Cyanobacteria (blue-green algae) are microorganisms that structurally resemble bacteria (they lack a nucleus and organelles ). However, unlike other bacteria, cyanobacteria contain chlorophyll a and conduct oxygenic photosynthesis. Cyanobacteria are approximately 2.5 billion years old and thus are the oldest oxygenic phototrophs on Earth. The early evolution of Earth's oxygen-rich atmosphere is most likely due to cyanobacterial photosynthesis.

Cyanobacteria are morphologically and physiologically diverse and broadly distributed in terrestrial and aquatic environments. Morphological groups include coccoid, filamentous nonheterocystous, and heterocystous genera. Heterocysts are specialized cells harboring nitrogen fixation, a process by which atmospheric nitrogen (N 2 ) is converted to a biologically useful form (NH 3 ). All heterocystous and some coccoid/filamentous cyanobacteria fix nitrogen. This enables cyanobacteria to exploit ecosystems devoid of nitrogen compounds, including those located in polar, open ocean, and desert regions. Cyanobacterial nitrogen fixation can be a significant source of biologically available nitrogen in these ecosystems.

Cyanobacteria move by gliding, using mucilaginous excretions as propellant, or, in the case of planktonic genera, by altering buoyancy through gas vesicle formation and collapse. Cyanobacteria exhibit remarkable ecophysiological adaptations to global change. They tolerate desiccation , hypersalinity , hyperthermal, and high ultraviolet light conditions, often for many years. Over their long evolutionary history, they have formed numerous endosymbiotic and mutualistic associations with microorganisms, higher plants, and animals, including lichens (fungi), ferns, cycads, diatoms, seagrasses, sponges, and even polar bears. Cyanobacteria have also exploited man-made pollution of aquatic environments, especially nutrient-stimulated primary productivity or eutrophication .

Cyanobacterial blooms are highly visible, widespread indicators of eutrophication. Because of the toxicity of some bloom taxa, blooms can pose serious water quality and animal and human health problems. Foul odors and tastes, oxygen depletion, fish kills, and drinking/recreational impairment are symptoms of bloom-infested waters. Finally, the large contribution of cyanobacterial blooms to phytoplankton biomass and ecosystem nutrient fluxes can alter biogeochemical cycling and food web dynamics.

SEE ALSO Eubacteria ; Photosynthesis ; Wetlands

Hans Paerl


Fogg, G. E., William D. P. Stewart, Peter Fay, and Anthony E. Walsby. The Blue-Green Algae. London: Academic Press, 1973.

Whitton, Brian A., and Malcolm Potts. The Ecology of Cyanobacteria. Dordrecht, Netherlands: Kluwer Academic Publishers, 2000.

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