Blue-green algae or Cyanophyceae are able to perform a photosynthesis that resembles that of green plants very much. The energy-converting processes take place at the outer cell membrane, the plasmalemma and at the intracellular membranes. Intracellular membrane systems do occur in much the same way as in all bacteria, but are not as dominant as in eucaryotic cells. The cell wall consists of murein. The single cells of most species form colonies that are surrounded by often voluminous jelly coats. Blue-green algae are photoautotrophic organisms and have a considerable share in the production of biomass. The ecological importance is emphasized by their dominance in many inshore waters. They are often the cause of "water bloom".
Among the secretions of the cells are, besides the already mentioned jelly coats of varying consistency, also clearly structured, often layered sheaths or capsules that may surround the trichomes. The production of joint sheaths enclosing more than two trichomes lead to a state described as illusory branching. Besides this type, coccal, i.e. non-trichome forming species occur.
In addition, many blue-green algae secrete a whole variety of low molecular metabolites like amino acids, sugars, and others as well as numerous toxic substances.
The trichomes of many species contain at least two functionally different cell types: the bluish green photoautotrophic cells and the usually somewhat more voluminous colourless heterocysts. Depending on the species, they occur either terminally or they alternate regularly with the photoautotrophic cells.
Heterocysts are the places where nitrogen fixation is performed. Just as in the case of nitrogen-fixing bacteria, this process is extremely sensitive towards oxygen and very energy-consuming. These features explain the evolution of a diversion of function within a trichome. The different functional units are separated in space. On the other hand, this separation makes an intercellular transport of materials necessary, otherwise
could not be explained. In electron-microscopic studies, cell-cell contacts in the form of thin transverse walls with pits emerged. Blue-green algae do often live in association with bacteria. In some cases, certain bacteria are associated with heterocysts. It is assumed that these bacteria consume oxygen thus establishing oxygen-free spaces which helps increasing the rate of nitrogen fixation.
Many blue-green algae form resistant stages like akinetes or spores.
Movement by flagella or cilia were found in no species. The typical movement of blue-green algae is a gliding that may set the trichomes into oscillation, the reason why a common genus is called Oscillatoria.
About 2000 recent species of blue-green algae have been described. Most are aquatic. Sweet-water species are predominant, marine species are rarer. Some few genera like Nostoc live on moist soils, others even in dry habitats. Some species live in symbiosis with fungi and form thus new groups of organisms, being either endosymbionts as in the case of Geosiphon or living in extracellular associations as in the lichen genera Peltigera, Collema, and Leptogium or in green plants. A species of Anabaena, for example, lives in symbiosis with the water fern Azolla and supplies it with nitrogen-containing compounds. Nostoc –species are known to live in the thalli of different liverworts, in the root cells of some Cycadean species, and in the rhizomes of Gunnera.
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