, 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 . Cyanobacteria (which are prokaryotes) used to be called "blue-green algae". They have been renamed 'cyanobacteria' in order to avoid the term "algae", which in modern usage is restricted to eukaryotes.
Cyanobacteria are aquatic and photosynthetic
, that is, they live in the water, and can manufacture their own food. Because they are bacteria, they are quite small and usually unicellular, though they often grow in colonies large enough to see.
They are also important providers of nitrogen fertilizer in the cultivation of rice and beans.