence of the accessory pigment fucoxanthinwithin the chloroplasts, masking chlorophylls-a and-c (Table 1.3). There are around 200 genera and 1000species of chrysophytes, present mainly in freshwateralthough some may be found in brackish and saltwaters.

1.9.1 Cytology

Distinguishing cytological features (Table 1.3)include the presence of two dissimilar (long andshort) flagella (heterokont condition) in motile organ-isms, plastids with four outer membranes plus tripletthylakoids, chrysolaminarin as the main storage prod-uct (present in special vacuoles) and cell walls com-posed of pectin – covered in some species with smallsilica spines or scales. Lipid droplets also frequentlyoccur, and an eyespot may be visible in some species,associated with the chloroplast. In order to sur-vive adverse conditions, round-walled cysts (stoma-tocysts) may be produced, with silica scales present insome species. Although most chrysophyte species arephotosynthetic, some may be partially heterotrophicor even fully phagotrophic (Pfandl et al., 2009). Many

species are small and are important members of thenanoplankton. They are widely distributed through-out freshwater systems and are of particular interestin relation to their morphological diversity, ecologyand as indicator organisms. Details of taxonomy andultrastructure are given by Kristiansen (2005).


Chrysophytes exhibit considerable diversity in theirgeneral organisation, ranging from unicellular tospherical (e.g. Synura – Fig. 4.37) and branching(e.g. Dinobryon – Fig. 4.2) colonial types. Thegreat morphological diversity shown by these organ-isms (Table 1.11) follows a similar diversity seen inother algal phyla and includes flagellates, amoeboidforms, cells in jelly (capsulate), filamentous algae andplate-like (thalloid) organisms. Each of these types,however, can transform into another phase (e.g. flag-ellates can become amoeboid) – so that grouping inrelation to morphology is dependent on the stage oflife cycle. The occurrence of a resistant stage (stoma-tocyst) phase (see previous section) is also a key partof the life cycle.

1.9.3 Ecology

Chrysophytes are ecologically important in a numberof ways (Kristiansen, 2005).

� In many ecosystems, they play an important role asprimary producers. This is particularly the case foradverse conditions – low nutrient and acid lakes(e.g. Nedbalova et al., 2006).

� They have a versatile nutrition, with many mem-bers (e.g. Dinobryon) being mixotrophic. Theseorganisms are able, in addition to photosynthesis,to obtain their carbon from organic sources – eitherby surface absorption of organic compounds or byphagocytosis of particulate organic matter.

� They can be nuisance algae, giving a fishy smellto drinking water reservoirs when they reach highpopulation levels.


1.9 CHRYSOPHYTES 33

Table 1.11 Range of Body Structure and Form in Chrysophytes.

General Structure Colony Status Examples

Flagellate cells (monads)Simple cell wall Single cells: Chromulina, Ochromonas

Colonies UroglenaSurrounded by envelope (lorica) Colony Dinobryon (Fig. 4.2)cells covered in silica scales Single cells Mallomonas

Colony Synura (Fig. 4.37)Non-flagellate unicells

Motile by pseudopod Golden amoebae ChrysamoebaNon-motile Cells surrounded by a jelly Chrysocapsa

Cells surrounded by solid cell wall StichogloeaColonies with distinctive

morphologySimple branched filaments Sphaeridiothrix

PhaeothamnionTwo-dimensional cell plates Phaeoplaca

1.9.4 Chrysophytes as bioindicators

Their diverse ecological preferences (Table 1.12)make chrysophyte species potentially useful as envi-ronmental indicator organisms – both in terms of con-temporary water assessment and sediment analysis.

Contemporary water quality

Although potentially very useful, chrysophytes arerarely used in monitoring projects. The main rea-son for this is that the best ecologically studied

Table 1.12 Chrysophyte Species as Bioindicators.

Strongly acid, oftenhumic conditions

Dinobryon pediforme, Synurasphagnicola, Mallomonaspaludosa, Mallomonashindonii, Mallomonas canina

Weakly buffered,slightly acidclear water lakes

Mallomonas hamata, Synuraechinulata, Dinobryonbavaricum var. vanhoeffenii

Alkaline conditions Mallomonas punctifera, Synurauvella

Alkaline/salineconditions

Mallomonas tonsurata,Mallomonas tolerans

species (with silica scales) require specialised lightmicroscopy (Cumming et al., 1992) or scanning elec-tron microscopy – neither of which may be availablefor quality assessment.

