Members of the phylum Cyanobacteria, also known as blue-green algae, include oxygenic photosynthetic bacteria, such as Nostoc and Spirulina. Chloroplasts, the organelles responsible for photosynthesis in green plants, are considered to be cyanobacteria that have developed a symbiosis with green plants.
Cyanobacteria are a microscopic and chemically diverse group of organisms. Evidence of cyanobacteria is found in the fossil record dating back 3.5 billion years, and cyanobacterial photosynthesis is believed to be responsible for generating the Earth's oxygen atmosphere during the Archaean and Proterozoic eras. Notable cyanobacteria include Anabaena, a class of nitrogen-fixing bacteria that forms a symbiotic relationship with floating ferns in rice paddies, and Oscillatoria, a reddish species that gives the Red Sea its characteristic color. A species of another cyanobacteria, Lyngbya, is responsible for the condition known as swimmer's itch.