Increased MeHg Concentrations Research Articles

Elevated CO2 exacerbates the risk of methylmercury exposure in consuming aquatic products: Evidence from a complex paddy wetland ecosystem

Elevated CO2 levels and methylmercury (MeHg) pollution are important environmental issues faced across the globe. However, the impact of elevated CO2 on MeHg production and its biological utilization remains to be fully understood, particularly in realistic complex systems with biotic interactions. Here, a complete paddy wetland microcosm, namely, the rice-fish-snail co-culture system, was constructed to investigate the impacts of elevated CO2 (600 ppm) on MeHg formation, bioaccumulation, and possible health risks, in multiple environmental and biological media. The results revealed that elevated CO2 significantly increased MeHg concentrations in the overlying water, periphyton, snails and fish, by 135.5%, 66.9%, 45.5%, and 52.1%, respectively. A high MeHg concentration in periphyton, the main diet of snails and fish, was the key factor influencing the enhanced MeHg in aquatic products. Furthermore, elevated CO2 alleviated the carbon limitation in the overlying water and proliferated green algae, with subsequent changes in physico-chemical properties and nutrient concentrations in the overlying water. More algal-derived organic matter promoted an enriched abundance of Archaea-hgcA and Deltaproteobacteria-hgcA genes. This consequently increased the MeHg in the overlying water and food chain. However, MeHg concentrations in rice and soil did not increase under elevated CO2, nor did hgcA gene abundance in soil. The results reveal that elevated CO2 exacerbated the risk of MeHg intake from aquatic products in paddy wetland, indicating an intensified MeHg threat under future elevated CO2 levels.

Selenium- and chitosan-modified biochars reduce methylmercury contents in rice seeds with recruiting Bacillus to inhibit methylmercury production

Biochar could reshape microbial communities, thereby altering methylmercury (MeHg) concentrations in rice rhizosphere and seeds. However, it remains unclear whether and how biochar amendment perturbs microbe-mediated MeHg production in mercury (Hg) contaminated paddy soil. Here, we used pinecone-derived biochar and its six modified biochars to reveal the disturbance. Results showed that selenium- and chitosan-modified biochar significantly reduced MeHg concentrations in the rhizosphere by 85.83% and 63.90%, thereby decreasing MeHg contents in seeds by 86.37% and 75.50%. The two modified bicohars increased the abundance of putative Hg-resistant microorganisms Bacillus, the dominant microbe in rhizosphere. These reductions about MeHg could be facilitated by biochar sensitive microbes such as Oxalobacteraceae and Subgroup_7. Pinecone-derived biochar increased MeHg concentration in rhizosphere but unimpacted MeHg content in seeds was observed. This biochar decreased the abundance in Bacillus but enhanced in putative Hg methylator Desulfovibrio. The increasing MeHg concentration in rhizosphere could be improved by biochar sensitive microbes such as Saccharimonadales and Clostridia. Network analysis showed that Saccharimonadales and Clostridia were the most prominent keystone taxa in rhizosphere, and the three biochars manipulated abundances of the microbes related to MeHg production in rhizosphere by those biochar sensitive microbes. Therefore, selenium- and chitosan-modified biochar could reduce soil MeHg production by these microorganisms, and is helpful in controlling MeHg contamination in rice.

Effect of peat and thiol-modified peat application on mercury (im)mobilization in mercury-polluted paddy soil

Mercury (Hg) pollution in paddy soil has gained special attention because methylmercury (MeHg) can accumulate in rice grains. Therefore, there is an urgent need to explore the remediation materials of mercury-polluted paddy soil. In this study, herbaceous peat (HP), peat moss (PM), and thiol-modified HP/PM (MHP/MPM) were selected to investigate the effects and probable mechanism of their application on Hg (im)mobilization in mercury-polluted paddy soil through pot experiments. The results showed that HP, PM, MHP and MPM addition increased MeHg concentrations in the soil, indicating that the addition of peat and thiol-modified peat might increase the exposure risk of MeHg in soil. The addition of HP could significantly decrease the total mercury (THg) and MeHg concentrations in rice, with average reduction efficiencies of 27.44% and 45.97%, respectively, while adding PM slightly increased the THg and MeHg concentrations in rice. In addition, the addition of MHP and MPM significantly decreased the bioavailable Hg concentrations in the soil and THg and MeHg concentrations in rice, with reduction efficiencies of rice THg and MeHg of 79.14∼93.14% and 82.72∼93.87%, respectively, indicating that thiol-modified peat had good remediation potential. The possible mechanism is that Hg can bind with thiols in MHP/MPM and form steady compounds in the soil, reducing Hg mobility in the soil and inhibiting its uptake by rice. Our study showed the potential value of HP, MHP and MPM addition for Hg remediation. Additionally, we must weigh the pros and cons when adding organic materials as remediation agents to mercury-polluted paddy soil.

