A phylum consisting of two groups of photosynthetic bacteria: the blue-green bacteria (formerly known as blue-green algae, or Cyanophyta), which comprise the vast majority of members, and the grass-green bacteria, or chloroxybacteria. Both groups obtain their food by photosynthesis in a manner very similar to that of green plants and true algae, producing oxygen in the process. All the blue-green bacteria contain the photosynthetic pigment chlorophyll a, plus accessory pigments called phycobiliproteins. The blue colour is caused by one class of these pigments, the phycocyanins; some also have red pigments (phycoerythrins). Blue-green bacteria are unicellular but sometimes become joined in colonies or filaments by a sheath of mucilage. They occur in all aquatic habitats. A few species fix atmospheric nitrogen and thus contribute to soil fertility (see heterocyst; nitrogen fixation). Others exhibit symbiosis (see lichens). The chloroxybacteria have been found in marine and freshwater habitats. They differ from the blue-green bacteria in containing chlorophyll a and chlorophyll b, but no phycobiliproteins—a combination like that found in plant chloroplasts. Fossil remains of cyanobacteria called stromatolites date back 3500 million years and are among the earliest evidence of life on earth. According to the endosymbiont theory, eukaryotic cells originally obtained their photosynthetic organelles by engulfing cyanobacteria.

http://www-cyanosite.bio.purdue.edu/ Webserver for cyanobacterial research, hosted by Purdue University: contains links to numerous images