Similar term(s): blue-green algae.
Cyanobacteria are one of the largest and most important groups of bacteria on Earth.
They live in water, and are photosynthetic. This means that they convert the carbon dioxide in the atmosphere into oxygen.
Cyanobacteria are mainly responsible for generating the oxygen that is found in the Earth's atmosphere today.
Although their common name is "blue-green algae", the cyanobacteria are bacteria, and are not related to algae.
Introduction to the Cyanobacteria