factors affecting algae biofilm growth and lipid production_a review

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Factors affecting algae biofilm growth and lipidproduction: A review

ARTICLE in RENEWABLE AND SUSTAINABLE ENERGY REVIEWS DECEMBER 2015

Impact Factor: 5.9 DOI: 10.1016/j.rser.2015.07.090

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Peter Schnurr

University of Toronto

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D. Grant Allen

University of Toronto

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Factors affecting algae biolm growth and lipid production: A review

Peter J. Schnurr, D. Grant Allen n

Department of Chemical Engineering& Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 3E5

a r t i c l e i n f o

Article history:

Received 21 May 2015

Received in revised form

7 July 2015

Accepted 17 July 2015

Keywords:

Microalgae

Biolm

Biofuels

Lipids

Growth parameters

Productivity

a b s t r a c t

Algae is recognized as a potentially valuable source of biofuels and biochemicals; however, a major

limitation to commercialization is in the high cost of harvesting, de-watering, and downstream

processing of dilute algae biomass when it is grown planktonically. Growing algae as a biolm offers

potential advantages for biomass processing because biolms are immobilized and orders of magnitude

more concentrated. For these reasons there has been an emerging interest in algae biolm biofuel

research over the past several years. Additionally, there has been a considerable amount of work on

understanding algae biolms in nature, and on using algae biolms for tertiary wastewater treatment.

This review paper draws from all of this literature to describe algae biolm composition, and their

growth responses to the key environmental factors affecting growth and internal lipid concentrations;

the emphasis being on optimizing biomass and lipid productivity. Additionally, the paper summarizes

key things known about planktonic algae growth and bacterial biolm growth in order to make

inferences about the potential growth of algae biolms. The paper identies many key knowledge gaps

in the potential for producing biomass and lipids from algae biolm growth systems.

& 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4182. Composition and structure of an algal biolm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419

2.1. Extracellular polymeric substances and matrices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419

2.2. Species and succession of photosynthetic biolms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420

3. Algae biolm biomass and lipid production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

3.1. Algae biolm attachment to growth materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

3.1.1. The affect of material properties on algae biolm growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

3.1.2. Biotic factors on biolm development and growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 422

3.2. Key growth parameters and their affect on algae biolm biomass productivities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

3.2.1. Light intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423

3.2.2. Carbon dioxide concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

3.2.3. Other growth factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

3.3. Algae biolm biomass lipid potential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

3.3.1. Lipids and lipid concentration enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

3.3.2. Algae biolm productivities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427

1. Introduction

Algae growth systems show great potential for the production

of biofuels and bioproducts. The main reasons for this are due to

their high growth rates doubling times as low as 78 h[1] and

Contents lists available atScienceDirect

jo ur nal ho me pag e: www.elsevier.com/locate/rser

Renewable and Sustainable Energy Reviews

http://dx.doi.org/10.1016/j.rser.2015.07.090

1364-0321/&2015 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: 1 416 978 8517.

E-mail addresses: [email protected] (P.J. Schnurr),

[email protected] (D.G. Allen).

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high concentrations of valuable biocompounds. Coupled together,

high biomass growth rates and biocompound concentrations

results in high overall productivities of valuable biocompounds.

For instance, Mata et al. [2] estimated that microalgae could

generate between 58,700 and 136,900 L/ha yr of biodiesel

depending on biomass lipid contents of 30% or 70% compared

to 5366 L/ha yr for the best terrestrial biodiesel crop (Palm Oil).

Other valuable biocompounds of interest within algae biomass are

amino acids, fatty acids, pigments, carbohydrates, polysaccharides,vitamins, and antioxidants, which can be used to generate nutra-

ceuticals, pharmaeucticals, functional foods and food additives,

farm and aquaculture feed, biofuels, cosmetics and bioplastics

[24]. In addition to high productivities, growing algae has

advantages over conventional biofuel sources because it can be

grown on non-arable lands, growth ponds and reactors can be

scaled vertically, and algae can be grown on wastewater[57]and

ue gas waste streams[8,9],thereby providing essential nutrients

for growth to the algae while simultaneously mitigating pollution

from these waste streams.

