Cyanobacteria

Cyanobacteria (/saɪˌænoʊbækˈtɪəri.ə/), also known as Cyanophyta, are a phylum of Gram-negative bacteria^[4] that obtain energy via photosynthesis. The name cyanobacteria refers to their color (from Ancient Greek κυανός (kuanós) 'blue'),^[5][6] giving them their other name, "blue-green algae",^{[7] [8]} though modern botanists restrict the term algae to eukaryotes and do not apply it to cyanobacteria, which are prokaryotes.^[9] They appear to have originated in freshwater or a terrestrial environment.^[10] 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.^[11]

Cyanobacteria use photosynthetic pigments, such as carotenoids, phycobilins, and various forms of chlorophyll, which absorb energy from light. Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed.^[12][13] 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.

Cyanobacteria are the first organisms known to have produced oxygen. 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",^[14] which dramatically changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.^[15]

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. Cyanobacteria produce a range of toxins known as cyanotoxins that can pose a danger to humans and animals.

Cyanobacteria are a very large and diverse phylum of photoautotrophic prokaryotes.^[17] They are defined by their unique combination of pigments and their ability to perform oxygenic photosynthesis. They often live in colonial aggregates that can take on a multitude of forms.^[18] Of particular interest are the filamentous species, which often dominate the upper layers of microbial mats found in extreme environments such as hot springs, hypersaline water, deserts and the polar regions,^[19] but are also widely distributed in more mundane environments as well.^[20]

Cyanobacteria are a group of photosynthetic bacteria evolutionarily optimized for environmental conditions of low oxygen.^[21] Some species are nitrogen-fixing and live in a wide variety of moist soils and water, either freely or in a symbiotic relationship with plants or lichen-forming fungi (as in the lichen genus Peltigera).^[22] They range from unicellular to filamentous and include colonial species. Colonies may form filaments, sheets, or even hollow spheres.