The Evolution of Cyanobacteria and Photosynthesis

Oxygenic photosynthesis, i.e. the biosynthesis of carbohydrate from CO2 supported by light-dependent ATP formation and production of O2, evolved very early, presumably about 2,300 million years before present (Ma) when the oxygen content in the atmosphere increased greatly owing to the activity of cyanobacteria. Cyanobacteria were present at least 2,700 Ma in the Archean and Proterozoic eons as evidenced by microfossils, biomarkers such as 2-methylhopanoids, and 13C/12C isotope fractionation. Molecular phylogenetic analyses have shown that the history of the photosystems and reaction centers differ from that of the pigment-synthesizing enzymes. The photosynthetic apparatus of cyanobacteria has a modular structure. It has been postulated that photosystems I and II evolved from an ancestral reaction center 1 in anoxygenic procyanobacteria by gene duplication under the evolutionary pressure of changing redox conditions in the environment. The reaction centers 1 and 2 of anoxygenic phototrophic bacteria may have derived from the ancient reaction center 1 by horizontal gene transfer and the selective pressure of the environment.