Microbial Mercury Methylation in Aquatic Environments: A Critical Review of Published Field and Laboratory Studies.

Methylmercury (MeHg) is an environmental contaminant of concern because it biomagnifies in aquatic food webs and poses a health hazard to aquatic biota, piscivorous wildlife and humans. The dominant source of MeHg to freshwater systems is the methylation of inorganic Hg (IHg) by anaerobic microorganisms; and it is widely agreed that in situ rates of Hg methylation depend on two general factors: the activity of Hg methylators and their uptake of IHg. A large body of research has focused on the biogeochemical processes that regulate these two factors in nature; and studies conducted within the past ten years have made substantial progress in identifying the genetic basis for intracellular methylation and defining the processes that govern the cellular uptake of IHg. Current evidence indicates that all Hg methylating anaerobes possess the gene pair hgcAB that encodes proteins essential for Hg methylation. These genes are found in a large variety of anaerobes, including iron reducers and methanogens; but sulfate reduction is the metabolic process most often reported to show strong links to MeHg production. The uptake of Hg substrate prior to methylation may occur by passive or active transport, or by a combination of both. Competitive inhibition of Hg uptake by Zn speaks in favor of active transport and suggests that essential metal transporters are involved. Shortly after its formation, MeHg is typically released from cells, but the efflux mechanisms are unknown. Although methylation facilitates Hg depuration from the cell, evidence suggests that the hgcAB genes are not induced or favored by Hg contamination. Instead, high MeHg production can be linked to high Hg bioavailability as a result of the formation of Hg(SH)2, HgS nanoparticles, and Hg-thiol complexes. It is also possible that sulfidic conditions require strong essential metal uptake systems that inadvertently bring Hg into the cytoplasm of Hg methylating microbes. In comparison with freshwaters, Hg methylation in open ocean waters appears less restricted to anoxic environments. It does seem to occur mainly in oxygen deficient zones (ODZs), and possibly within anaerobic microzones of settling organic matter, but MeHg (CH3Hg+) and Me2Hg ((CH3)2Hg) have been shown to form also in surface water samples from the euphotic zone. Future studies may disclose whether several different pathways lead to Hg methylation in marine waters and explain why Me2Hg is a significant Hg species in oceans but seemingly not in most freshwaters.

Linking cellulose fiber sediment methyl mercury levels to organic matter decay and major element composition.

Methylation of mercury (Hg) to highly toxic methyl Hg (MeHg), a process known to occur when organic matter (OM) decomposition leads to anoxia, is considered a worldwide threat to aquatic ecosystems and human health. We measured temporal and spatial variations in sediment MeHg, total Hg (THg), and major elements in a freshwater lagoon in Sweden polluted with Hg-laden cellulose fibers. Fiber decomposition, confined to a narrow surface layer, resulted in loss of carbon (C), uptake of nitrogen (N), phosphorus (P), and sulfur (S), and increased MeHg levels. Notably, fiber decomposition and subsequent erosion of fiber residues will cause buried contaminants to gradually come closer to the sediment-water interface. At an adjacent site where decomposed fiber accumulated, there was a gain in C and a loss of S when MeHg increased. As evidenced by correlation patterns and vertical chemical profiles, reduced S may have fueled C-fixation and Hg methylation at this site.

Mercury in a boreal forest stream--role of historical mercury pollution, TOC, temperature, and water discharge.

Over a one-year study period (2003), we monitored total Hg (HgT) and methyl Hg (MeHg) at two sites in a Swedish forest stream located above (Site(ref)) and below a stretch of Hg-contaminated sediments (SiteHg). We also monitored HgT, MeHg, and ancillary water chemistry in peat water close to the stream and HgT in open field wet deposition. Despite the presence of historical Hg contaminants, direct atmospheric Hg deposition and transfer of Hg from the catchment explained more than half of the annual HgT load at SiteHg. The concentrations of both HgT and MeHg were sensitive to changes in water discharge (Q) and water temperature (T) at both sites, suggesting that the stream HgT and MeHg load can change dramatically in response to changing weather conditions. The 2003 data together with data from 1996 disclosed intersite differences and temporal variation in the relationships between HgT, MeHg, and TOC (total organic carbon), reflecting variable sources of HgT, MeHg, and TOC and temporal changes in factors affecting Hg speciation.