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Published August 27, 2008 | Updated
Journal Article Open

Paleoproterozic Icehouses and the Evolution of Oxygen Mediating Enzymes: The Case for a Late Origin of Photosystem -- II


Two major geological problems regarding the origin of oxygenic photosynthesis are: (1) identifying a source of oxygen predating biological oxygen production and capable of driving the evolution of oxygen tolerance, and (2) determining when oxygenic photosynthesis evolved. One solution to the first problem is the accumulation of photochemically-produced H2O2 at the surface of glaciers and its subsequent incorporation into ice. Melting at the glacier base would release H2O2, which interacts with seawater to produce O2 in an environment shielded from the lethal levels of ultraviolet radiation needed to produce H2O2. Answers to the second problem are controversial and range from 3.8 to 2.2 Ga. A skeptical view, based on metals that have redox potentials close to oxygen, argues for the late end of the range. The preponderance of geological evidence suggests little or no oxygen in the late Archaean atmosphere (< 1 ppm). The main piece of evidence for an earlier evolution of oxygenic photosynthesis comes from lipid biomarkers. Recent work, however, has shown that 2-methylhopanes, once thought to be unique biomarkers for cyanobacteria, are also produced anaerobically in significant quantities by at least two strains of anoxygenic phototrophs. Sterane biomarkers provide the strongest evidence for a date ≥2.7 Ga but could also be explained by the common evolutionary pattern of replacing anaerobic enzymes with oxygen-dependent ones. Although no anaerobic sterol synthesis pathway has been identified in the modern biosphere, enzymes that perform the necessary chemistry do exist. This analysis suggests that oxygenic photosynthesis could have evolved close in geological time to the Makganyene Snowball Earth Event and argues for a causal link between the two.

Additional Information

Copyright (C) 2008 Joseph L. Kirschvink and Robert E. Kopp. An edited version of this paper was published by the Royal Society. Citation: J. L. Kirschvink and R. E. Kopp (2008). Palaeoproterozoic ice houses and the evolution of oxygen-mediating enzymes: the case for a late origin of photosystem II. Phil. Trans. Roy. Soc. B 363, 2755-2765. doi: 10.1098/rstb.2008.0024. Published online 16 May 2008. Discussion Meeting Issue 'Photosynthetic and atmospheric evolution' organized by Euan Nisbet, Derek Bendall, Christopher Howe and Ellen Nisbet. Philosophical Transactions of the Royal Society B: Biological Sciences, Volume 363, Number 1504 / August 27, 2008. We thank John Abelson, Mel Simon, John H. Richards, Ann Pearson, Woodward Fischer, Roger Summons, Jochen Brocks, Timothy D. Raub, and David Fike for helpful discussions. They bear no responsibility for the opinions expressed herein, but have helped us greatly strengthen our arguments. The Agouron Institute, the NASA Astrobiology and Exobiology programs, and the NSF graduate fellowship programs provided support during the development of these ideas. We thank Jim O'Donnell and Tony Diaz of the Caltech Geology Library for helping track down some of the now-obscure early references we discuss.

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