Although chrysophytes have traditionally beenheld to indicate oligotrophic conditions, the situa-tion is more complex. Studies by Kristiansen (2005)have shown that the presence of a few species suchas Uroglena and Dinobryon may indicate oligotro-phy, but greater chrysophyte species diversity atlower overall biomass is more indicative of eutrophicconditions. The value of individual chrysophytespecies as ecological indicators varies considerably(Kristiansen, 2005). Synura petersenii, with a broadecological range, is of limited use – but other narrow-range species (Table 1.12) can be used particularly inrelation to pH and salinity.

Sediment analysis

The main application of chrysophytes as environ-mental bioindicators is in paleoecology. Their majoradvantage for this (as with diatoms) is the presenceof non-degradable silica cell wall material, so thatthey remain undamaged in the lake sediments. Theuse of these algae in paleoecology depends on



accurate means of classification and identification,undisturbed sediment layers and absolute datingof the layers – for example by the lead isotopePb210. Techniques for the extraction and countingof chrysophyte remains from sediment cores, withdetermination of concentrations using microspheres,are described by Laird et al. (2013).

Two main types of chrysophyte remains are usefulin freshwater sediments – silica scales (from vege-tative planktonic cells) and stomatocysts (resistantspores). Stomatocysts have the advantage of bet-ter preservation compared to scales, but the disad-vantage that (with a few exceptions) they cannotbe referred to particular species. Although stom-atocysts cannot be identified in terms of species,they can be recorded as distinct morphological types(morphotypes). These can be correlated with othermicrofossils (e.g. diatoms) and with pollen to obtainenvironmental indicator values. Quantitative assess-ment of past aquatic environments in relation to fossilchrysophytes (scales, stomatocysts) parallels that ofdiatoms (see Section 3.2.2) and can be obtained inrelation to species ratios (environmental indices),transfer functions (Facher and Schmidt, 1996), groupanalysis and total planktonic chrysophyte populations(flux).

Species ratios The ratio of chrysophytes to otheralgal groups may provide a useful index to assessenvironmental change. Studies by Smol (1985) on thesurface sediments of Sunfish Lake (Canada) demon-strated a marked increase in the ratio of diatoms tostomatocysts, coincident with the arrival of settlersto the lake catchment area. The diatom/stomatocystratio is a trophic index, signalling an increase ineutrophic status of the lake at the time of human set-tlement. This ratio change resulted from a populationdecrease in the chrysophyte Mallomonas (indicatingoligotrophic conditions) and an increase in eutrophicdiatoms. The increase in trophic ratio parallels anincrease in the pollen count of ragweed, an indicatorof forest clearance and farming activity.

Transfer functions The environmental proper-ties of stomatocyst morphotypes can be assigned asnumerical descriptors or ‘transfer functions’. Thesedefine each morphotype in relation to a range of

environmental data (pH, conductivity, phosphoruscontents and maximal depth of the lake).

The transfer functions have been derived by multi-variate analysis of sediment calibration sets, obtainedfrom a series of North American lakes in whichboth environmental data and stomatocyst assem-blages were available (Charles and Smol, 1994).

Group analysis The various environmentalfactors which influence correlations between mor-photypes can be identified by statistical analysis –particularly principle component analysis (PCA).Studies by Duff and Smol (1995) on 181 morphotypesobtained from 71 different lakes in the United Statesshowed that differences in pH and lake morphometryhad the greatest influence in determining statisticalassociations. PCA separated morphotypes into threemain groups relating primarily to acid environments,neutral/alkaline environments and lake morphometry(e.g. lake depth).

Total Chrysophyte populations Assessment ofoverall changes in planktonic chrysophytes can bedetermined from sediments by multiplying the con-centration of chrysophyte scales by the annualsediment rate. Laird et al. (2013) used thisapproach in their analysis of limnological changesof Saskatchewan (USA) lakes, showing that anincreased flux of chrysophyte scales was consistentwith recent climatic changes deduced from changesin diatom assemblages.

