Net Methylmercury Research Articles

Net Methylmercury Production as a Basis for Improved Risk Assessment of Mercury-contaminated Sediments

Sediments contaminated by various sources of mercury (Hg) were studied at 8 sites in Sweden covering wide ranges of climate, salinity, and sediment types. At all sites, biota (plankton, sediment living organisms, and fish) showed enhanced concentrations of Hg relative to corresponding organisms at nearby reference sites. The key process determining the risk at these sites is the net transformation of inorganic Hg to the highly toxic and bioavailable methylmercury (MeHg). Accordingly, Hg concentrations in Perca fluviatilis were more strongly correlated to MeHg (p < 0.05) than to inorganic Hg concentrations in the sediments. At all sites, except one, concentrations of inorganic Hg (2-55 microg g(-1)) in sediments were significantly, positively correlated to the concentration of MeHg (4-90 ng g(-1)). The MeHg/Hg ratio (which is assumed to reflect the net production of MeHg normalized to the Hg concentration) varied widely among sites. The highest MeHg/Hg ratios were encountered in loose-fiber sediments situated in southern freshwaters, and the lowest ratios were found in brackish-water sediments and firm, minerogenic sediments at the northernmost freshwater site. This pattern may be explained by an increased MeHg production by methylating bacteria with increasing temperature, availability of energy-rich organic matter (which is correlated with primary production), and availability of neutral Hg sulfides in the sediment pore waters. These factors therefore need to be considered when the risk associated with Hg-contaminated sediments is assessed.

Evaluation of Wetland Methyl Mercury Export as a Function of Experimental Manipulations

Mercury associated with natural enrichment, historic mining, and ore processing is a contaminant of concern in watersheds of the western USA. In this region, water is a highly managed resource and wetlands, known to be important sites of methyl mercury production, are often an integral component of watersheds. This study applied controlled manipulations of four replicated experimental wetland designs with different water and soil mercury concentrations to determine the potential impacts on methyl mercury export. Wetlands were manipulated by drying and wetting, changing hydraulic retention time, and adding sulfate and nitrate to influent waters. In a summer drying and wetting manipulation, an immediate increase in total methyl mercury release was observed with rewetting, however, concentrations decreased quickly. Drying all wetlands over the winter and rewetting in the spring resulted in high net methyl mercury output relative to that observed before drying. Net methyl mercury output was not influenced by changes in hydraulic retention time from 4 to 8 h or to 30 min, or by increasing the nitrate concentration from 0.1 to 10 mg L(-1). The addition of sulfate to the inlet waters of two mesocosms to increase concentrations from approximately 100 to 250 mg L(-1) did not result in a clear effect on methyl mercury output, most likely due to sulfate concentrations being higher than optimal for methyl mercury production. Despite the lack of response to sulfate amendments, the change in sulfate concentration between the inlet and outlet of the mesocosms and temperature were the parameters best correlated with methyl mercury outputs.

Net methyl mercury production versus water quality improvement in constructed wetlands: Trade-offs in pollution control

In a system with mercury contamination, there are trade-offs between beneficial functions of a wetland and environmental risk of methyl mercury (MeHg) production. This project used five wetland mesocosms with three different experimental designs to assess the potential for nutrient, sediment, and total mercury (THg) removal and MeHg production associated with a proposed a large-scale wetland system. The latter was suggested for the mouth of Steamboat Creek (Nevada, USA) at the confluence with the Truckee River. Steamboat Creek has been documented to have high mercury concentrations and is a major source of nutrients to the river. Mesocosms that had creek sediments as the base and creek water as inflows resulted in decreasing THg concentration by 72–82%. Average percent nitrogen and phosphorus and suspended solids removal were 43%, 30%, and 70%, respectively. Net MeHg production was observed during spring, summer, and fall months; however, in the winter, these mesocosms acted as a sink. One wetland mesocosm with sediments low in mercury and creek water showed similar trends. Mesocosms with creek sediments and water low in mercury were a source of MeHg year round, with outflow concentrations 10 to 200 times that in the inflow. Based on the developed data, the environmental risk of the proposed large-scale wetland would be an increase of methyl mercury concentration in creek water that reaches the Truckee River by as much as 20 to 75%. However, the wetland would also be a significant sink for nutrients, suspended solids, and total mercury, decreasing the amount of mercury available for methylation downstream.

