Absence of magnetic isotope fractionation for Hg during dark biological processes: experimental evidence and theoretical considerations

The complex biogeochemistry and toxicity of Hg compounds warrants the search for new strategies that could be used to decipher the relative importance of its multiple abiotic and microbial transformations in ecosystems. In this regard, the emerging mercury isotope systematics is showing tremendous potential. We have studied the extent of fractionation of Hg stable isotopes during 1) degradation of MMHg and 2) Hg(II) reduction by multiple Hg(II) reducing strains, and irrespective of the extent of mass dependent fractionation (MDF) we did not observe any mass independent fractionation (MIF) of Hg isotopes. On the other hand, photo-chemical degradation of MMHg and reduction of Hg(II) cause a very high extent of MIF (Bergquist and Blum, 2007). Because there are many more unexamined microbial processes that influence Hg cycling in addition to the microbial transformations examined experimentally, and because some of these (e.g., oxidative degradation of MMHg) are not amenable to pure culture studies, a crucial question facing Hg biogeochemists is "Can microbial/biological processes cause MIF or are MIF signatures unique to photo- chemical transformations?" Based on the high spin orbit coupling in Hg compounds, the low likelihood of suppression of spin orbit coupling during dark biological processes, and the nature of known enzyme-Hg and microbe-Hg interactions, we suggest that the nuclear spin dependent MIF is unlikely to occur during dark biological processes. Because of the important implications of the absence of MIF during biological processes on Hg isotope systematics, we will also discuss experimental strategies that could be used to confirm this suggestion (Kritee et al., 2008). Bergquist B. A. and Blum J. D. (2007) Mass-dependent and mass-independent fractionation of Hg isotopes by photo-reduction in aquatic systems. Science 318(5849), 417-420. Kritee K., Barkay T., and Blum J. D. (2008) Mass dependent mercury stable isotope fractionation during microbial degradation of methylmercury. (submitted).