1.10 Diatoms

Diatoms (Bacillariophyta) are a very distinct groupof algae, identifiable under the light microscope bytheir yellow-brown coloration (Fig. 1.13) and bythe presence of a typically thick silica cell wall.This normally appears highly refractive under thelight microscope, giving the cell a well-definedshape. Removal of surface organic material bychemical oxidation (see Section 2.5.2) often revealsa complex cell-wall ornamentation – as illustratedin the light microscope images comparing living anddigested cells of Stephanodiscus (Fig. 4.58). Cell-wall ornamentation can be seen even more clearly


1.10 DIATOMS 35

with the higher resolution of the scanning electronmicroscope and is important in species identification.

Diatoms occur as non-flagellate single cells, simplecolonies or chains of cells, and are widely distributedin both marine and freshwater environments. Theirsuccess in colonising and dominating a wide rangeof aquatic habitats is matched by their genetic diver-sity – with a worldwide total of 285 recorded genera,encompassing 10,000–12,000 species (Round et al.,1990; Norton et al., 1996). Diatoms are also veryabundant in both planktonic and benthic freshwaterenvironments, where they form a major part of thealgal biomass, and are a major contributor to primaryproductivity.

1.10.1 Cytology

Distinctive cytological features (Table 1.3) include:

� Plastids with periplasmic endoplasmic reticulumand the presence of girdle lamellae.

� Chrysolaminarin and lipid food reserves, presentoutside the plastid.

� A distinctive cell wall, the frustule, composedof opaline silicon dioxide (silica) together withorganic coatings.

Silica cell wall

The presence of silica in the diatom cell wall canbe detected by cold digestion in a strongly oxidisingacid to remove organic matter (see Section 2.5.2).If the cell wall resists this treatment it is probablysilica. The cell wall of diatoms differs from thatof other algae in being almost entirely inorganic incomposition. It has evolved as an energy-efficientstructure, requiring significantly less energy to man-ufacture than the cellulose, protein and mucopeptidecell walls of other algae (Falkowski and Raven, 1997).This gives diatoms a major ecological advantage attimes when photosynthesis is limited (e.g. early inthe seasonal cycle), but has a number of potentialdisadvantages.

� The cell wall of diatoms is very dense. Theseorganisms are only able to stay in suspension inturbulent conditions, limiting the development ofextensive planktonic diatom populations to wellmixed unstratified waters.

� The formation of the diatom cell wall is dependenton an adequate supply of soluble silica (silicic acid)in the surrounding water. The diatom spring bloomof temperate lakes strips out large quantities ofsilica from the water, reducing concentrations to alevel that becomes limiting.

� Unlike other types of cell wall material, silica isrigid and unable to expand. This means that daugh-ter cells are unable to enlarge and progressive celldivisions result in a gradual decrease in cell size.Ultimately this decrease reaches a critical level,at which point sexual reproduction is required tocompletely shed the original cell wall and formnew, large daughter cells.

Cell wall structure

The frustule is composed of two distinct halves – theepitheca and hypotheca – which fit together ratherlike the lid and base of a pill-box (Fig. 1.12). Atcell division, two new walls are formed internallyat the cell equator ‘back to back’, and become thehypothecae of the two daughter cells. The originalepitheca and hypotheca of the parent diatom becomethe epithecae of the two daughter cells, so the epithecais always the oldest part of the frustule.

In both centric and pennate diatoms the epithecaconsists of two main parts – a circular disc (the epi-valve) with a rim (the mantle) plus an attached bandor girdle (the epicingulum). The hypotheca corre-spondingly consists of a hypovalve, hypovalve mantleand hypocingulum. When the epitheca and hypothecafit together to form the complete frustule, thehypocingulum fits inside the epicingulum to give anoverlap.