Decrease in Net Mercury Methylation Rates Following Iron Amendment to Anoxic Wetland Sediment Slurries

The rate of mercury methylation in anoxic wetland sediments is affected by the concentration of bioavailable complexes between Hg and sulfide. Previous research with pure bacterial cultures has shown that addition of ferrous iron reduces the net rate of mercury methylation by decreasing the concentration of dissolved sulfide. To assess the possibility of using this approach to decrease net mercury methylation in restored and constructed wetlands, laboratory experiments were conducted by adding Hg(II) and Fe(II) to sediments collected from six sites in five estuarine wetlands. Addition of 30 mM (0.07 mmol g(-1) or 3.9 mg g(-1)) Fe(II) decreased net mercury methylation relative to that of unamended controls by a factor of 2.1-6.6. In all cases, the observed decrease in net mercury methylation was accompanied by a decrease in the concentrations of sulfide and filterable mercury in the water overlying the sediments. When iron was added to one of the sediment samples at doses that were small relative to the concentration of sulfide present, net mercury methylation either increased slightly or was unaffected. Comparison of the results to speciation model predictions suggests that dissolved reduced sulfur-containing species play a role in the formation of uncharged, bioavailable Hg complexes. Although further research is needed to determine the long-term effect of iron amendment, these results suggest that iron addition decreases mercury methylation in authentic wetland sediments.

Potential for Increased Mercury Accumulation in the Estuary Food Web

Present concentrations of mercury in large portions of San Francisco Bay (Bay), the Sacramento-San Joaquin Delta (Delta), and the Sacramento and San Joaquin rivers are high enough to warrant concern for the health of humans and wildlife. Large scale tidal wetland restoration is currently under consideration as a means of increasing populations of fish species of concern. Tidal wetland restoration activities may lead to increased concentrations of mercury in the estuarine food web and exacerbate the existing mercury problem. This paper evaluates our present ability to predict the local and regional effects of restoration actions on mercury accumulation in aquatic food webs. A sport fish consumption advisory is in place for the Bay, and an advisory is under consideration for the Delta and lower Sacramento and San Joaquin rivers. Mercury concentrations in eggs of several water bird species from the Bay have exceeded the lowest observed effect level. A variety of mercury sources, largely related to historic mercury and gold mining, is present in the watershed and has created a spatially heterogeneous distribution of mercury in the Bay-Delta Estuary. Mercury exists in the environment in a variety of forms and has a complex biogeochemical cycle. The most hazardous form, methylmercury, is produced at a relatively high rate in wetlands and newly flooded aquatic habitats. It is likely that distinct spatial variation on multiple spatial scales exists in net methylmercury production in Bay-Delta tidal wetlands, including variation within each tidal wetland, among tidal wetlands in the same region, and among tidal wetlands in different regions. Understanding this spatial variation and its underlying causes will allow environmental managers to minimize the negative effects of mercury bioaccumulation as a result of restoration activities. Actions needed to reduce the uncertainty associated with this issue include a long term, multifaceted research effort, long term monitoring on local and regional scales, and careful evaluation of individual restoration projects with regard to potential increase of food web mercury.

Reduction of net mercury methylation by iron in Desulfobulbus propionicus (1pr3) cultures: implications for engineered wetlands.

Although one potential drawback of wetland construction and restoration is the formation of monomethylmercury, it may be possible to decrease net mercury methylation with the use of an appropriate sediment amendment. Using pure cultures of the sulfate-reducing bacterium Desulfobulbus propionicus (1pr3), we tested the hypothesis that adding ferrous iron to sulfidic wetland sediments decreases mercury solubility and bioavailability and, therefore, net methylation. In sediment-free cultures, net mercury methylation decreased with increasing [Fe(II)]. After 72 h of incubation, more than four times as much net methylmercury formed in the lowest ([Fe(II)] = 10(-6) M) treatment (180 +/- 33 pM) as compared with the highest ([Fe(II)] = 10(-2) M) treatment (42 +/- 14 pM). In cultures containing a model wetland sediment, more than three times as much methylmercury was observed in 10(-6) M Fe(II) treatments (1,010 +/- 95 pM) as compared with treatments amended with 10(-2) M Fe(II) (300 +/- 46 pM). Initial filterable mercury measurements and chemical equilibrium speciation predictions suggest that the lower net methylmercury production in the high-iron treatments was due to a decrease in sulfide activity and a concomitant decrease in the concentration of dissolved mercury. Although iron amendments could potentially minimize net mercury methylation in engineered wetland sediments, further research under field conditions is required to assess the efficacy of this approach.

