The Evolution of Chlorophylls and Photosynthesis

Abstract

Photosynthesis evolved very early on the Earth, but after respiration, and probably after the metabolic pro- cesses for methanogenesis and sulfur oxidation. This occurred in ancestors of anoxygenic photosynthetic bacteria. An ancestral reaction center of Photosystem I and II (RCI/II) type of photosynthesis arose in which a fi ve membrane-spanning helix (MSH) protein bound two molecules of chlorophyll (Chl)/bacteriochlorophyll (BChl) in a special pair and had a Chl/quinone primary acceptor, and this protein fused, early on, with a six MSH antenna protein. Logic suggests that the earliest photopigments were protoporphyrin IX, followed by Mg protochlorophyllide a, followed by Chl/BChl. It is not clear whether Chl or BChl came fi rst. The evolution of the modern RCI type occurred later but it is not clear under what selection pressure it arose, possibly when ferric salts and sulfur compounds became more available in the Proterozoic Eon.