Because of frustule morphology (Fig. 1.12),diatoms can be observed from either a girdle (side)view or a valve (face) aspect – with girdle and valveviews of live diatoms being clearly distinguishable



A

B C

D E

1

2

3

4

5aa

aa

ta ta

e

h

e

h

eh

Figure 1.12 Diatom frustule struc-ture – comparison of centric andpennate diatoms. Centric diatom:A. Separate views of epitheca (e:1. Epivalve, 2. Mantle of epivalve,3. Epicingulum) and hypotheca (h:4. Hypocingulum, 5. Hypovalve). B.Complete frustule – girdle or sideview (showing overlap of cingula). C.Valve or face view of epivalve. Pen-nate diatom: D. Complete frustule –girdle view. E. Valve view, showingapical or longitudinal axis (aa) andtransapical or transverse axis (ta).

under the light microscope (Fig. 1.13). The two pos-sible valve views, of the top (epivalve) or bottom(hypovalve) of the diatom, are not distinguishable inmany genera – but do differ in some cases (e.g. Coc-coneis, Achnanthes). Axes of symmetry in pennatediatoms are shown in Figs. 1.12 and 1.13.

Frustule markings

A wide range of surface markings can be seen onthe face (epivalve and hypovalve) of diatoms. Thesehave been recorded in considerable detail (Barber andHaworth, 1981; Round et al., 1990; Wehr and Sheath,2003) and form the basis for the classification andidentification of these organisms (see Chapter 4).

Clear visualisation of the frustule markingsrequires the high resolution of either oil immersion(light microscopy) or scanning electron microscopy,and is normally carried out after the removal ofsurface organic matter by chemical (acid diges-tion) cleaning. Chemical fixation for scanning elec-tron microscopy may also strip away some of the

overlying organic material to reveal frustule surfacestructure.

The terminology of diatom morphology (see Glos-sary, Chapter 4) includes various descriptors offrustule markings – including eye-shaped structures(ocelli), small pores (punctae) and fine lines (striae).Illustrations of diatom surface markings are showndiagrammatically in Figs. 1.13–1.14, and in variousfigures and plates in Chapter 4. Although the bio-logical significance of much of this surface detailis obscure, the presence of one major surface struc-ture – the raphe – is clearly associated with locomo-tion. The secretion of mucus from this channel orcanal promotes movement on solid surfaces. In somediatoms such as Nitzschia (Fig. 4.70a,b), the rapheis elevated from the main diatom surface as a keel,allowing more intimate contact between the rapheand substrata. Such keeled diatoms are able to moveparticularly well on fine sediments, and reach theirmaximum abundance in the epipelon of pools andslowly flowing streams (Lowe, 2003). A raphe is notalways present in pennate diatoms and is never seenin centric diatoms.


1.10 DIATOMS 37

a a

r

t a pa pa

v v

v p

v p

v p

e

e

p a

vj

g

v

vr

h

g

p

g g

15 μm

p

a a

n

r

(A) h (B)

t a

c n

Figure 1.13 Details of frustule structure in a pennate diatom (Pinnularia). Top: Diagrams of cell wall structure, withprincipal axes. Top left: Valve view. aa, apical axis; ta, transapical axis; pn, polar nodule; r, raphe. Top middle: Girdleview. pa, pervalvular axis; vp, valvar plane; h, hypotheca; e, epitheca; g, girdle. Top right: Transverse section. Fritsch,1956. Reproduced with permission from CUP. Bottom: Light microscope images of fresh (unfixed) cells in valve (left)and girdle (right) view.



1 2 3 5

876

12 13

15 16 17

18 19 20

14

9 1110

4

Figure 1.14 Diatom valve markings: (1) internal septa;(2) transapical costae; (3) raphe in thickened ribs; (4) nor-mal raphe; (5) shortened raphe in ribs; (6) parallel striae;(7) radiate striae; (8) central area round; (9) central areatransverse; (10) central area small; (11) central area acuteangled; (12) punctae in radial rows interspersed with sub-radial rows; (13) coarse areolae; (14) areolae grouped insegments; (15) sections of valve alternately raised anddepressed giving shaded appearance; (16) central regionraised or depressed; (17) pennate with raised or depressedareas; (18) ocelli on surface; (19) rudimentary raphe, asin Eunotia; (20) raphe curved to point at centre, as inEpithemia.


The morphological diversity of diatoms can beconsidered in relation to two main aspects – thedistinction between centric/pennate diatoms and therange of unicellular to colonial forms.

