Introduction
Cyanobacteria are photosynthetic prokaryotes that capture sunlight for energy using chlorophyll a and various accessory pigments. They are common in lakes, ponds, springs, wetlands, streams, and rivers, and they play a major role in the nitrogen, carbon, and oxygen dynamics of many aquatic environments. The exact timing of appearance of the first cyanobacterialike microbes on Earth is still unclear because of controversy over the interpretation of Precambrian
fossils; however, much of their present diversity was achieved more than 2 billion years ago, and they likely
played a major role in the accumulation of oxygen in the Earth’s early atmosphere. In addition to their
remarkably long persistence as free-living organisms, cyanobacteria also form symbiotic associations with
more complex biota; for example, the nitrogen-fixing species Anabaena (or Nostoc) azollae forms a symbiotic
association with the floating fern Azolla, which is widely distributed in ponds and flooded soils. The chloroplasts in plants and algae appear to be originally derived from an endosymbiosis in which a cyanobacterium was engulfed and retained within a colorless eukaryotic cell.