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In spite of the significant impact of biomethylation on the mobility and toxicity of metals and metalloids in the environment, little is known about the biological formation of these methylated metal(loid) compounds. While element-specific methyltransferases have been isolated for arsenic, the striking versatility of methanoarchaea to methylate numerous metal(loid)s, including rare elements like bismuth, is still not understood. Here, we demonstrate that the same metal(loid)s (arsenic, selenium, antimony, tellurium, and bismuth) that are methylated by Methanosarcina mazei in vivo are also methylated by in vitro assays with purified recombinant MtaA, a methyltransferase catalyzing the methyl transfer from methylcobalamin [CH 3 Cob(III)] to 2-mercaptoethanesulfonic acid ( Biomethylation and hydride generation of group 15 and 16 metals and metalloids (As, Se, Sb, Te, and Bi) by microorganisms are widespread phenomena in anaerobic habitats including landfills, sewage sludge fermentation, alluvial soils, and, as recently shown, the gut of mice and humans (5-6, 20, 22-23, 29). While these processes have a drastic effect on metal(loid) mobility and toxicity, little is known about the pathways involved in the biological formation of methyl and hydride metal(loid) species. For the methylation of arsenic, a metal(loid)-specific methyltransferase has been identified (2,24,28,33). For instance, genes encoding arsenite methyltransferases such as ArsM, which catalyzes the stepwise methylation of arsenic in S-adenosyl methionine (SAM)-dependent reactions, are found in numerous prokaryotic and eukaryotic genomes. As ArsM confers resistance against increased arsenic concentrations and is expressed in response to elevated arsenic concentrations, arsenic methylation by ArsM is considered a deliberate detoxification mechanism (24). Furthermore, methylcobalamin [CH 3 Cob(III)]-dependent methylation of As, Se, Sb, Te, Hg, and Bi has been reported for numerous anaerobic prokaryotes (4,19,21). In particular, autotrophic sulfate-reducing bacteria as well as methanoarchaea were suggested to be responsible for this process, as CH 3 Cob(III) and CH 3 Cob(III)-dependent enzymes are integral parts of physiological pathways such as carbon fixation via the reductive acetyl-coenzyme A (CoA) pathway and methanogenesis. Hence, these organisms contain high concentrations of corrinoids (17). However, it is unclear whether the different metal(loid)s are methylated by the same pathways and whether metal(loid) methylation is a deliberate enzymatic process, as in the case of ArsM, or whether it arises from side reactions of the basal physiological pathways. Interestingly, nonenzymatic methylation of some metal(loid)s, like As and Hg, by CH 3 Cob(III) under reductive conditions was assumed by some authors (32,34).To identify metal(loid) methylation pathways which could increase understanding of the multielement biomethylation observed in anaerobic habitats, we focused on methanoarchaea. Almost all methanoarchaea studied are capable of methylating a bro...

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Floodplain soils are often contaminated with toxic elements such as Ba, Cr, Sr, and V. For an adequate risk assessment of such contaminated sites, the assessment of factors affecting the leaching of those elements from riverine soils into water is fundamental. Since the redox potential (E-H) can be important within this context, we aimed to assess the impact of pre-set redox conditions on the dynamics of Ba, Cr, Sr, and V in a floodplain soil. To achieve this aim, we used an automatic biogeochemical microcosm system allowing computer-controlled regulation of E-H by adapting the supply of N-2 or O-2 to the soil suspension. With this system, the effect of E-H on the dynamics of Ba, Cr, Sr, and V was studied mechanistically. Chromium and V were negatively correlated with E-H possibly due to co-precipitation of these metals with Fe (hydr)oxides at high E-H. Vanadium might additionally be oxidized from more soluble V(IV) to less soluble V(V) with rising E-H. Barium and Sr were positively correlated with E-H, which might be attributed to their association with dissolved organic carbon (DOC). The influence of pH on the dynamics of the studied elements seemed to be of minor importance in our study. A significant negative correlation was observed between the specific UV absorbance at 254 nm (SUVA (254)) and E-H indicating that oxidizing conditions favored the removal of aromatic DOC molecules from solution via binding to Fe (hydr)oxides. Redox potential is important for metal fate in the current study. Results imply an absorption of Cr and V to Fe (hydr)oxides and an interaction with DOC while particularly Ba and Sr dynamics may be influenced by DOC. In the future, different V species in wetland soils under pre-definite redox conditions should be determined and further studies should elucidate the specific role of DOC and sulfur (S) chemistry on the dynamics of the studied metals