Cyanobacteria are a morphologically diverse group of photosynthetic prokaryotic microorganisms that form a closely related phylogenetic lineage of eubacteria. Historically, cyanobacteria were classified with plants and called blue-green algae, although true algae are eukaryotic . Cyanobacteria appear early in the fossil record with some examples approximately 3.5 billion years old. Stromatolites are large, often fossilized colonies of cyanobacteria that build up layer upon layer. Cyanobacteria contributed to the conversion of Earth's atmosphere from an anoxic -reducing environment to one rich in oxygen. Commonly studied genera include Anabaena, Lyngbya, Microcystis, Nostoc, Oscillatoria, Synechococcus, and Synechocystis.
Marine and freshwater aquatic environments (including aquaria) are rich in cyanobacteria, either free-living, in biofilms, or in mats. Cyanobacterial species (Microcystis or Oscillatoria ) that produce compounds (e.g., micro-cystins) toxic to humans and animals are sometimes associated with large-scale blooms in aquatic systems. Curling crusts on soils are often due to cyanobacteria. Pioneer communities on bare rock surfaces often include cyanobacteria or lichens, the latter existing as symbiotic associations of cyanobacteria and fungi. Cyanobacteria are found in extreme environments, including hot springs, desert sands, hypersaline ponds, and within the rocks of dry Antarctic valleys. Urban cyanobacteria are found as biofilms on concrete, brick buildings, and wooden fences.
Cyanobacteria are morphologically diverse, including unicellular and filamentous forms (branched and unbranched). Some filamentous species produce specialized cells including heterocysts, trichomes, hormogonia, and akinetes. As prokaryotes , cyanobacteria lack a nucleus and membrane-bound organelles . Photosynthetic thylakoid membranes and polyhedral bodies (carboxysomes) are visible using an electron microscope. Cyanobacteria may contain gas vacuoles, polyphosphate granules, and inclusions of cyanophycin, a nitrogen storage polymer .
A distinguishing feature of cyanobacteria is their photosynthetic pigment content. In addition to chlorophyll a, cyanobacterial thylakoids include phycobilin-protein complexes (phycobilisomes) containing mixtures of phycocyanin, phycoerythrin, and allophycocyanin, which give cyanobacteria their characteristic blue-green coloration. Phycobilisomes harvest light at wavelengths (500 to 650 nanometers ) not absorbed by chlorophylls. Most cyanobacteria perform oxygenic photosynthesis like higher plants. A few species perform anoxygenic photosynthesis, removing electrons from hydrogen sulfide (H2 S) instead of water (H2 O). There is a general dependence on carbon dioxide as a carbon source, although some cyanobacteria can live heterotrophically by absorbing organic molecules. The reductive pentose phosphate pathway predominates for carbon assimilation, as cyanobacteria have an incomplete tricarboxylic acid (Krebs) cycle.
Many species of cyanobacteria fix atmospheric dinitrogen (N2) into ammonia (NH3) using nitrogenase, an enzyme that is particularly sensitive to the presence of oxygen. In filamentous cyanobacteria, such as Anabaena and Nostoc, certain cells differentiate into heterocysts (thick-walled cells that do not photosynthesize), in which nitrogen fixation occurs under reduced oxygen concentrations. Cyanobacterial nitrogen fixation produces bioavailable nitrogen compounds that are important in nitrogen-limited aquatic ecosystems and plays an important role in global nitrogen cycling.
No other group of microbes participates in as many symbioses as cyanobacteria, including extra- or intracellular relationships with plants, fungi, and animals. This phenomenon, coupled with the plantlike photosynthesis of cyanobacteria, suggests that cyanobacteria were the progenitors of chloroplasts . Endosymbiotic theory holds that ancestral eukaryotic cells engulfed the ancient cyanobacteria that evolved into modern plastids. Better candidates may be prochlorophytes, oxygenic photosynthetic bacteria that contain chlorophyll a and b and form an evolutionarily related group with cyanobacteria and plastids.
see also Aquatic Ecosystems; Endosymbiosis; Eubacteria; Nitrogen Fixation; Photosynthesis, Carbon Fixation and; Photosynthesis, Light Reactions and; Wetlands.
Mark A. Schneegurt
Cyanosite: A Webserver for Cyanobacteria Research. [Online] Available at http://wwwcyanosite.bio.purdue.edu/index.html.
Fogg, G. P., W. D. P. Stewart, P. Fay, and A. E. Walsby. The Blue-Green Algae. London: Academic Press, 1973.
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 (N2) is converted to a biologically useful form (NH3). 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
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.