ABSTRACT
About 3.5 billion years ago, the first light utilizing organisms had only one ‘‘cyclic’’ photosystem. Thus, for the purpose of carbon fixation they had to use exogenous reductants such as hydrogen sulfide or hydroxylamine. The first ‘‘energy crisis’’ arose when these reduced compounds were exhausted (oxidized) in aqueous environments. The solution of the problem for the ancestors of cyanobacteria was the ‘‘invention’’ of photosystem II, i.e., a second photosystem containing a modified chlorophyll with an E00 as high as þ830mV, thus allowing them to utilize water as an inexhaustible electron source at zero cost. This novel photosystem produced oxygen, protons, and electrons in a light-dependent reaction involving manganese as catalytic redox converter as an electron trap with water as donor. This strategy was so efficient that it allowed the assembly of high densities of organisms, accumulating as pure carbon, geologically designated as graphite (coal is approximately 2.5-3 billion years younger!).
