cyanophyceae by dr. s.n. suresh
TRANSCRIPT
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