Researchers have identied several key limitations to commer-

cialization of biofuels and bioproducts from algae growth systems,

including optimizing growth rates and internal biochemical compo-

sitions through genetic modication of species, optimization of

growth conditions and reactor design, and developing the biorenery

concept for algae[10].Many researchers agree that one of the most

signicant economic limitations to commercialization, however, is

the harvesting and dewatering of algae biomass grown planktonically

estimated to be up to 30% of the production costs [1113]. By

growing algae as a biolm there is potential to signicantly reduce

these costs because the biomass is immobilized and much more

concentrated 0.4% (g/g) for planktonic systems[10], compared to

816% in biolms[14,15]. The immobilized nature may also make

downstream processing easier and more economical.

There has, however, been a limited amount of work conducted

on the production of biofuels and biochemicals with algae biolms

compared to planktonic growth systems. Of the work that has been

done on algae biolms for biofuel production, much of it revolves

around the study of the attachment, and subsequent growth, of

algae biolms to various growth materials [14,1623]. There has

also recently been a signicant amount of algae biolm research on

novel reactor designs studying biomass and lipid production rates

[14,1718,20,21,2429]. Other studies on algae biolms have related

not specically to biofuel and bioproduct production, but rather, on

using them to treat wastewaters of nitrogen and phosphorus using

algal turf scrubbers and novel biolm growth systems[56,3038].

Additionally, there are many studies on algae biolms from a

fundamental perspective i.e. oxygen proles under various condi-

tions, species composition and succession, affects of shear rate,

affects of temperature, etc., which is helpful in understanding how

to optimize growth rates and lipid concentrations for biofuel and

biochemical production. Lastly, there is a large breadth of knowl-

edge on bacteria biolms and planktonic algae growth systems thatmay provide insights into algal biolm systems.

This review paper summarizes the current knowledge of algae

biolms, and how it relates to the production of biomass and

biofuels. The overall objective of this paper is to provide an over-

view for what we currently know about algae biolm development

and composition, and growth parameters that affect growth rates

and internal biomass lipid concentrations. The review will include

studies done on algae biolms for biofuel and biochemical produc-

tion, on algae biolms studied from a fundamental perspective, and

on algae biolms used to treat wastewater streams. It will also draw

from the extensive literature on bacterial biolms and planktonic

algae growth systems to make inferences about algae biolm

growth systems. This review will not focus on reactor design, since

reactor designs were covered extensively by Berner et al. [39]. A

review such as this is important as algae biolm biofuel and

biochemical growth systems have emerged as a promising biotech-

nology over the past 5 years, evidenced by the growing number of

groups working on such systems. To the best of our knowledge no

such review has been written to date.

2. Composition and structure of an algal bio

lm

Biolms are communities of microorganisms attached to each

other and a solid growth substratum. They form virtually any-

where water exists for extended periods of time. Photosynthetic

biolms in nature are often referred to as periphytic or algae

biolms, which are mostly composed of algae, cyanobacteria, and

heterotrophic bacteria living in symbiosis. The composition and

structure of photosynthetic biolms will vary according to abiotic

and biotic factors within the environment.

2.1. Extracellular polymeric substances and matrices

In biolms, extracellular polymeric substances (EPS) are bio-

molecules and inert solids that bind cells to each other and to solid

materials. Extracellular polymeric substances are located on the

outside of cells, generated through active secretion, cell lysis,

shedding of cell surface material, and adsorption from the envir-

onment [40,41]. The predominant EPS are polysaccharides and

proteins; however, nucleic acids, lipids, and suspended solids can

also make up the EPS matrix [42].

