Cyanobacteria /saɪˌænoʊbækˈtɪəriə/, also known as Cyanophyta, are a phylum consisting of free-living photosynthetic bacteria and the endosymbiotic plastids, a sister group to Gloeomargarita, that are present in some eukaryotes. They commonly obtain their energy through oxygenic photosynthesis.[4] The oxygen gas in the atmosphere of earth is produced by cyanobacteria of this phylum, either as free-living bacteria or as the endosymbiotic plastids.[5] The name cyanobacteria comes from the color of the bacteria (Greek: κυανός, romanized: kyanós, lit. 'blue').[6][7] Cyanobacteria, which are prokaryotes, are also called "blue-green algae",[4][8] though some modern botanists restrict the term algae to eukaryotes.[9] Cyanobacteria appear to have originated in freshwater or a terrestrial environment.[10]
Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed.[11][12]
Phototrophic eukaryotes such as green plants perform photosynthesis in plastids that are thought to have their ancestry in cyanobacteria, acquired long ago via a process called endosymbiosis. These endosymbiotic cyanobacteria in eukaryotes then evolved and differentiated into specialized organelles such as chloroplasts, etioplasts and leucoplasts.
By producing and releasing oxygen (as a byproduct of photosynthesis), cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxygenation Event and the "rusting of the Earth",[13] which dramatically changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.[14]
Cyanobacteria produce a range of toxins known as cyanotoxins that can pose a danger to humans and animals.
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.
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