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...