Microbially-Mediated Transformations of Mercury

Collaborators: Tamar Barkay (Rutgers University), Charley Driscoll (Syracuse University), Jadran Faganeli (Marine Biological Station and Institute of Biology, Slovenia), Milena Horvat (Josef Stefan Institute, Slovenia)




The biogeochemistry of mercury (Hg) is important because of the toxicity of methylmercury (MeHg), the accumulation of MeHg in biota, and its biomagnification in aquatic food chains. Hg attacks the central nervous system, and concerns about Hg are based on its effects on both ecosystem and human health. The principle pathway for human exposure is the consumption of contaminated fish. Therefore, a knowledge of the concentration, transport, and dynamics of MeHg in aquatic ecosystems is needed to predict the potential impact on human, as well as aquatic life.

We have been studying the methylation Hg to MeHg and the demethylation of MeHg. Both processes are catalyzed by bacteria. Although most previous work focused on the methylation of Hg, the accumulation of MeHg is controlled in part by its formation and degradation. Hence, when the former exceeds the latter, MeHg can accumulate readily. We have been applying radiotracer methods and field and laboratory experiments to better understand these competing processes. This project is currently part of a multi-disciplinary, multi-institutional project funded by the Biocomplexity Program at the National Science Foundation (http://www.ecs.syr.edu/faculty/driscoll/biocomplexity/). Previous work was also funded by the NSF (Environmental Geochemistry and Biogeochemistry Program and International Programs (see our Slovene work for more information on our international projects)

Our studies have been conducted in many locations in the northern hemisphere (Figure 1) including regions contaminated by Hg such as Hg and gold mining regions. In addition, we have studied processes occurring in rather pristine locations such as soils, sediments and wetlands. Hg deposition from the atmosphere has increased greatly since the advent of the industrial revolution so Hg has been deposited all over the world, including very remote regions.

Figure 1. Locations were some of the Hg studies have been conducted.

Hg methylation is primarily an anaerobic process catalyzed by sulfate and iron-reducing bacteria. However, the demethylation of MeHg is conducted by a variety of bacteria under both aerobic and anaerobic conditions. Therefore, redox alone can have a strong effect on the ability of a habitat to produce (and degrade) MeHg. For example, when riverbank sediments are incubated aerobically or anaerobically, Hg methylation responds rapidly to changes in oxygen (Figure 2). Wet, anaerobic sediment begins methylating an added tracer immediately, whereas anaerobic sediment that is incubated aerobically quickly ceases methylation. Conversely, dry, oxic sediment begins to methylate Hg soon after oxygen is removed and this occurs even if the sediment has been dried for several months to years. Sediments retain a high potential to form MeHg even when kept under inappropriate conditions for extended periods of time. Hence, riverbanks are able to rapidly form MeHg when flooded, which is bad news for river biota and organisms that eat them, like us.

Figure 2. Wet riverbank sediments begin methylation Hg immediately when incubated under anaerobic conditions (wet/anoxic), but methylation ends rapidly under aerobic conditions (wet/oxic). Sediments that were stored dry for several months retained the ability to methylate once anaerobic conditions were resumed (dry/anoxic). Sediments were incubated as slurries and subsamples removed daily for rate determinations.

The demethylation of MeHg responds very differently to changes in redox. Demethylation occurs via two main pathways: 1) oxidative in which the methyl group is oxidized to carbon dioxide and the the Hg moiety is presumably released as Hg2+; 2) reductive in which the methyl group is released as methane and the hg moiety is then reduced to Hg(0). The reductive path is catalyzed by genes of the bacterial mer operon. Hg released during the oxidative pathway (Hg2+) can be remethylated under anaerobic conditions, whereas the Hg(0) released during reductive demethylation cannot be readily methylated and can leave the system as a gas. Hence, the pathway affects the ability of Hg to accumulate since remethylation can keep MeHg levels high. Slurry experiments demonstrated that the oxidative path dominates under anaerobic conditions, whereas the reductive path "kicks in" rapidly when oxygen is introduced (Figure 3). In this way, Hg can be recycled under anaerobic conditions since Hg released during MeHg degradation (Hg2+) can be methylated by anaerobic bacteria, while Hg liberated from MeHg under aerobic conditions cannot and may be lost to the atmosphere. Hence, the introduction of oxygen not only stops methylation of Hg, but it also allows for demethylation to continue and for Hg loss, both of which should lead to a decrease in MeHg.

Figure 3. Demethylation of MeHg responds to redox changes differently than the methylation process. Under anoxic conditions, Hg is methylated rapidly, but ceases when oxygen is introduced (top figure). MeHg demethylation occurs under both oxic and anoxic conditions (bottom figure). However, demethylation occurs via the oxidative pathway under anoxic conditions, but rapidly switches to the reductive pathway (CH4 formation) when oxygenated.

The reversal of the process depicted in Figure 3, i.e., the restoration of anaerobic conditions, causes demethylation to revert to the oxidative pathway again. However, this reversion takes almost three months to occur as opposed to the almost immediate switch to the reductive path that occurs upon oxygen addition. Therefore, if a flooded riverbank sediment dries out within a month or two after flooding, the reductive path will continue unabated and Hg will not be able to be recycled into more MeHg (good news for biota). However, continued flooding and persistent anoxia will eventually result in Hg cycling again (bad news for biota)

.Studies are underway at Sunday Lake located in the western Adirondacks in upstate New York.(43.86N; 75.10W) to determine sites of Hg methylation and factors controlling MeHg accumulation (http://www.ecs.syr.edu/faculty/driscoll/biocomplexity/). These lakes are full of MeHg due to increased atmospheric deposition of Hg from upwind coal-fired power plants. In addition, these system are acidic, which enhances MeHg accumulation in fish. The lake is adjacent to a wetland that includes riparian areas, a boggy wetland with a floating mat, fen edge and bog that is not floating. Experiments have shown that all of the wetland areas except for a Carex fen, are capable of rapid methylation of Hg (Figure 4). Methylation activity is greatly affected by water table height, so that all sites methylate at high water, but only the boggy areas, especially the floating mat, methylate when the water table is low. None of the wetland sites demethylate very well suggesting that even low rates of methylation may lead to high accumulation of MeHg since it is not degraded significantly. Conversely, the sediments in the lake demethylate rapidly suggesting that the wetlands are the main sources of MeHg for lake biota. We are studying factors that might limit demethylation in wetlands including links to trophic status and methanogenesis as described on another page at this site (http://biogeochemistry.uml.edu/pages/Acetate.html).

Figure 4. Sampling sites (left) and Hg methylation potential (right) for wetlands adjacent to Sunday Lake in the Adirondacks of New York. SURF and SURN refer to riparian areas with upper as top 20 cm and lower as 20-40 cm. Soil is from an upland site near the riparian area. Data are for samples collected when the system was wet. SURF and SURN sites exhibit very low rates of methylation when the water table is low and the sites are rather dry.