Cyanobacteria are prokaryotes that encompass a wide range of photosynthesing bacteria, which as the name suggests belongs to the domain of bacteria.

Cyanobacteria are regarded as critical in the formation of the atmosphere that we have today, even though they were not the first organisms capable of photosynthesis. Some have a vital role within ecology, for example Heterocysts which produce enzymes capable of fixing Nitrogen. Other species can be endosymbiotic and some are known to produce toxins know as cyanotoxins.

References

A Biology of the Algae, Sze, 3ed, McGraw-Hill, 1998, New York

Microbiology, Prescott et al, 6ed, McGraw-Hill, 2005, New York