"Cyanobacteria 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. Cyanobacteria, which are prokaryotes, are also called "blue-green algae", though the term "algae" in modern usage is restricted to eukaryotes." (Wikipedia)