Mercury methylation in macrophytes, periphyton, and water -- comparative studies with stable and radio-mercury additions.

Comparative tests of net mercury methylation potentials, with cultivated and macrophyte-associated periphyton and using stable ((200)HgCl(2) and CH(3)(199)HgCl) and labeled ((203)HgCl(2)) mercury, have been conducted in the Everglades nutrient removal area (Florida, USA) and in a tropical coastal Brazilian lake (RJ, Brazil). More methylmercury was formed by macrophyte-associated (up to 17% of added (203)Hg(II)) than cultivated (up to 1.6%) periphyton and methylmercury formation was lower in periphyton exposed to light (0.2%). High methylation was also observed for samples incubated with stable mercury isotopes (1.5-7.7% of added (200)Hg(II)), confirming the results obtained with labeled mercury. Simultaneous addition of (200)HgCl(2) and CH(3)(199)HgCl indicated that CH(3)(199)HgCl had no inhibitory effect on Hg methylation. The elevated methylation potentials observed in macrophytes, because of their root-associated periphyton, might contribute significantly to the high levels of methylmercury observed in Everglades biota. Comparative mercury methylation tests were also conducted in the water of a stratified temperate lake (Wisconsin, USA). Similar trends were observed for both stable and radioisotopes, with increasing mercury methylation along the depth profile. The highest levels (0.9% (203)Hg(II) and 0.8% (200)Hg(II)) were obtained below the oxic/anoxic boundary, where sulfide starts to increase, probably as a result of the intense activity of sulfate-reducing bacteria in the anoxic layer.

Mercury methylation along a lake–forest transect in the Tapajós river floodplain, Brazilian Amazon: seasonal and vertical variations

The seasonal and spatial variations of net methylmercury production in sediments, soils and other sites were evaluated by assays with 203Hg at different depths and locations along a lake–forest transect at lake Enseada Grande, Tapajós river. Soil and sediment samples were taken at the surface and at different depths up to 9 cm. Fresh samples and acidified controls (1–3 g dry wt.) were slurried with local water and incubated in the dark at 25–28°C for 3 days with 0.5–1.6 μg Hg g −1 (dry wt.) added as 203HgCl 2. CH 3 203Hg was extracted and measured in scintillation cocktail after acid leaching. Methylmercury production varied by orders of magnitude among sites and among sediment or soil layers. Seasonal variations were smaller than those with sample depth and location. In both seasons, MeHg formation in sediment and soil or flooded soil decreased with depth and was, in the top layers, one order of magnitude higher in the C-rich littoral macrophyte zone (2.3–8.9%) and flooded forest (3.2–4.5%) than in the center of the lake (0.2–0.56%). Similar MeHg production was found in slurried dry soils (dry season) and in soils already flooded for months. In the macrophyte zone soil (dry season), methylation was mainly associated with the thin Paspalum sp. rootlet layer. In the forest site, vertical variation in methylation was less pronounced in flooded than in dry soils and during the inundation the higher methylation rate was found in the flocculent sediment settled over the litter layer. The roots of floating Paspalum sp. were an important Hg methylation site, particularly those heavily colonized with periphyton (3.4–5.4%). Methylation in surface or near-bottom water was undetectable (<3×10 −2%) at all sites. Flooded forests and macrophyte mats are specific features of the Amazon and are important links between Hg inputs from natural and manmade sources and MeHg exposure of local populations through fish intake.

