All blue-green algae (Figure 1.25) and prochlorophytes (Figure 1.26) are non-motile Gramnegative
eubacteria. In structural diversity, blue-green algae range from unicells through branched
and unbranched filaments to unspecialized colonial aggregations and are possibly the most widely
distributed of any group of algae. They are planktonic, occasionally forming blooms in eutrophic
lakes, and are an important component of the picoplankton in both marine and freshwater systems;
benthic, as dense mats on soil or in mud flats and hot springs, as the “black zone” high on the seashore,
and as relatively inconspicuous components in most soils; and symbiotic in diatoms, ferns,
lichens, cycads, sponges, and other systems. Numerically these organisms dominate the ocean ecosystems.
There are approximately 1024 cyanobacterial cells in the oceans. To put that in perspective,
the number of cyanobacterial cells in the oceans is two orders of magnitude more than all
the stars in the sky.
Pigmentation of cyanobacteria includes chlorophyll a, blue and red
phycobilins (phycoerythrin, phycocyanin, allophycocyanin, and phycoerythrocyanin), and carotenoids.
These accessory pigments lie in the phycobilisomes, located in rows on the outer surface of the
thylakoids. Their thylakoids, which lie free in the cytoplasm, are not arranged in stacks, but singled and equidistant, in contrast to prochlorophytes and most other algae, but similar to Rhodopyta and Glaucophyta.
The reserve polysaccharide is cyanophycean starch, stored in tiny granules lying between the
thylakoids. In addition, these cells often contain cyanophycin granules, that is, polymer of arginine
and asparagine. Some marine species also contain gas vesicles used for buoyancy regulation. In
some filamentous cyanobacteria, heterocysts and akinetes are formed. Heterocysts are vegetative
cells that have been drastically altered (loss of photosystem II, development of a thick, glycolipid
cell wall) to provide the necessary anoxygenic environment for the process of nitrogen fixation
(Figure 1.27). Some cyanobacteria produce potent hepato- and neurotoxins.
Prochlorophytes can be unicellular or filamentous, and depending on the filamentous species,
they can be either branched or unbranched. They exist as free-living components of pelagic nanoplankton and obligate symbionts within marine didemnid ascidians and holothurians, and are
mainly limited to living in tropical and subtropical marine environments, with optimal growth
temperature at about 24°C.
Prochlorophytes possess chlorophylls a
similar to euglenoids
and land plants, but lack phycobilins, and this is the most significance difference between these and cyanobacteria;
other pigments are β-carotene and several xanthophylls (zeaxanthin is the principal
one). Their thylakoids, which lie free in the cytoplasm, are arranged in stacks. Prochlorophytes have a starch-like reserve polysaccharide. These prokaryotes contribute a large percentage of the
total organic carbon in the global tropical oceans, making up to 25–60% of the total chlorophyll
a biomass in the tropical and subtropical oceans. They are also able to fix nitrogen, though not
in heterocysts. Both blue-green algae and prochlorophytes contain polyhedral bodies (carboxysomes)
containing RuBisCo (ribulose bisphospate carboxylase/oxygenase, the enzyme that
converts inorganic carbon to reduced organic carbon in all oxygen evolving photosynthetic
organisms), and have similar cell walls characterized by a peptoglycan layer. Blue-green
algae and Prochlorophytes can be classified as obligate photoautotrophic organisms. Reproduction
in both divisions is strictly asexual, by simple cell division of fragmentation of the colony or
|FIGURE 1.25 Trichome of Arthrospira sp.
(Bar: 20 µm.)
||FIGURE 1.26 Cells of Prochloron sp.
(Bar: 10 µm.)
|FIGURE 1.27 Heterocyst (arrow) of Anabaena azollae. (Bar: 10 µm.)