Centric and pennate diatoms

Diatoms can be separated into two major group-ings – centric and pennate diatoms, based primarilyon cell shape and frustule morphology (Fig. 1.12).Centric diatoms typically have a discoid or cylindri-cal shape, with a radial symmetry when seen in face or‘valve’ view. Pennate diatoms have an elongate bilat-eral symmetry, with longitudinal and transverse axes,and often have the appearance of a ‘feathery’ (hence‘pennate’) ornamentation when seen in surface view(Fig. 1.13). Pennate diatoms have a range of shapes(Barber and Haworth, 1981), with broad division(Fig. 1.15) into isopolar (ends of valve similar in sizeand shape), heteropolar (ends of valve differing in sizeand shape), asymmetrical (outline dissimilar eitherside of the longitudinal axis) and symmetrical (outlineof valve similar either side of the longitudinal axis).

The fundamental differences between centric andpennate diatoms in relation to structure and symme-try reflect a range of other distinguishing features(Table 1.13) – including motility, number and sizeof plastids, sexual reproduction and ecology. Centricdiatoms are mainly planktonic algae, and the occur-rence of oogamy – with the production of a largenumber of motile sperms, is regarded as a strategy forincreasing the efficiency of fertilisation in open-waterenvironments. Pennate diatoms are almost alwaysisogamous, with equal-sized non-flagellate gametes(two per parent cell). In these algae, efficiency offertilisation is promoted by the pairing of gameteparental cells prior to gamete formation, a possibleadaptation to more restricted benthic environments.Fossil and molecular evidence suggests that centricdiatoms arose prior to pennate ones, implying thatin these algae isogamy has been phylogeneticallyderived from oogamy (Edlund and Stoermer, 1997).This is in contrast to the evolutionary sequence nor-mally accepted for other algal groups – particularlygreen and brown algae.

Unicellular and colonial diatoms

The genetic diversity of diatoms is not matched bythe complexity of their morphological associations –which are limited to small chains and groups of cells.


1.10 DIATOMS 39

12

3

5

d

b

s

4

a

c

Figure 1.15 Diatom morphology:(1) pennate diatom with raphe; (2)pennate diatom with pseudoraphe; (3)sulcus (s) on Melosira; (4) spines onmargin of Stephanodiscus; (5) valveshapes: (a) isopolar, (b) heteropolar,(c) asymmetrical, (d) symmetrical.

In planktonic diatoms, simple filamentous (e.g. Aula-coseira – Fig. 4.9) and radial (e.g. Asterionella – Figs.1.16 and 4.42) colonies have evolved, but there is noformation of spherical globular colonies that are akey feature of other algal groups.

Colonial forms of diatoms arise by means of simplelinkages, which are of three main types.

� Mucilage pads. These occur at the ends of cellsto join diatom cells in various ways. Asterionella

Table 1.13 Centric and Pennate Diatoms.

Characteristic Centric Diatoms Pennate Diatoms

Symmetry Radial BilateralExamples Stephanodiscus (Fig. 4.58)

Cyclotella (Fig. 4.62)Pinnularia (Figs. 1.13 and 4.71)Navicula (Fig. 4.73)

Gliding Motility Non-motile Some (raphid) diatoms are activelymotile.

Plastids Many discoid plastids Two large plate-like plastidsSexual reproduction

(Hasle and Syvertsen,1997)Gamete formation Independent formation of gametes from

parent cellsPairing of parental cells prior to gamete

productionEgg cells Oogamous – production of one or two

eggs per parent cellIsogamous

Sperm cells 4–128 sperms per parent cell. Each witha single flagellum bearing two rowsof mastigonemes.

Few amoeboid, non-flagellate spermcells.

Ecology Mainly planktonic, typical of openwater

Planktonic, epiphytic and benthic forms



100 μm

Figure 1.16 Spring diatom bloom in a eutrophic lake.SEM view of lake surface phytoplankton sample(March), showing numerous stellate colonies of Aste-rionella (A) with some filamentous Aulacoseira (Au).The large amount of debris (d) occurs due to the com-plete mixing of the water column at this time of year. Seealso Figs. 2.8 (seasonal cycle), 4.9 (live Aulacoseira) and4.42 (live Asterionella).

has elongate cells which are joined at their innerends to form stellate colonies. In Tabellaria, cellsare linked valve to valve as short stacks, with con-nection of the stacks at frustule edges by mucilagepads to form a zig-zag pattern. Mucilage pads areseen particularly clearly in Fig. 2.28, where chainsof Tabellaria are attached to Cladophora.

