Blue-green algae

Cyanobacteria ( ) are a group of autotrophicgram-negative bacteria of the phylumCyanobacteriota 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" (from Ancient Greek κύανος (kúanos) 'blue') refers to their bluish green (cyan) color, which forms the basis of cyanobacteria's informal common name, blue-green algae.