FRACTIONATION OF MERCURY STABLE ISOTOPES DURING MICROBIAL METHYLMERCURY PRODUCTION IN PURE CULTURES

The biological production of monomethylmercury (MeHg) in soils and sediments is an important process leading to the accumulation of mercury (Hg) in aquatic and terrestrial food webs. Advances in the analysis of Hg stable isotopes in environmental samples have allowed for the tracking of discrete Hg sources and the examination of photochemical and microbial transformations. However, the fractionation of Hg stable isotopes during biologically mediated methylation has not been comprehensively examined due to previous limitations on the separation of MeHg from complex matrices. In order to assess the extent of Hg stable isotopic fractionation during biological methylation and determine whether that fractionation is affected by metabolic differences, three methylating strains were studied in pure culture Geobacter sulfurreducens PCA , Desulfovibrio desulfuricans ND132, and the archaeal species Methanospirillum hungatei JF-1. Among the three microorganisms examined, the sulfate reducing strain ND132 had the fastest rate and highest yield of mercury methylation. For the two bacteria, biologically produced MeHg was initially 0.3 to 0.6 ‰ depleted in δ²⁰²Hg with respect to starting composition of total Hg(II), indicating that lighter isotopes are preferentially methylated as shown in previous work. The isotopic composition of MeHg produced later in the time course was enriched in ²⁰²Hg in comparison to starting inorganic δ²⁰²Hg values indicating that there was a separate pool of inorganic Hg (enriched in ²⁰²Hg in comparison to the bulk solution) that was preferentially utilized during the methylation process. Hg fractionation factors, calculated based on the bioavailable pool of inorganic Hg, were similar for G. sulfurreducens and D. desulfuricans (α_r/p = 1.0006 and 1.0011, respectively) despite differences in methylation rate and cellular metabolism. Methylation by archaeal species M. hungatei is currently under investigation to determine whether it displays a similar Hg isotopic fractionation compared to bacterial strains. Quantifying changes in the stable isotopic composition of MeHg during its production under different electron accepting conditions is critical to the evaluation of MeHg cycling in natural systems using stable isotope techniques.