Cyanobacteria , also known as Cyanophyta, are a phylum of bacteria that obtain their energy through photosynthesis and are the only photosynthetic prokaryotes able to produce oxygen. The name cyanobacteria comes from the color of the bacteria (Greek: κυανός, translit. kyanós, lit. 'blue'). Cyanobacteria, which are prokaryotes, are also called "blue-green algae", though the term algae in modern usage is restricted to eukaryotes.
Unlike heterotrophic prokaryotes, cyanobacteria have internal membranes. These are flattened sacs called thylakoids where photosynthesis is performed.
Phototrophic eukaryotes perform photosynthesis by plastids that may have their ancestry in cyanobacteria, acquired long ago via a process called endosymbiosis. These endosymbiotic cyanobacteria in eukaryotes may have evolved or differentiated into specialized organelles such as chloroplasts, etioplasts and leucoplasts.
By producing and releasing oxygen (as a byproduct of photosynthesis), cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxygenation Event and the "rusting of the Earth", which dramatically changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.