Cyanobacteria: Phylum of prokaryotes

They are not algae, though they were once called blue-green algae. It is a phylum of bacteria, with about 1500 species. In endosymbiont theory, chloroplasts (plastids) are descended from cyanobacteria. Their DNA profile is evidence for this.

Cyanobacteria Temporal range: 3500 mya – Recent
Oscillatoria sp
Scientific classification
Domain:	Bacteria
Phylum:	Cyanobacteria
Orders
The taxonomy is under revision Unicellular forms: Chroococcales (suborders Chamaesiphonales and Pleurocapsales) Filamentous (colonial) forms: Nostocales (= Hormogonales or Oscillatoriales) True-branching (budding over multiple axes): Stigonematales

Cyanobacteria have an extremely long fossil record, starting at least 3,500 million years ago. They were the main organisms in the stromatolites of the Archaean and Proterozoic eons.

The ability of cyanobacteria to perform oxygenic photosynthesis is highly significant. The early atmosphere on Earth was largely reducing, that is, without oxygen. The cyanobacteria in stromatolites were the first known organisms to photosynthesise and produce free oxygen. After about a billion years, the effect of this photosynthesis began a huge change in the atmosphere. The process, called the Great Oxygenation Event, took a long time. Eventually, it killed off most of the organisms which could not live in oxygen, and led to the kinds of environment we know today, where most organisms use and need oxygen.

Cyanobacteria have a way of detecting light. Conrad Mullineaux, of Queen Mary University of London, said, "It has a way of detecting where the light is; we know that because of the direction that it moves".

"In a single-celled pond slime, they observed how incoming rays are bent by the bug's spherical surface and focused in a spot on the far side of the cell. By shuffling along in the opposite direction to that bright spot, the microbe moves towards the light".

• Ice algae