Chlorophyceae

By Dr. S.N. Suresh

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Chlorophyceae

Green algae (Chlorophyta)

These algae live in wide variety of habitats, marine to fresh water to damp soil.

General Characteristics

(i) These are unicellular (Chlamydomonas), colonial (Volvox) or filamentous. When filamentous they are unbranched (Spirogyra, Ulothrix) or branched (Chara).

(ii) The cell wall consists of an inner layer of cellulose and outer layer of pectic compounds and may be covered by a gelatinous sheath.

(iii) The protoplasm is divisible into cytoplasm and nucleus. Cytoplasm contains one or more vacuoles. Chlorophyll is present in chloroplasts, the shape and number of which are characteristic of each alga.

(iv) Pigments chlorophyll-a and chlorophyll-b are predominant. However, carotene and xanthophyll are also present.

(v) Food reserve is in the form of starch surrounding the proteinaceous refractile bodies called pyrenoids.

(vi) Reproduction is vegetative by mitotic cell division in unicellular forms or by fragmentation in filamentous forms; asexual by formation of spores such as zoospores, aplanospores, hypnospore and akinates and sexual simply by conjugation or by gamete formation by isogamy or anisogamy or oogamy.

(vii) The life-cycle is of haplontic type showing alternation of dominant haploid stage with short-lived diploid stage.

Examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra, Ulva etc.

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Morphology

The simplest morphology in the cyanobacteria is that of unicells, free-

living or enclosed within a mucilaginous envelope Subsequent evolution

resulted in the formation of a row of cells called a trichome. When the

trichome is surrounded by a sheath, it is called a filament. It is possible to

have more than one trichome in a filament. The most complex thallus is the

branched filament. Such a branched filament can be uniseriate (composed of

a single row of cells) or multiseriate (composed of one or more rows of

cells). Pigments and photosynthesis

The major components of the photosynthetic light-harvesting system

of the cyanobacteria are chlorophyll a in the thylakoid membrane, and the

phycobiliproteins, which are water-soluble chro mo proteins assembled into

macro molecular aggregates (phycobili somes) attached to the outer

thylakoid membranes. Some cyano -bacteria contain chlorophyll b and the

cyanobacterium Acaryochloris marina contains chlorophyll d. At one time

those cyanobacteria containing chlorophyll b surface of the thylakoid

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membranes. Some cyano - bacteria contain chlorophyll b and the

cyanobacterium Acaryochloris marina contains chlorophyll d. At one time

those cyanobacteria containing chlorophyll b. Asexual reproduction

Asexual reproduction occurs by the formation of hormogonia or

baeocytes or fragmentation of colonies Hormogonia (or hormogones), which

are characteristic of all truly filamentous cyanobac - teria, are short pieces of

trichome that become detached from the parent filament and move away by

gliding, eventually developing into a separate filament. Hormogonia are

distinguished from vegetative filaments by their gliding motility, the small

size of their cells. and the absence of heterocysts (Meeks and Elhai, 2002).

In some species, hormogonia con tain gas vacuoles, which control buoyancy.

In some filamentous algae, such as Oscillatoria and Cylindrospermum, the

entire filament may break up, whereas in others the hormogonia are

produced at the tips of special branches. In some algae, specialized

separation discs or necridia are involved in the breaking of the hormogone

from the parent filament, whereas in others, the filament just fractures.

Reproduction methods in Chlorophyceae members