Mercury methylation in a tropical macrophyte: influence of abiotic parameters

Sediment has been considered to be one of the most important mercury methylation sites, but recent studies have demonstrated a new site that is relevant, i.e. the roots of floating aquatic macrophytes, where high methylation is observed. The effects of temperature, pH and electric conductivity on net mercury methylation were studied in the roots of the water-hyacinth Eichhornia crassipes of a freshwater coastal lagoon (Lagoinha, RJ, Brazil). Root samples were incubated, over three days, with 203HgCl2 addition, at different temperatures (10–90 °C), pH values (3–8) and different electrolytic solutions (KClO4, KCl and CaCl2, at 1, 5, 10, 25 and 50 mM, ranging between 18 and 760 µS cm−1). 203Hg-labeled methylmercury (Me203Hg) was extracted in toluene, after acid leaching, and measured by β-counting. Up to 35% of mercury added was converted to MeHg Methylation increased from 10 to 35 °C, and decreased thereafter. The process was completely inhibited at 90 °C. At pH values of 6 and 7 methylation was stimulated and a significant decrease was verified at pH 8. Increasing KClO4 concentrations led to a significant decrease in the methylation rates, while for KCl and CaCl2 solutions only a slight decrease was observed. Copyright © 1999 John Wiley & Sons, Ltd.

A simplified radiochemical technique for measurements of net mercury methylation rates in aquatic systems near goldmining areas, Amazon, Brazil : J. R. D. Guimaraes, O. Malm & W. C. Pfeiffer, Science of the Total Environment, 175(2), 1995, pp 151–162

A simplified radiochemical technique for measurements of net mercury methylation rates in aquatic systems near goldmining areas, Amazon, Brazil : J. R. D. Guimaraes, O. Malm & W. C. Pfeiffer, Science of the Total Environment, 175(2), 1995, pp 151–162

A simplified radiochemical technique for measurements of net mercury methylation rates in aquatic systems near goldmining areas, Amazon, Brazil

Current Hg 0 emissions from goldmining activities in the Amazon basin reach 100 tons·year −1, but very little is known about Hg methylation rates in local aquatic systems. Simplified versions of the Furutani and Ruddi (1980) radiochemical method were tested and applied in the determination, by in-situ incubations, of specific net Hg methylation rates in water and surface sediments of the Madeira river goldmining area. Efficiency for Me 203 Hg extraction from water and sediment was in the 75.3–96.77% range, depending on sample type and extraction procedure. Methylation was not detectable in water but rates in surface sediments ranged from 10 −5 to 10 −1% · g −1. h −1, the higher rates being found in sites in a reservoir area. Methylation was absent or strongly depressed in sterilized sediments. The effect of delays between sampling, incubation and extraction on methylation rates and Me 203Hg recovery was documented. Despite their limited spatial and temporal range, these data are further evidence that concerns about the potential toxicological impacts of Hg emissions in the Amazon basin are justified.

Importance of Wetlands as Sources of Methyl Mercury to Boreal Forest Ecosystems

Wetlands were found to be important sources of methyl mercury to the boreal forest ecosystem. Yields of methyl mercury were about 26–79 times higher from wetland portions of catchments (1.84–5.55 mg∙ha−1∙yr−1) than from purely upland areas (0.07 mg∙ha−1∙yr−1). Mass-balance estimates using methyl mercury inputs in wet deposition and outputs in runoff water indicated that purely upland catchments and lakes were sites of methyl mercury retention or demethylation, while catchments with wetland areas were sites of net methyl mercury production. These observations may explain the high concentrations of mercury in fish taken from lakes that are high in colour because they receive water from wetlands. There was no relationship between the concentration of total mercury and the concentration of methyl mercury in runoff water. Total mercury yields were low from a wetland-dominated catchment, higher from a combination upland/riparian wetland catchment, and highest from a purely upland catchment. The opposite was true for methyl mercury yields from these same catchments. This indicates that environmental factors other than total mercury concentration are controlling the production and loss of methyl mercury from catchments.

