POSSIBLE BIOLOGICAL IMPLICATIONS OF MID-PROTEROZOIC OCEAN ANOXIA

Mid-Proterozoic ocean anoxia may have been widespread compared to today, potentially encompassing the bulk of the oceans. This concept was first suggested by Canfield (1998) based on the sulfur stable isotope record. What are the biological implications of such ocean chemistry?

Expanded ocean anoxia has potential implications for the availability of bioessential elements with redox-sensitive aqueous chemistry, and hence for ocean productivity. In particular, Anbar and Knoll (2002) hypothesized that the coupled scarcity of Fe and Mo in a relatively reducing mid-Proterozoic "Canfield ocean" could have limited biological N₂-fixation because of the importance of these elements in the most common form of the nitrogenase enzyme that catalyzes conversion of N₂ to biologically useful forms. In the resultant N-deficient oceans, eukaryotes would have been at a particular disadvantage relative to prokaryotes because eukaryotes are unable to fix N₂. This hypothesis may explain the delayed diversification of eukaryotes observed in the fossil record.

Here we revisit the Proterozoic metal limitation hypothesis in light of the recent Mo isotope study of mid-Proterozoic black shales by Arnold et al. (2004). This study greatly strengthens the case for widespread mid-Proterozoic ocean anoxia. It also provides insight into the extent of Mo depletion relative to modern seawater. When compared to younger black shales, the reported Mo abundances suggest that seawater Mo concentrations were likely within an order of magnitude of the modern value (~ 100 nM). Although this depletion is not extreme, a small number of prior gene expression experiments under metal-limited conditions indicate that even such moderate depletion of Mo could have consequences for N-fixation by N-fixing microbes (Joerger and Bishop, 1988). This talk will provide an integrated overview of these geochemical and microbiological studies and suggest directions for future microbiological research motivated by questions in Proterozoic ocean biogeochemistry.

A. D. Anbar and A. H. Knoll, Science 297, 1137 (2002); G. L. Arnold, A. D. Anbar, J. Barling and T. W. Lyons, Science 304, 87 (2004); D. E. Canfield, Nature 396, 450 (1998); R. D. Joerger and P. E. Bishop, Crit. Rev. Microbiol. 16, 1 (1988)