Methylmercury oxidative degradation potentials in contaminated and pristine sediments of the carson river, nevada

Abstract

Sediments from mercury-contaminated and uncontaminated reaches of the Carson River, Nevada, were assayed for sulfate reduction, methanogenesis, denitrification, and monomethylmercury (MeHg) degradation. Demethylation of [ 14 C]MeHg was detected at all sites as indicated by the formation of 14 CO 2 and 14 CH 4. Oxidative demethylation was indicated by the formation of 14 CO 2 and was present at significant levels in all samples. Oxidized/reduced demethylation product ratios (i.e., 14 CO 2 / 14 CH 4 ratios) generally ranged from 4.0 in surface layers to as low as 0.5 at depth. Production of 14 CO 2 was most pronounced at sediment surfaces which were zones of active denitrification and sulfate reduction but was also significant within zones of methanogenesis. In a core taken from an uncontaminated site having a high proportion of oxidized, coarsegrain sediments, sulfate reduction and methanogenic activity levels were very low and 14 CO 2 accounted for 98% of the product formed from [ 14 C]MeHg. There was no apparent relationship between the degree of mercury contamination of the sediments and the occurrence of oxidative demethylation. However, sediments from Fort Churchill, the most contaminated site, were most active in terms of demethylation potentials. Inhibition of sulfate reduction with molybdate resulted in significantly depressed oxidized/reduced demethylation product ratios, but overall demethylation rates of inhibited and uninhibited samples were comparable. Addition of sulfate to sediment slurries stimulated production of 14 CO 2 from [ 14 C]MeHg, while 2-bromoethanesulfonic acid blocked production of 14 CH 4. These results reveal the importance of sulfate-reducing and methanogenic bacteria in oxidative demethylation of MeHg in anoxic environments.

Periphyton and Flocculent Materials Are Important Ecological Compartments Supporting Abundant and Diverse Mercury Methylator Assemblages in the Florida Everglades

“…Along with Hg methylation, MeHg demethylation is likely to be a crucial determinant of net concentrations of MeHg, which had been well documented in the three compartments of the Everglades (31,34). Two pathways have been proposed for microbial MeHg demethylation: the reductive pathway that first cleaves MeHg to Hg(II) and CH 4 by an organomercurial-lyase enzyme encoded by merB and then to Hg(0) by mercuric reductase encoded by merA of Hg-resistant bacteria (46)(47)(48)(49)(50), as well as by the oxidative pathway that completely oxidizes MeHg to CO 2 and Hg(II) by SRP and methanogens (51)(52)(53)(54). More recently, a methanotroph was also reported to degrade MeHg via the potential activity of methanol dehydrogenase (55).…”

Effect of salinity on mercury methylating benthic microbes and their activities in Great Salt Lake, Utah

“…In addition to MPP, MDP was also strongly correlated with the abundance of 16S rRNA gene transcripts affiliated with Desulfobacterium. Previous studies have shown that SRB and methanogens are responsible for the majority of oxidative MeHg demethylation in anoxic sediments (Marvin-DiPasquale and Oremland, 1998;Oremland et al, 1995), with SRB apparently dominating in estuarine sediments (Oremland et al, 1991). If Desulfobacterium is involved in both Hg(II)-methylation and oxidative demethylation of MeHg, the next critical question becomes what determines the relative importance of MeHg production and degradation in these organisms since it appears as though both are sensitive to extreme salinity.…”

The Methylation of Metals and Metalloids in Aquatic Systems

“…However, while methane and Hg 0 are the primary products of mer-mediated Hg demethylation, CO 2 has also been observed as a major demethylation product in many studies [24,94,95] and this oxidative demethylation is not considered an active detoxification pathway. A variety of aerobes and anaerobes have been implicated in carrying out oxidative demethylation which has been observed in freshwater, estuarine and alkaline-hypersaline sediments [24,94,96].…”