Sulfate stimulation of mercury methylation in freshwater sediments

The relationship between bacterial sulfate reduction and mercury methylation, as well as the in situ distribution of methylmercury in sediments, was studied in Quabbin Reservoir, MA. Fish methylmercury levels in Quabbin and other lakes affected by acid deposition are often elevated. However, the cause of acceleration of net methylmercury production or bioaccumulation in these lakes is poorly understood. Experimental additions of sulfate to either anoxic sediment slurries or lake water above intact sediment cores resulted in increased microbial production of methylmercury from added inorganic mercury

Influence of Dissolved Organic Carbon, pH, and Microbial Respiration Rates on Mercury Methylation and Demethylation in Lake Water

Effects of changes in DOC concentrations, pH, and microbial respiration rates on specific rates of mercury methylation and demethylation in lake water were studied using radioisotopic techniques. Increased concentrations of DOC resulted in decreased specific rates of net methylation, possibly as a result of complexation of inorganic mercury with DOC. A reduction in pH from 7.0 to 5.0 had the greatest effect, causing large to moderate increases in net methylation rate at both low and high DOC concentrations (500–2600 μM). Rates of respiration (indicative of general rates of microbial activity), which were insensitive to pH change over the range tested (5.0–7.0), had the smallest effect on net methyl mercury production rates. We propose the following explanations for three situations in which high mercury concentrations are commonly found in fish. (1) in acidified dilute clear-water lakes, high fish mercury concentrations may be a result of enhanced in-lake methylation; (2) in brown-water circumneutral lakes, where in-lake methylation is inhibited by high DOC concentrations, terrestrial inputs of methyl mercury may be most important; and (3) in brown-water, low-pH lakes, both in-lake and terrestrial sources of methyl mercury may contribute to elevated mercury concentrations in fish.

Specific rates of net methylmercury production in lake sediments

Specific rates of Hg (203HgCl2) methylation and McHg (14CH3HgI) demethylation in aerobic and anaerobic conditions were determined in samples of surface sediments (0 to 2 cm) taken from five small headwater lakes in Southern Finland. The highest rates of methylation were measured in anaerobic conditions. However, the importance of aerobic methylation increased with increasing Fe and Mn content in sediment. There was little difference between aerobic and anaerobic demethylation. The results demonstrate that the net McHg production in lake sediments depends on the individual characteristics of the lake, particularly pH and and sediment properties. These characteristics seem to affect demethylation in anaerobic conditions and methylation in aerobic conditions.

Laboratory Study of Chemical Speciation of Mercury in Lake Sediment and Water under Aerobic and Anaerobic Conditions

Chemical speciation and partitioning of radiolabeled HgCl(2) were studied in model aquatic systems consisting of undisturbed eutrophic lake sediment and water in plastic cylinders. The cylinders were either gradually made anaerobic by a gentle flow of N(2)-CO(2) or kept aerobic by air flow. The proportion of methylated Hg was significantly higher, in both water and sediment, in the anaerobic systems than in the aerobic systems. The composition and total concentration of fatty acids originating from bacterial phospholipids, as well as the concentration of vitamin B(12), including related cobalamins, were similar in sediments from the anaerobic and aerobic systems. Bacterial cell numbers were, on average, 3.6 times higher in the anaerobic water columns than in the aerobic ones. Volatilization of Hg occurred in all systems except in an autoclaved control and was of similar magnitudes in the anaerobic and aerobic systems. Incorporation of Hg into the sediment was significantly faster in the aerobic systems than in the anaerobic systems. These results suggest that episodes of anoxia in bottom waters and sediment cause an increase in net mercury methylation and, hence, an increase in bioavailable mercury.

Measurements of Specific Rates of Net Methyl Mercury Production in the Water Column and Surface Sediments of Acidified and Circumneutral Lakes

Specific rates of mercury methylation and demethylation were determined for water and surficial sediment samples taken from several lakes located in the Experimental Lakes Area, northwestern Ontario. Specific rates of mercury methylation were found to increase with decreasing pH in epilimnetic water samples in which pH was adjusted prior to incubation and in epilimnetic water samples taken from lakes of different pH. Reduction of pH also increased methyl mercury production at the sediment surface. Both increases and decreases in pH reduced specific rates of mercury demethylation. However, these changes were smaller than for methylation. Proportionally, specific rates of methylation increased faster than increasing concentrations of Hg2+, while specific rates of mercury demethylation increased linearly with increasing concentrations of methyl mercury. Overall, this study predicts that the net rate of methyl mercury production in the water column and at the sediment–water surface will increase as a result of lake acidification, and this may at least partially explain why the mercury concentration of fish appears to increase during lake acidification.