Evolution of Photosynthetic System

Abstract

Cyanobacteria and chloroplasts in plants and algae possess two different light-driven engines, designated as photosystem I (PS I) and II (PS II). Each photosystem contains chlorophylls as a photosynthetic pigment and has a principal importance in the photosynthetic electron transport system. They photooxidize water as an electron donor, and oxygen is evolved as a result, which is called oxygenic photosynthesis. In the living world, however, there is another type of photosynthesis without evolving oxygen, i.e., anoxygenic photosynthesis. The anoxygenic phototrophs cannot use water as an electron donor but use various reductive compounds such as hydrogen sulfide and hydrogen instead of water. Although oxygenic photosynthesis includes two photosystems, PS I and PS II, anoxygenic phototrophs have either one of the photosystems. Anoxygenic phototrophs are widely distributed among the bacteria, whereas oxygenic photosynthesis is limited to the cyanobacterial lineage. The phylogenetic analysis strongly suggests that oxygenic photosynthesis has emerged from anoxygenic photosynthesis. Before emergence of oxygenic photosynthesis, ancestral PS I and PS II have evolved in anoxygenic phototrophs. Emergence of oxygenic photosynthesis has a close relation to the coexistence of these different photosystems in a cyanobacterial ancestor 2.5 G years ago. The coexistence occurred by lateral gene transfer (LGT), such a LGT was frequently found in the evolutionary process of anoxygenic photosynthesis. The frequent LGT of photosystems formed the phylogenetic divergence of anoxygenic phototrophs and contributed the emergence of oxygenic photosynthesis.