Cyanobacteria are examples of the prokaryotic cell type. They are photosynthetic and were thought to be examples of bluegreen algae until it was discovered that they had no membrane-bound nucleus and had no chloroplasts. Their metabolism utilizes energy from the sun and they release oxygen into the atmosphere. In fact, it is thought that they are some of the oldest forms of life and may have played a major role in giving the Earth an oxygen-containing atmosphere.
Enger & Ross