Diagram of typical cyanobacterium
Cyanobacteria also known as Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. 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, although other definitions of algae encompass prokaryotic organisms.
By producing gaseous oxygen as a byproduct of photosynthesis, cyanobacteria are thought to have converted Terra's early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Terra by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to theory, the chloroplasts found in plants and eukaryotic algae evolved from cyanobacterial ancestors via endosymbiosis.