Cyanobacteria are a group of autotrophic gram-negative bacteria of the phylum Cyanobacteriota that can obtain biological energy via oxygenic photosynthesis. Cyanobacteria originated in a freshwater or terrestrial environment, and first appeared in the middle Archean eon. They are probably the most numerous taxon to have ever existed on Earth. The name "cyanobacteria" refers to their bluish green (cyan) color, which forms the basis of cyanobacteria's informal common name, blue-green algae.