� Interlinking spines. In diatoms such as Aulaco-seira, chains of cells are joined valve to valve byspines. The valves are of two main types (Daveyand Crawford, 1986):

– Separation valves, with long tapering spines andstraight rows of pores

– Linking valves, with short spines and curvedrows of pores

Cells are strongly linked into chains via the shortspines on the linking valves, but weakly linked viathe long spines of the separation valves – where fil-ament breakage easily occurs (Fig. 4.9). The lengthof filaments within a population of Aulacoseira

depends on the proportion of separation valves tolinking valves.

� Gelatinous stalks. Biofilm diatoms such asRhoicosphenia (Fig. 4.67) and Gomphonema(Fig. 2.29) have gelatinous stalks, which attach thediatom to the substratum and join small groups ofcells together as a single colony. In these diatoms,the stalk material is secreted via apical pores. Somespecies of Cymbella also produce a gelatinoustube-like filament.

1.10.3 Ecology

Diatoms are ubiquitous in both standing and runningfreshwaters – occurring as planktonic, benthic, epi-phytic (on higher plants and other algae) and epizoic(on animals such as zooplankton) organisms. Theyare equally successful as free-floating and attachedforms, where they may be respectively important inplankton bloom formation and biofilm development.

Planktonic diatoms: bloom formation

In many temperate lakes, diatoms such as Asteri-onella and Tabellaria dominate the phytoplanktonpopulation in Spring and early Summer (Fig. 1.16),at a time when inorganic nutrients (N, P, Si) are at highconcentration, light and temperature levels are risingand lake turbulence is maintained by moderate windaction. At this point in the seasonal cycle, diatomsare able to out-compete other microalgae due to theirtolerance of low temperature and low light condi-tions, coupled with their ability to grow in turbulentwater. Autumn blooms of Fragilaria and Asterionellaare also a common feature of many lakes, with thediatoms part of a mixed phytoplankton population.

Diatoms such as Aulacoseira italica are regardedas meroplanktonic, with major growth in veryearly Spring, but spending the rest of the year asresistant cells on lake sediments. Other diatoms,such as Asterionella and Tabellaria also form majorblooms in Spring, but are additionally present asminor constituents within the mixed phytoplanktonpopulation over much of the annual cycle – and areregarded as holoplanktonic.


1.11 RED ALGAE 41

Attached diatoms: biofilm formation

Diatoms are major components of biofilms, wherethey are present early in the colonisation sequenceand also within the mature periphyton community.See later for examples of diatoms in river biofilm(Fig. 2.23), reed biofilm (Fig. 2.29) and as attachedepiphytes (Fig. 2.28).

1.10.4 Diatoms as bioindicators

The major use of diatoms as bioindicators of waterquality (both lotic and lentic systems) is detailed inChapter 3. These algae can be isolated from existinglive populations (contemporary analysis) or from sed-iments (fossil diatoms), and quantitative informationon water quality from species counts can be obtainedfrom taxonomic indices, multivariate analysis, trans-fer functions and species assemblage analysis.

Contemporary analysis

Many diatom species have distinct ecological prefer-ences and tolerances, making them useful indicatorsof contemporary ecological conditions. These prefer-ences relate to degree of water turbulence, inorganicnutrient concentrations, organic pollution, salinity,pH and tolerance of heavy metals. Round (1993) alsolists a number of other advantages in using diatomsas indicators – including easy field sampling, highsensitivity to water quality but relative insensitivityto physical parameters in the environment and easycell counts. Diatoms are also the most diverse groupof algae present in freshwaters, making them themost ideal assemblage for calculation of bioindices(Section 3.4.5).

Diatoms, once cleaned and mounted for identifi-cation, make excellent permanent slides which canform an important historical record for a location.Although there are hundreds of diatom species thatmay be present, only dominant species need to beused in an assessment. Benthic diatoms have beenparticularly useful in assessing water quality of rivers,

and the wide range of indices that have been devisedis discussed in Chapter 3.

