Cyanobacteria
A cyanobacterium, 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 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 the early reducing atmosphere into an oxidizing one, causing the "rusting of the Earth"[6] and dramatically changing the composition of life forms on Earth by stimulating biodiversity and leading to the near-extinction of oxygen-intolerant organisms. According to endosymbiotic theory, the chloroplasts found in plants and eukaryotic algae evolved from cyanobacterial ancestors via endosymbiosis.