Cyanobacteria (also known as blue-green algae, blue-green bacteria, and Cyanophyta) is a phylum of bacteria that obtain their energy through photosynthesis.
Prokaryotic plankton, blue-green algae are abundant in both oceans and freshwater. Their outer membrane is very hard to digest; not many larger plankton eat them until that membrane is destroyed by a few species of bacteria and virus. Some blue-greens form large floating colonies (mats), which are the major phytoplankton nuisances in fresh water
The ability of cyanobacteria to perform oxygenic photosynthesis is thought to have converted the early reducing atmosphere into an oxidizing one, which dramatically changed the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to endosymbiotic theory, chloroplasts in plants and eukaryotic algae have evolved from cyanobacterial ancestors via endosymbiosis.