Cyanobacteria /saɪˌænoʊbækˈtɪəriə/, also known as Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis.^[3] The name “cyanobacteria” comes from the color of the bacteria (Greek: κυανός (kyanós) = blue). They are often called blue-green algae, but some consider that name a misnomer as cyanobacteria are prokaryotic and algae should be eukaryotic,^[4] although other definitions of algae encompass prokaryotic organisms.^[5]

By producing oxygen as a gas as a by-product of photosynthesis, cyanobacteria are thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction ofoxygen-intolerant organisms. According to endosymbiotic theory, the chloroplasts found in plants andeukaryotic algae evolved from cyanobacterial ancestors via endosymbiosis.

Cyanobacteria can be found in almost every terrestrial and aquatic habitat—oceans, fresh water, damp soil, temporarily moistened rocks in deserts, bare rock and soil, and evenAntarctic rocks. They can occur as planktonic cells or formphototrophic biofilms. They are found in almost every endolithic ecosystem.^[6] A few are endosymbionts in lichens, plants, various protists, or sponges and provide energy for the host. Some live in the fur of sloths, providing a form of camouflage.^[7]

Aquatic cyanobacteria are known for their extensive and highly visible blooms that can form in both freshwater and marine environments. The blooms can have the appearance of blue-green paint or scum. These blooms can be toxic, and frequently lead to the closure of recreational waters when spotted. Marine bacteriophages are significant parasites of unicellular marine cyanobacteria.^[8]

Cyanobacteria are a photosynthetic nitrogen fixing group that survive in wide variety of habitat, soil and water. In this group photosynthetic pigments are cyanophycin, allo-phycocyanine and erythro-phycocyanine. Their thallus varies from unicellular to filamentous, filamentous heterocystous. they fix atmospheric nitrogen in aerobic condition by heterocyst, specialized cell, and in anaerobic condition.

(From Wikipedia, April 29th, 2014)