In well-developed biolms the EPS forms a matrix that creates a

microenvironment for the cells. This microenvironment protects cells

from environmental stress such as dehydration, and uctuations of

pH, temperature, and nutrient concentrations[41,43,44].Additionally,

EPS matrices are known to act as a nutrient reservoir as enzymes

within the matrix digest EPS and inert solids,[45],and the EPS acts as

an ion exchange resin as it traps nutrients through sorption [40,46]. In

these ways the EPS matrix helps accumulate and concentrate

nutrients from the bulk medium. Although EPS is 99% water and

collapses upon itself when dehydrated[47], it can compose up to 90%

of the organic matter in some (bacterial) biolms[46].

During biolm formation and growth, microalgae will respond

to environmental circumstances by increasing or decreasing the

expression of specic promoters that affect EPS production. For

instance, Becker [48] demonstrated that the diatom Amphora

coffaeformis increases EPS production when it is in contact with

materials that have good adhesion strength with EPS. A growth

material effect on algae EPS production was also demonstrated by

Shen et al. [28]. Additionally, Domozych[49]and Shen et al. [28]

demonstrated that increasing nutrient concentrations, particularly

nitrogen, would increase EPS production from diatom and green

algae species. This is likely because a signicant fraction of EPS iscomposed of proteins [28], allowing the cells to over-produce

amino acids while nitrogen is abundant and environmental condi-

tions are favorable. There is some evidence that suggests algae cells

increase their EPS production as their colonies age and mature

[28,48]. This could be a result of mature colonies allocating less

resources into reproduction i.e. reaching a stationary growth phase,

and more into stabilizing their biolm community. Lastly, there is

evidence that temperature stress and mineral (calcium) accumula-

tion stress adversely affect EPS production from algal cells [49].

Although it is clear that algae cells produce EPS according to

environmental stimuli, compared to bacterial systems, the literature

on EPS production and EPS matrices in axenic and mixed commu-

nity algal biolms is limited. This is an opportunity for fundamental

algal biolm knowledge development.

P.J. Schnurr, D.G. Allen / Renewable and Sustainable Energy Reviews 52 (2015) 418 429 419
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2.2. Species and succession of photosynthetic biolms

Photosynthetic biolms have many different algae, bacteria,

cyanobacteria, protozoa, and multicellular microorganism species

present within them. Diatoms, green algae, and lamentous algae

are groups of algae that usually compose signicant fractions of

biolm biomass; some of these species are known to grow both

autotrophically and heterotrophically[50,51].Bacteria species within

biolms include cyanobacteria, and heterotrophic and autotrophic

bacteria. Organisms within biolms form symbiotic relationships

with one another, whereby heterotrophic bacteria supply carbon

dioxide during respiration, and photosynthetic organisms utilize the

carbon dioxide to generate biomass, thus producing the oxygen for

bacteria during respiration [5255]. Additionally, carbohydrates,

vitamins and organic compounds are excreted by, and become

nutrients for, both algae and bacteria within the biolm[52]. Mature

biolms reach a steady state with organisms within the biolms and

the environment in which it exists.

Biolm maturity affects the succession of species present i.e.

abundance and proportions of algae, bacteria and EPS (Fig. 1). Early

stage development photosynthetic biolms have a high proportion

of EPS and bacteria compared to algae and cyanobacteria [56,57].

Some researchers refer to this as conditioning of the biolm

growth surface (Fig. 1A). Mack and Anderson [58] reported that,

after the establishment of the EPS matrix, algae cells begin to

rapidly grow in the upper layers (Fig. 1B and C) of the matrix

causing bacteria to form aerial colonies (away from the substrate) to

compete for nutrients. If the environmental conditions are favorable

(discussed later), a mature algae biolm will form, consisting of a

dense and diverse population dominated by algae cells (Fig. 1D)[56,57,59].The biolm matrix facilitates the retainment and sharing

of nutrients, as symbiotic relationships exist between the hetero-

trophic and autotrophic microorganisms (Fig. 1C and D).