Lake sediments

Diatoms, more than any other group of algae, are usedto monitor historical ecological conditions from sed-iment analysis – as discussed in Section 3.2.2. Themajor role of diatoms in this lies in the resistance ofthe diatom frustule to biodegradation, coupled withease of identification from frustule morphology andthe wide range of species with clear ecological pref-erences seen in freshwater environments.

The importance of diatoms in environmentalanalyses is indicated by the European DiatomDatabase Initiative (EDDI). This key web-basedsite includes electronic images and data handlingsoftware and is particularly useful for lake sedimentanalysis (see Section 3.2.2). Other databases, suchas the stream benthic database of Gosselain et al.(2005), are more applicable to contemporary envi-ronmental analysis. Techniques for the collection ofsediment cores, preparation of diatom samples andprocedures for making counts are widely described(e.g. Laird et al., 2013). Methods for cleaningdiatoms in preparation for microscopy are given inSection 2.5.2.

1.11 Red algae

Red algae (Rhodophyta) are predominantly marinein distribution, with only 3% of over 5000 speciesworldwide occurring in true freshwater habitats(Wehr and Sheath, 2003). Although termed red-algae, the level of accessory pigments (phycoery-thrin and phycocyanin) may not be sufficient to maskthe chlorophyll – resulting in an olive-green to blue(rather than red) coloration. The relatively commonriver alga Batrachospermum, for example, typicallyappears bluish-green in colour (Fig. 4.4) – giving noclue to its rhodophyte affinity.

Major cytological features of red algae (Table1.3) include the absence of flagella, presenceof floridean starch as the major food reserve,



characteristic photosynthetic pigments (chlorophyll-a only, presence of phycobilins) and distinctiveplastid structure (unstacked thylakoids, no externalendoplasmic reticulum).

Many of the freshwater species (which occurmainly in streams and rivers) are quite large, beingvisible to the naked eye when occurring in rea-sonable numbers (Sheath and Hambrook, 1990).The range of morphologies includes gelatinous fila-ments (e.g. Batrachospermum), pseudoparenchyma-tous forms (e.g. Lemanea) and a flat thallus of tieredcells (e.g. Hildenbandia). Many of these shapes canbe an advantage in resisting the forces exerted inswiftly flowing waters.

Although freshwater red algae (such as the large fil-amentous alga, Batrachospermum) are largely foundin streams and rivers, Rhodophyta may also occur asmarine invaders of lakes and brackish environments.Certain freshwater red algae in the littoral zones ofthe Great Lakes Basin (USA), for example, appear tobe originally marine and to have lost the capacity forsexual reproduction. These include the filamentousred alga Bangia atropurpurea (Lin and Blum, 1977),which reproduces only by asexual monospores –in contrast to marine species which undergoalternation of generations and carry out sexual repro-duction. Attached red algae (e.g. Chroodactylon

ramosum) also contribute to the epiphytic flora oflake periphyton.

1.12 Brown algae

As with red algae, brown algae (Phaeophyta) arealmost entirely marine – with less than 1% of speciespresent in freshwater habitats (Wehr and Sheath,2003). These species are entirely benthic, either inlakes or rivers, and have a very scattered distribution.

Cytological features of this group (Table 1.3)include heterokont flagella (reproductive cells only),presence of laminarin as the major food reserve,characteristic photosynthetic pigments (chlorophyll-a and -c, β- and ε-carotenes) and distinctive plastidstructure (triple thylakoids, enclosing endoplasmicreticulum). Freshwater brown algae include generasuch Pleurocladia and Heribaudiella and are the leastdiverse of all freshwater algae. Their morphologiesare based on a relatively simple filamentous struc-ture (tufts or crusts), and they lack the complexmacro-morphology typical of the brown seaweeds.The freshwater brown-algae have not been fully stud-ied and their ecological characteristics are not wellknown. All are benthic and may be recognized bytheir possession of large sporangia (Wehr, 2002).

introductiontofreshwateralgae · 2020. 1. 16. · 7.chrysophytes (chrysophyta a, c 1, c 2, c 3 α,...

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