Though still called blue-green algae, cyanobacteria are prokaryotes. They are bacteria. The release of oxygen from primeval seas by cyanobacteria was instrumental in the natural history of the Earth in permitting oxidative metabolism and establishing the carbon cycle. Nearly three billion years ago cyanobacteria were responsible for converting the early reducing atmosphere into an oxidizing atmosphere.

This is not really MCAT material. So shoot me. I do hope that 'reducing atmosphere' and 'oxidizing atmosphere' come through vividly.