| Abstract
| - Mercury (Hg) undergoes systematic stable isotopicfractionation; therefore, isotopic signatures of Hg mayprovide a new tool to track sources, sinks, and dominantchemical transformation pathways of Hg in the environment.We investigated the isotopic fractionation of Hg byHg(II) resistant (HgR) bacteria expressing the mercuricreductase (MerA) enzyme. The isotopic composition of boththe reactant Hg(II) added to the growth medium andvolatilized product (Hg(0)) was measured using cold vaporgeneration and multiple collector inductively coupledplasma mass spectrometry. We found that exponentiallydividing pure cultures of a gram negative strain Escherichiacoli JM109/pPB117 grown with abundant electron donorand high Hg(II) concentrations at 37, 30, and 22 °C, and anatural microbial consortium incubated in natural sitewater at 30 °C after enrichment of HgR microbes, preferentiallyreduced the lighter isotopes of Hg. In all cases, Hgunderwent Rayleigh fractionation with the best estimatesof α202/198 values ranging from 1.0013 to 1.0020. In the culturesgrown at 37 °C, below a certain threshold Hg(II)concentration, the extent of fractionation decreasedprogressively. This study demonstrates mass-dependentkinetic fractionation of Hg and could lead to developmentof a new stable isotopic approach to the study of Hgbiogeochemical cycling in the environment.
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