PAVING THE WAY FOR OXYGENIC PHOTOSYNTHESIS: GLACIAL PEROXIDES, THE PALEOPROTEROZIC SNOWBALL, AND BIOCHEMICAL EVOLUTION

Understanding the origin of oxygenic photosynthesis is one of the major puzzles for molecular biochemistry, evolutionary biology, and Earth history, as photosystem (PS)-II is now almost the sole source of oxygen on Earth. Oxygen-mediating enzymes must have evolved before PS-II, or the first mutant, oxygen-releasing bug would have promptly killed itself. Two major geological problems have been (1) to find a source of inorganic oxygen capable of exposing trace quantities of this poison to the biosphere, and (2) to determine when in Earth History PS-II arose. In answer to this first problem, the UV production of oxidants has often been suggested, principally because most plausible anoxic atmosphere models would permit intense UV radiation to reach the surface. However, UV radiation energetic enough to make peroxides is, by itself, lethal, and hence incapable of driving biochemical evolution. Liang et al. [1] have solved this problem by demonstrating that peroxides produced by the interaction of UV radiation with water vapor at the surface of glaciers would gradually build up in the ice. Upon melting at the base of the ice sheet, these peroxides react with seawater to release molecular oxygen, in a UV-free environment. Up to 1 bar of oxygen could be released after the Makganyene snowball at 2.2 Ga. Possible answers to the second problem are more controversial. Within the last 5 years, geochemical data have been used to support the presence of oxygenic photosynthesis as far back as 3.8 Ga, or as late as 2.2 Ga. A conservative view - based on metals like Mn and Ce with redox potentials close to oxygen (unlike U, Mo, Fe, and many others) argues for the younger option. Sterol biomarkers, which support an early origin, are also not good oxygen indicators because of the well-known replacement of anaerobic enzymes for oxygen-dependent ones [2].

[1] M.-C. Liang et al., Proc. Natl. Acad. Sci. 103: 18896-18899. [2] J.L. Kirschvink, Geoch.Cosmo. acta. 70 (18): A320-A320, 2006.