Mercury Stable Isotope Fractionation during Reduction of Hg(II) by Different Microbial Pathways

Abstract

Mercury (Hg) stable isotope fractionation has recently been developed as a tool in biogeochemistry. In this study, the extent of Hg stable isotope fractionation during reduction of ionic mercury [Hg(II)] by two Hg(II)-resistant strains, Bacillus cereus 5 and the thermophile Anoxybacillus sp. FB9 [which actively detoxify Hg(II) by the mer system] and a Hg(II)-sensitive metal-reducing anaerobe, Shewanella oneidensis MR-1 [which reduces Hg(II) at low concentrations], was investigated. In all cases, barring suppression of fractionation that is likely due to lower Hg(II) bioavailability, the Hg(II) remaining in the reactor became progressively enriched with heavy isotopes with time and underwent mass-dependent Rayleigh fractionation with alpha202/198 values of 1.0016 +/- 0.0004 (1 SD). Based on a multistep framework for the Hg(II) reduction pathways in the three strains, we constrain the processes that could contribute toward fractionation and suggest that for Hg(II)-resistant strains, reduction by mercuric reductase is the primary step causing fractionation. The proposed framework helps explain the variation in the extent of Hg stable isotope fractionation during microbial reduction of Hg(II), furthering the promise of Hg isotope ratios as a tool in determining the role of microbial Hg transformations in the environment.

Reduced isotope fractionation by denitrification under conditions relevant to the ocean

“…As described previously, 500-600 ml of autoclaved medium components (except as noted below) were assembled aerobically in a pre-sterilized 1 L reactor, and the assay medium was bubbled with N 2 for 45 min to purge any dissolved O 2 (Kritee et al, 2008). To initiate the assays, cell pellets from starter cultures were re-suspended in the assay medium and were also bubbled with N 2 for 45 min.…”

Mercury isotope fractionation during photoreduction in natural water is controlled by its Hg/DOC ratio

“…However, only a few have been characterized in any detail. Early studies focusing on processes only found mass dependent fractionation (MDF), such as volatilization of dissolved gaseous mercury (Zheng et al, 2007) and microbial reduction of Hg(II) (Kritee et al, 2007(Kritee et al, , 2008. Evidence of mass independent fractionation (MIF) was recently observed in various aquatic food web organisms (Jackson et al, 2008), fish , mosses and peat cores (Ghosh et al, 2008), and coal deposits (Biswas et al, 2008), suggesting that MIF of Hg can be caused by natural processes, such as photoreduction and liquid-vapour evaporation (Estrade et al, 2009).…”

Investigation of Hg uptake and transport between paddy soil and rice seeds combining Hg isotopic composition and speciation

“…Hg isotopic signature, a powerful tool for the investigation of biogeochemical processes and the potential identification of pollution sources Perrot et al, 2010;Sonke, 2011), can also be used to investigate biological processes Kritee et al, 2008;Kwon et al, 2012;Perrot et al, 2015;Rodŕiguez-Gonźalez et al, 2009 ) that can undergo mass dependent fractionation (MDF) due to specific reactions like methylation/ demethylation, reduction as well as transport and metabolic processes (Bergquist and Blum, 2007;Kritee et al, 2008;Yin et al, 2010Yin et al, , 2013b. Hg can additionally exhibit mass independent fractionation (MIF) (Bergquist and Blum, 2007).…”