Unravelling biogeochemical drivers of methylmercury production in an Arctic fen soil and a bog soil.

Arctic tundra soils store a globally significant amount of mercury (Hg), which could be transformed to the neurotoxic methylmercury (MeHg) upon warming and thus poses serious threats to the Arctic ecosystem. However, our knowledge of the biogeochemical drivers of MeHg production is limited in these soils. Using substrate addition (acetate and sulfate) and selective microbial inhibition approaches, we investigated the geochemical drivers and dominant microbial methylators in 60-day microcosm incubations with two tundra soils: a circumneutral fen soil and an acidic bog soil, collected near Nome, Alaska, United States. Results showed that increasing acetate concentration had negligible influences on MeHg production in both soils. However, inhibition of sulfate-reducing bacteria (SRB) completely stalled MeHg production in the fen soil in the first 15 days, whereas addition of sulfate in the low-sulfate bog soil increased MeHg production by 5-fold, suggesting prominent roles of SRB in Hg(II) methylation. Without the addition of sulfate in the bog soil or when sulfate was depleted in the fen soil (after 15 days), both SRB and methanogens contributed to MeHg production. Analysis of microbial community composition confirmed the presence of several phyla known to harbor microorganisms associated with Hg(II) methylation in the soils. The observations suggest that SRB and methanogens were mainly responsible for Hg(II) methylation in these tundra soils, although their relative contributions depended on the availability of sulfate and possibly syntrophic metabolisms between SRB and methanogens

Nanomolar Copper Enhances Mercury Methylation by <i>Desulfovibrio desulfuricans</i> ND132

Methylmercury (MeHg) is produced by certain anaerobic microorganisms, such as the sulfate-reducing bacterium <i>Desulfovibrio desulfuricans</i> ND132, but environmental factors affecting inorganic mercury [Hg­(II)] uptake and methylation remain unclear. We report that the presence of a small amount of copper ions [Cu­(II), <100 nM] enhances Hg­(II) uptake and methylation by washed cells of ND132, while Hg­(II) methylation is inhibited at higher Cu­(II) concentrations because of the toxicity of copper to the microorganism. The enhancement or inhibitory effect of Cu­(II) is dependent on both time and concentration. The presence of nanomolar concentrations of Cu­(II) facilitates rapid uptake of Hg­(II) (within minutes) and doubles MeHg production within a 24 h period, but micromolar concentrations of Cu­(II) completely inhibit Hg­(II) methylation. Metal ions such as zinc [Zn­(II)] and nickel [Ni­(II)] also inhibit but do not enhance Hg­(II) methylation under the same experimental conditions. These observations suggest a synergistic effect of Cu­(II) on Hg­(II) uptake and methylation, possibly facilitated by copper transporters or metallochaperones in this organism, and highlight the fact that complex environmental factors affect MeHg production in the environment

Structure determination of the HgcAB complex using metagenome sequence data: insights into microbial mercury methylation.

Bacteria and archaea possessing the hgcAB gene pair methylate inorganic mercury (Hg) to form highly toxic methylmercury. HgcA consists of a corrinoid binding domain and a transmembrane domain, and HgcB is a dicluster ferredoxin. However, their detailed structure and function have not been thoroughly characterized. We modeled the HgcAB complex by combining metagenome sequence data mining, coevolution analysis, and Rosetta structure calculations. In addition, we overexpressed HgcA and HgcB in Escherichia coli, confirmed spectroscopically that they bind cobalamin and [4Fe-4S] clusters, respectively, and incorporated these cofactors into the structural model. Surprisingly, the two domains of HgcA do not interact with each other, but HgcB forms extensive contacts with both domains. The model suggests that conserved cysteines in HgcB are involved in shuttling HgII, methylmercury, or both. These findings refine our understanding of the mechanism of Hg methylation and expand the known repertoire of corrinoid methyltransferases in nature

Global Proteome Response to Deletion of Genes Related to Mercury Methylation and Dissimilatory Metal Reduction Reveals Changes in Respiratory Metabolism in <i>Geobacter sulfurreducens</i> PCA

<i>Geobacter sulfurreducens</i> PCA can reduce, sorb, and methylate mercury (Hg); however, the underlying biochemical mechanisms of these processes and interdependent metabolic pathways remain unknown. In this study, shotgun proteomics was used to compare global proteome profiles between wild-type <i>G. sulfurreducens</i> PCA and two mutant strains: a Δ<i>hgcAB</i> mutant, which is deficient in two genes known to be essential for Hg methylation and a Δ<i>omcBESTZ</i> mutant, which is deficient in five outer membrane <i>c</i>-type cytochromes and thus impaired in its ability for dissimilatory metal ion reduction. We were able to delineate the global response of <i>G. sulfurreducens</i> PCA in both mutants and identify cellular networks and metabolic pathways that were affected by the loss of these genes. Deletion of <i>hgcAB</i> increased the relative abundances of proteins implicated in extracellular electron transfer, including most of the <i>c</i>-type cytochromes, PilA-C, and OmpB, and is consistent with a previously observed increase in Hg reduction in the Δ<i>hgcAB</i> mutant. Deletion of <i>omcBESTZ</i> was found to significantly increase relative abundances of various methyltransferases, suggesting that a loss of dissimilatory reduction capacity results in elevated activity among one-carbon (C1) metabolic pathways and thus increased methylation. We show that <i>G. sulfurreducens</i> PCA encodes only the folate branch of the acetyl-CoA pathway, and proteins associated with the folate branch were found at lower abundance in the Δ<i>hgcAB</i> mutant strain than the wild type. This observation supports the hypothesis that the function of HgcA and HgcB is linked to C1 metabolism through the folate branch of the acetyl-CoA pathway by providing methyl groups required for Hg methylation