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Cyanobacteria

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.