Biolm maturity also affects the succession and proportions of

specic algal groups. Researchers have demonstrated that diatoms

make up a signicant fraction of early biolms i.e. rst 1520 days of

growth, while lamentous chlorophytes become predominant after-

wards[60,61].However, Sekar[62]reported that early phase biolm

succession (14 days) is dominated by green algae, followed by a

second phase (59 days) dominated by diatoms, and lastly, a third

phase dominated by cyanobacteria (1015 days). Zippel and Neu[57]

also demonstrated that cyanobacteria are a late successional micro-

organism. The differences of succession of algae species is likely

attributed to the types of species initially present in the growth

medium, and how they respond to the particular abiotic growth

conditions of the experiment. The literature suggests that non-axenic

photosynthetic biolms eventually become dominated by chloro-

phytes (single celled andlamentous) and cyanobacteria given enough

time, and provided any form of seeding mechanism. This has

implications for long-term operation of algae biolm growth systems,

as species type has signicant potential to affect overall biomass and

lipid productivities, as will be demonstrated later in this paper.

Nutrient concentrations and light intensity have a strong effect

on the abundance of algal species in a biolm, compared to

heterotrophic bacteria, EPS and inert solids. Organic and inorganic

carbon concentrations in the growth medium will affect the

abundance and proportions of algae compared to bacteria high

inorganic and low organic carbon results in algae dominated

cultures [55]. In addition to carbon species concentrations, Villa-

nueva et al. [63] and Kebede-Westhead et al. [35] demonstrated

that increases in nitrogen and phosphorus loading rates cause

signicantly greater photosynthetic biomass accumulation com-

pared to bacteria. With adequate nutrients and light, photosynthetic

biolms will be greater than 75% algae biomass [56,57]; however,

when biolms become too thick, or light intensities insufcient,

light limitation will occur and the biolm layer furthest from the

light source (light limited) may become dominated by bacteria, EPS,

and other non-photoautotrophic materials, as was demonstrated by

Kuhl et al.[64]and Guariento et al. [65].Fig. 2demonstrates these

photosynthetically inactive layers and their respective depths

under different light regimes increased light intensity increases

the photosynthetically active regions of the biolm, and the overall

amount and proportions of algae biomass. This idea is discussed

later in the light intensity section of this paper.Light intensity, temperature, nutrient concentrations, and shear

rates affect the succession of photosynthetic biolms in terms of

what algal species are predominant. Congestri et al.[59]and Davies

et al. [66]concluded that seasonal light and temperature uctua-

tions in wastewater treatment plants affected the proportions of

specic algal groups/species in biolms. More specically, Kebede-

Westhead et al. [35]reported that diatoms were more prominent

under low light conditions (270 mol/m2/s) compared to high light

conditions (390 mol/m2/s), and Villanueva et al.[63]reported that

diatoms were a major biomass fraction in lower temperatures (7

11 1C) conditions compared to higher (1115 1C) temperature.

Moreover, cyanobacteria proportions were shown to increase rela-

tive to the diatom and green algae populations when temperatures

were increased from 12, 18 and 24 1C[67].This is not surprising as

Fig. 1. Development of a mixed community algal biolm: (A) growth surfaces are rst conditionedwith bacteria cells that excrete the initial EPS matrix; (B) various species

of algae cells present in the bulk medium then begin to colonize the EPS matrix; (C) the algae cells grow and reproduce, forming a symbiotic relationship with the bacteria

present in the EPS matrix; and (D) a mature biolm matrix is densely populated with algae cells, particularly cyanobacteria and chlorophytes, and retains nutrients in the EPS

matrix.

P.J. Schnurr, D.G. Allen / Renewable and Sustainable Energy Reviews 52 (2015) 418429420
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cyanobacteria is known to thrive under higher temperatures

optimal temperature between 30 and 35 1C for Aphanothece micro-

scopica Nagaliand Spirulina platensis grown planktonically[68,69].

