Olof Regnell

Mercury methylation in the hypolimnetic waters of lakes with and without connection to wetlands in northern Wisconsin

Rates of Hg methylation and demethylation were measured in anoxic hypolimnetic waters of two pristine Wisconsin lakes using stable isotopes of Hg as tracers. One of the lakes is a clear-water seepage lake situated in sandy terrain with minimal wetland influence. The other is a dark-water lake receiving channelized inputs from a relatively large terrestrial wetland. Methyl mercury (MeHg) accumulated in the anoxic hypolimnia of both lakes during summer stratification, reaching concentrations of 0.8 ng center dot L-1 in the clear-water lake and 5 ng center dot L-1 in the dark-water lake. The stable isotopic assays indicated that rate constants of Hg-(II) methylation (K-m) ranged from 0.01 to 0.04 center dot day(-1) in the clear-water lake and from 0.01 to 0.09 center dot day(-1) in the dark-water lake, depending on the depth stratum. On average, K-m was threefold greater in the dark-water lake. Hypolimnetic demethylation rate constants (K-dm) averaged 0.03 center dot day(-1) in the clear-water lake and 0.05 center dot day(-1) in the dark-water lake. These methylation rates were sufficient to account for the observed accumulation of MeHg in hypolimnetic water during summer in both lakes. Despite substantial export of MeHg from the wetland to the dark-water lake, our study indicates that in-lake production and decomposition of MeHg dominated the MeHg cycle in both lakes.

The effect of pH and dissolved oxygen levels on methylation and partitioning of mercury in freshwater model systems.

Radio-labelled mercury, 203HgCl2, was added to water overlying sediment in freshwater model systems. The partitioning of 203Hg between water, sediment and air and the concentration of methyl 203Hg in the water did not differ significantly between systems with a water pH of 5.8+/-0.07 and 6.6+/-0.05. When the systems were made anaerobic and microbial activity was stimulated by the addition of tryptic soybroth (TSB), the level of methyl 203Hg in the water increased by two orders of magnitude. There was also an increase in the concentration of total 203Hg in the water, which was greater than the increase in methyl 203Hg, indicating release of both methyl 203Hg and other 203Hg species from sediment to water. In the systems that were kept aerobic, there was no increase in either total or methyl 203Hg in the water after stimulating the microbial activity with TSB. However, volatilization of 203Hg increased, which was not the case in the anaerobic systems.

Coupling of methyl and total mercury in a minerotrophic peat bog in southeastern Sweden

During most of an annual cycle, we studied the temporal variation of total mercury (HgT) and methyl mer- cury (MeHg) in unfiltered and filtered (0.45 µm) peat water from a minerotrophic peat bog in southeastern Sweden. MeHg in bulk water ((MeHgT)) and total Hg in filtered water in discharge water from the peat bog ((HgD)) were an order of magnitude higher than in upland runoff water entering the peat bog. At the discharge end, peat-water (HgD) and (MeHgD) ranged from 8 to 54 pmol·L -1 and from 1 to 32 pmol·L -1 , respectively. Whereas the variation of (MeHgT) was explained by changes in (MeHgD), the variation of inorganic HgT (IHgT )=( HgT) - (MeHgT )w as ex- plained by changes in particle-bound IHg (IHgP) = (IHgT) - (IHgD). Filterable organic matter and sulfide in the water both correlated poorly with (HgD). Neither did the amount of HgT in precipitation and upland runoff water correlate well with the estimated discharge of HgD from the peat bog. However, there was a strong correlation between (HgD) and (MeHgT) in the peat water (r = 0.96). Furthermore, a significant fraction of HgD was MeHg (mean 28%; range 8- 60%). These results suggest that methylation increased the mobility of Hg.

Increasing concentrations of iron in surface waters as a consequence of reducing conditions in the catchment area

Recent studies report trends of strongly increasing iron (Fe) concentrations in freshwaters. Since Fe is a key element with a decisive role in the biogeochemical cycling of major elements, it is important to understand the mechanisms behind these trends. We hypothesized that variations in Fe concentration are driven mainly by redox dynamics in hydraulically connected soils. Notably, Fe(III), which is the favored oxidation state except in environments where microbial activity provide strong reducing intensity, has several orders of magnitude lower water solubility than Fe(II). To test our hypothesis, seasonal variation in water chemistry, discharge, and air temperature was studied in three Swedish rivers. Methylmercury and sulfate were used as indicators of seasonal redox changes. Seasonal variability in water chemistry, discharge, and air temperature in the Eman and Lyckeby Rivers implied that the variation in Fe was primarily driven by the prevalence of reducing conditions in the catchment. In general, high Fe concentrations were observed when methylmercury was high and sulfate was low, indicative of reducing conditions. The Fe concentrations showed no or weak relationships with variations in dissolved organic matter concentration and aromaticity. The seasonal variation in Fe concentration of the Ume river was primarily dependent on timing of the snowmelt in high- versus low-altitude areas of the catchment. There were long-term trends of increasing temperature in all catchments and also trends of increasing discharge in the southern rivers, which should increase the probability for anaerobic conditions in space and time and thereby increase Fe transport to the aquatic systems.

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.

Comment on “Anaerobic Mercury Methylation and Demethylation by Geobacter bemidjiensis Bem”

S the discovery in the midsixties that inorganic Hg is converted to highly toxic CH3Hg (MeHg) in lake sediment, thousands of papers have been published on microbial Hg methylation. Yet, this process is far from completely understood. The results presented by Lu et al. could help reveal important aspects of the Hg methylation process. This comment concerns interpretations of the methylation/demethylation assays. An iron reducer, Geobacter bemidjienis Bem, was shown to both methylate Hg (added as HgCl2) and demethylate MeHg. Hg was identified as a product of demethylation. The observation that more Hg than MeHg was produced from HgCl2 suggests that the rate of demethylation (Kdemeth) was higher than the rate of methylation (Kmeth) (Figure 1 ), unless Hg was a result mainly of direct reduction of the added Hg. It is common to determine Kmeth and Kdemeth from the equation