Cyanobacteria

Cyanobacteria Temporal range: 2100–0 Ma Pha. Proterozoic Archean Had. (Possible Paleoarchean records)
Microscope image of Cylindrospermum, a filamentous genus of cyanobacteria
Scientific classification
Domain:	Bacteria
Kingdom:	Bacillati
Phylum:	Cyanobacteriota Oren et al., 2022[1]
Classes[2][3] and genera
Cyanophyceae "Ca. Sericytochromatia" Vampirovibrionophyceae †Archaeosphaeroides †Collenia †Gunflintia †Obruchevella †Ozarkcollenia †Rothpletzella
Synonyms[note 1]
List Chloroxybacteria Margulis & Schwartz, 1982 "Cyanobacteria" Woese et al., 1985[4] "Cyanophycota" Parker, Schanen & Renner, 1969 "Cyanophyta" Steinecke, 1931 "Diploschizophyta" Dillon, 1963 "Endoschizophyta" Dillon, 1963 "Exoschizophyta" Dillon, 1963 Gonidiophyta Schaffner, 1909 "Phycobacteria" Cavalier-Smith, 1998 Phycochromaceae Rabenhorst, 1865 Prochlorobacteria Jeffrey, 1982 Prochlorophycota Shameel, 2008 Prochlorophyta Lewin, 1976 Chroococcophyceae Starmach, 1966 Chamaesiphonophyceae Starmach, 1966 "Cyanobacteriia" Cyanophyceae Sachs, 1874 Cyanophyta Steinecke, 1931 Hormogoniophyceae Starmach, 1966 Myxophyceae Wallroth, 1833 Nostocophyceae Christensen, 1978 Pleurocapsophyceae Starmach, 1966 Prochlorophyceae Lewin, 1977 Scandophyceae Vologdin, 1962 Phycochromaceae Rabenhorst, 1865 Oxyphotobacteria Gibbons & Murray, 1978 Schizophyceae Cohn, 1879

Cyanobacteria (/saɪˌænoʊbækˈtɪəriə/ sy-AN-oh-bak-TEER-ee-ə) are a group of autotrophic gram-negative bacteria^[5] of the phylum Cyanobacteriota^[1] that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" (from Ancient Greek κύανος (kúanos) 'blue') refers to their bluish green (cyan) color,^[6][7] which forms the basis of cyanobacteria's informal common name, blue-green algae.^{[8][9][10][note 2]}

Cyanobacteria are probably the most numerous taxon to have ever existed on Earth and the first organisms known to have produced oxygen,^[11] having appeared in the middle Archean eon and apparently originated in a freshwater or terrestrial environment.^[12][13] Their photopigments can absorb the red- and blue-spectrum frequencies of sunlight (thus reflecting a greenish color) to split water molecules into hydrogen ions and oxygen. The hydrogen ions are used to react with carbon dioxide to produce complex organic compounds such as carbohydrates (a process known as carbon fixation), and the oxygen is released as a byproduct. By continuously producing and releasing oxygen over billions of years, cyanobacteria are thought to have converted the early Earth's anoxic, weakly reducing prebiotic atmosphere, into an oxidizing one with free gaseous oxygen (which previously would have been immediately removed by various surface reductants), resulting in the Great Oxidation Event and the "rusting of the Earth" during the early Proterozoic,^[14] dramatically changing the composition of life forms on Earth.^[15] The subsequent adaptation of early single-celled organisms to survive in oxygenous environments likely had led to endosymbiosis between anaerobes and aerobes, and hence the evolution of eukaryotes during the Paleoproterozoic.

Cyanobacteria use photosynthetic pigments such as various forms of chlorophyll, carotenoids, phycobilins to convert the photonic energy in sunlight to chemical energy. Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed.^[16][17] Photoautotrophic eukaryotes such as red algae, green algae and plants perform photosynthesis in chlorophyllic organelles that are thought to have their ancestry in cyanobacteria, acquired long ago via endosymbiosis. These endosymbiont cyanobacteria in eukaryotes then evolved and differentiated into specialized organelles such as chloroplasts, chromoplasts, etioplasts, and leucoplasts, collectively known as plastids.

Sericytochromatia, the proposed name of the paraphyletic and most basal group, is the ancestor of both the non-photosynthetic group Melainabacteria and the photosynthetic cyanobacteria, also called Oxyphotobacteria.^[18]

The cyanobacteria Synechocystis and Cyanothece are important model organisms with potential applications in biotechnology for bioethanol production, food colorings, as a source of human and animal food, dietary supplements and raw materials.^[19] Cyanobacteria produce a range of toxins known as cyanotoxins that can cause harmful health effects in humans and animals.

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