Increased loading rates of nitrogen and phosphorus also seem to

favor cyanobacteria, composing up to 65% of the biolm under

these conditions [63]. Besemer et al. [60] reported the affects of

ow regime on succession. Particularly, it was shown that coccal

chlorophytes were more abundant under laminar and transitional

ow than in turbulent ow, and that operational taxonomic units

(number of species) decreased as ows were increased.

Biolm succession is extremely complicated, but is clearly

affected by biotic factors i.e. the types of species present, and

abiotic factors of the growth environment. It appears that high

nitrogen and phosphorus loadings, inorganic carbon concentrations,

and light intensities increase photosynthetic biomass accumulation

and proportions compared to non-photosynthetic biomass. Addi-

tionally, matured cultures, and high temperature and nutrient

loading rates cause signicant increases in cyanobacteria propor-

tions in photosynthetic biolms. Because some species present in

photosynthetic biolms produce signicantly less lipids and other

valuable compounds, it is important to know how biotic and abiotic

conditions affect the proportions and abundances of these species.

Future studies might consider studying (long-term) succession of

algae biolms to control for high proportions of high productivity

species both in terms of biomass productivities and of lipid and

other high value product biomass concentrations.

3. Algae biolm biomass and lipid production

Compared to planktonic algae growth systems, there has been a

limited amount of work on algal biolm growth systems for theproduction of biofuels and biochemicals. Of the algal biolm

biofuel research conducted to date, many researchers have focused

on the affects of growth materials on biolm attachment and

biomass accumulation [14,1623], and on novel reactor designs

and the biomass and lipid productivities they can produce

[14,17,18,20,21,2429]. Additionally, there has been some work

on how different growth parameters e.g. light intensity

[18,26,35,70], CO2 concentrations [26,29,71], etc., affects biomass

accumulation, and on quantifying and manipulating biomass lipid

concentrations [2427,72,73]. Additionally, much research has

been done from a fundamental understanding of algae biolms

in nature [57,63,64,74,75], and in using biolms to treat waste-

water of contaminants [5,6,3038]. The section below draws on

these studies, and on the extensive amount of literature on

planktonic algae growth, to determine how to potentially max-

imize productivity of algae biolm growth systems.

3.1. Algae biolm attachment to growth materials

Because biolms, by nature, are attached to a solid (growth)

material, it is important to understand if material properties affect

biolm formation, growth, and development. Growth material

properties studied by researchers are surface tension/surface wett-

ability/water contact angle/hydrophobicity, polar surface energies,

and surface micropatterning. In addition to material properties, cell

recruitment and overall biolm growth is a result of biotic factors

such as the presence ofrst inhabitors and extracellular polymeric

substances [53,56,58,76], and the re-growth of biolms already

acclimated and succeeded to the growth conditions[14,18,20].

3.1.1. The affect of material properties on algae biolm growth

Some researchers have studied differences in biolm growth

with different materials without quantifying the material proper-

ties, and found there are differences in growth rates. For instance,

Johnson and Wen[20]studied biolm growth rates on polystyrene

foam, cardboard, polyethylene fabric, and loofah sponge. They

found that polystyrene foam yielded signicantly higher biomass

productivities than the other materials tested, but gave no poten-

tial reason for this higher yield. Similarly, Christenson and Sims

[14]and Gross et al. [18] found that cotton rope and cotton duct,

respectively, were the best materials they each tested for growth.

It is clear different materials do affect biolm growth and devel-

opment, but understanding the properties of these materials is

critical to understand and predict biolm formation and growth.

