Oxidation of Hg(0) to Hg(II) by diverse anaerobic bacteria

Redox cycling between elemental [Hg(0)] and divalent [Hg(II)] mercury is a key control on the fate and transport of Hg in groundwater systems. In this study we tested the ability of anaerobic bacteria to oxidize dissolved Hg(0) to Hg(II). Controlled laboratory experiments were carried out with the obligate anaerobic bacterium Geothrix fermentans H5 and the facultative anaerobic bacteria Shewanella oneidensis MR-1 and Cupriavidus metallidurans AE104. Under anoxic conditions all three bacterial strains reacted with dissolved gaseous Hg(0) to form non-purgeable Hg. In mass balance experiments the formation of non-purgeable Hg corresponded to the loss of volatile Hg. To determine if the non-purgeable Hg was oxidized we performed ethylation experiments on Hg(0)-reacted cell suspensions and X-ray absorption near edge structure (XANES) spectroscopy on Hg(0)-reacted cells. Derivatization of non-purgeable Hg to diethylmercury and the Hg L-III-edge position of the XANES spectra demonstrated that the reacted bacterial samples contained Hg(II). XANES analysis also revealed that cell-associated Hg(II) was covalently bound to bacterial functional groups most likely to thiol moieties. Finally experiments with metabolically active and heat-inactivated cells indicated that both live and dead cells oxidized Hg(0) to Hg(II). Hg(0) oxidation rates for metabolically active cultures increase in the order S. oneidensis MR-1 (1.6 x 10(-4) fg/cell/min) C. metallidurans AE104 (2.5 x 10(-4) fg/cell/min) and G. fermentans H5 (23.1 x 10(-4) fg/cell/min). The results of this study suggest that reactivity towards Hg(0) is widespread among diverse anaerobic bacteria and passive microbial oxidation of Hg(0) may play an important role in the redox transformation of mercury contaminants in subsurface environments. (C) 2013 Elsevier B. V. All rights reserved.