Mass dependent stable isotope fractionation of mercury during its microbial transformations

Mercury (Hg) is often cited in fish consumption advisories across the world due to the extreme neurotoxicity of its methylated forms. Given the complex biogeochemical cycling of Hg, a differentiation between local vs. global and natural vs. anthropogenic sources of Hg(0) and determination of transformations that are dominant in a given ecosystem is critical. Mercury has seven stable isotopes and Hg isotope ratios can become a novel biogeochemical tool to track sources and transformations of Hg in the environment. However, development of a stable isotope based tool requires the determination of the extent of fractionation during individual biotic and abiotic transformations that can occur in the environment. This thesis reports the extent of fractionation of Hg isotopes during two biological transformations: (1) degradation of monomethyl-Hg (MMHg) via the mercury resistance (mer) pathway in Escherichia coli JM109/pPB117; and (2) Hg(II) reduction by four Hg(II) reducing strains, including three Hg(II) resistant strains (E. coli JM109/pPB117, Bacillus cereus Strain 5 and Anoxybacillus spp. FB9) and a Hg(II) sensitive strain (Shewanella oneidensis MR-1). Using a multi-collector inductively coupled plasma mass spectrometer, it was found that MMHg and Hg(II) that remained in the reactors became progressively heavier (increasing delta202Hg) with time and underwent mass dependent Rayleigh-type fractionation with average fractionation factors of 1.0004 and 1.0016, respectively. Mass independent fractionation (MIF) was not observed and based on the nature of microbe-Hg interactions, it is suggested that the nuclear spin dependent MIF is unlikely to occur during biological processes. A multi-step framework for understanding the extent of fractionation seen during the mer mediated MMHg degradation and Hg(II) reduction experiments is provided, and based on the biochemistry and kinetics of the steps involved in the two pathways, the steps in the process that could contribute to the observed extent of fractionation are suggested in the thesis. A clear effect of Hg(II) bioavailability on the extent of fractionation of Hg was observed and is also discussed. The framework discussed here can guide future experiments on Hg isotope fractionation during other transformations in its biogeochemical cycle, and ultimately facilitate a more rigorous development of a Hg isotope based geochemical tool.