Research conducted on the affects of material properties on algalbiolm growth is somewhat inconclusive. Some researchers have

demonstrated a correlation between hydrophobic surfaces and

biolm formation and growth. Specically, they conclude that

hydrophobic surfaces are ideal for the growth of biolms

[16,22,77]. The theory behind the affects of hydrophobicity is that

hydrophobic molecules, particles, and cells, prefer a hydrophobic

environment, and will therefore adhere to each other to minimize

their contact with water[78].Other researchers, on the other hand,

found no correlation or weak correlations between hydrophobicity

and algal biolm formation and growth [17,19]. The difference

between these groups of researchers and their ndings is the length

of the growth period and the density of the biolm grown.

Researchers reporting an affect of material properties on growth

did not observe growth over the long-term i.e. they observed initial

Low Light

Intensity

GrowthMaterial

GrowthMaterial

GrowthMaterial

Medium Light

Intensity

High Light

Intensity

Photosynthetically

Active Region Photosynthetically

Active RegionPhotosynthetically

Active Region

Photosynthetically

Inactive Region

Photosynthetically

Inactive Region

Photosynthetically

Inactive Region

Fig. 2. Schematic of light proles through an algae biolm at various light intensities: photon penetration through algae biolms increases with increasing photon ux

density. Thick algae biolms have relatively thin photosynthetically active regions adjacent to their source of light, but comparatively thick photosynthetically inactive

regions opposite the light source (A); as light intensity increases the photosynthetically active region increases due to increased photon penetration, and the subsequent

reduction of antennae size/number within this regions (B) and (C).

P.J. Schnurr, D.G. Allen / Renewable and Sustainable Energy Reviews 52 (2015) 418 429 421
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recruitment to the growth material; however, the groups reporting

no effect grew thick biolms for relatively long periods of time i.e.

the cells were eventually growing on top of each other. Genin et al.

[17] attributed differences in overall algal biolm productivity to

differences in colonization time, and that that colonization time i.e.

recruitment, is highly correlated to polar surface energies of growth

materials. Corresponding to the above conclusions, Ozkan and

Berberoglu [22] reported differences in cell attachment between

diatoms and green algae according to the hydrophobicity of thesurface, and that hydrophobic cells adhered to hydrophobic surfaces

more strongly than hydrophilic cells did to hydrophobic and hydro-

philic surfaces. They also concluded that, above all else, acidbase

interactions were the dominating mechanism for cell attachment to

substrata and each other. From the research on material properties it

would appear that polar surface energies, and cellcell and cell

substrata hydrophobicity and acidbase interactions are important

parameters to consider for biolm formation and short-term growth.

Once cells have adhered to a subsurface and conuence is reached,

however, cells are growing on top of each other and long-term

biolm biomass growth is a function of other growth parameters i.e.

nutrient concentrations, light availability, etc.

Surface roughness and micropatterns affect cell recruitment and

short-term biolm biomass accumulation rates, but not long-term

growth trends. A preliminary study conducted by Cao et al. [79]

showed algae cell populations were enhanced by micropatterning

stainless steel growing surfaces. Similarly, Sekar et al. [77]demon-

strated more biolm cell attachment with increasing surface rough-

ness of algae grown on both titanium and stainless steel growth

substratum. In both of the above studies it is important to consider

that the differences observed were after short growth periods and

low population densities i.e. attachment and recruitment rather

than growth. It is likely that surface roughness differences do not

actually affect overall biolm productivity rates over long term

growth periods when cell densities are high i.e. after cell conuence

is reached. This was demonstrated by Irving and Allen [19] and

Blanken et al. [71] as they concluded no statistically signicant

long-term algae biolm productivity gains from patterning the

surface. Their data does also, however, suggest signicant short-

term gains (rst 23 days of growth) from micropatterned surfaces.

Micropattern dimension and depth, compared to cell size, may be

a signicant factor in micropatterning over short-term growth

periods. Sathananthan et al. [23] tested 3 different micropatterns

on algal biolms grown for 10 days and did not observe increased

growth (compared to smooth surfaces) in shallow (1.5

factors affecting algae biofilm growth and lipid production_a review

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