Fractionation of mercury isotopes during methylation by sulfate-reducing bacteria

Measurements of natural fractionation of Hg isotopes are a powerful tool to directly identify biogeochemical processes controlling the fate of mercury in the environment. In this study, the likelihood for mercury (Hg) isotopes to undergo biological fractionation through the methylation processes performed by Sulfate-Reducing Bacteria (SRB) was investigated.; To experimentally observe fractionation, pure cultures of Desulfobulbus proprionicus 1pr3 and Desulfovibrio desulfuricans 27774 were spiked with inorganic Hg and incubated in appropriate conditions. MeHg was extracted for isotope ratio analysis using distillation/aqueous phase derivitization/solid phase pre-concentration. Hg isotope ratio measurements were performed by GC-MC-ICP/MS. Several methods for isotope ratio determination were evaluated for their precision and accuracy from the transient signals. MeHg fractionation was observed during the GC elution process for all isotope pairs, which necessitated integration of signals prior to the isotope ratio calculation. A newly developed average peak ratio method yielded the closest isotope ratio in relation to certified values and the most precise external replication. delta198/202Hg values of +1.1 to +1.7‰ were measured in the MeHg created by the SRB. This is the first report demonstrating fractionation during methylation. The external precision for replicates is currently ∼150 ppm for the 198/202Hg ratio using the optimized operating conditions.