Elevated methylmercury production in mercury-contaminated paddy soil resulted from the favorable dissolved organic matter variation created by algal decomposition

Algae-derived organic matter (AOM) may considerably regulate methylmercury (MeHg) production and accumulation in the paddy fields by changing the soil-dissolved OM (SDOM) properties. In this study, a 25-day microcosm experiment was performed to compare the responding mechanisms of MeHg production in the Hg-contaminated paddy soil-water system to the input of algae-, rice-, and rape-derived OMs. Results showed that algal decomposition could release much more cysteine and sulfate than crop straws. Compared with crop straw-derived OMs, AOM input greatly increased the dissolved organic carbon concentrations in soil but resulted in a greater decrease in tryptophan-like fractions while accelerated the formation of high-molecular-weight fractions in soil DOM. Moreover, AOM input significantly increased MeHg concentrations in the pore water by 19.43%–3427.66% and 52.81%–5846.57% compared to rape- and rice-derived OMs, respectively (P < 0.05). And, a similar MeHg changing pattern was also observed in the overlying water (10–25 d) and the soil solid-phase particles (15–25 d) (P < 0.05). Correlation analysis revealed that MeHg concentrations in the AOM-added soil-water system had significantly negative and positive relationships with the tryptophan-like C4 fraction and molecular weight (E2/E3 ratio) of soil DOM, respectively (P < 0.01). These findings suggest that AOM has a higher capacity than crop straw-derived OMs to promote MeHg production and accumulation in the Hg-contaminated paddy soils by creating a favorable soil DOM variation and providing more microbial electron donors and receptors.

Inorganic versus organic fertilizers: How do they lead to methylmercury accumulation in rice grains

Both inorganic and organic fertilizers are widely used to increase rice yield. However, these fertilizers are also found to aggravate mercury methylation and methylmercury (MeHg) accumulation in paddy fields. The aim of this study was to reveal the mechanisms of inorganic and organic fertilizers on MeHg accumulation in rice grains, which are not yet well understood. Potting cultures were conducted in which different fertilizers were applied to a paddy soil. The results showed that both inorganic and organic fertilizers increased MeHg concentrations rather than biological accumulation factors (BAFs) of MeHg in mature rice grains. Inorganic fertilizers, especially nitrogen fertilizer, enhanced the bioavailability of mercury and the relative amount Hg-methylating microbes and therefore intensified mercury methylation in paddy soil and MeHg accumulation in rice grains. Unlike inorganic fertilizers, organic matter (OM) in organic fertilizers was the main reason for the increase of MeHg concentrations in rice grains, and it also could immobilize Hg in soil when it was deeply degraded. The enhancement of MeHg concentrations in rice grains induced by inorganic fertilizers (5.18–41.69%) was significantly (p < 0.05) lower than that induced by organic fertilizers (80.49–106.86%). Inorganic fertilizers led to a larger increase (50.39–99.28%) in thousand-kernel weight than MeHg concentrations (5.18–41.69%), resulting in a dilution of MeHg concentrations in mature rice grains. Given the improvement of soil properties by organic fertilizer, increasing the proportion of inorganic fertilizer application may be a better option to alleviate MeHg accumulation in rice grains and guarantee the rice yield in the agricultural production.

Sulfur-driven methylmercury production in paddies continues following soil oxidation

Methylmercury (MeHg) production in paddy soils and its accumulation in rice raise global concerns since rice consumption has been identified as an important pathway of human exposure to MeHg. Sulfur (S) amendment via fertilization has been reported to facilitate Hg methylation in paddy soils under anaerobic conditions, while the dynamic of S-amendment induced MeHg production in soils with increasing redox potential remains unclear. This critical gap hinders a comprehensive understanding of Hg biogeochemistry in rice paddy system which is characterized by the fluctuation of redox potential. Here, we conducted soil incubation experiments to explore MeHg production in slow-oxidizing paddy soils amended with different species of S and doses of sulfate. Results show that the elevated redox potential (1) increased MeHg concentrations by 10.9%−35.2%, which were mainly attributed to the re-oxidation of other S species to sulfate and thus the elevated abundance of sulfate-reducing bacteria, and (2) increased MeHg phytoavailability by up to 75% due to the reductions in acid volatile sulfide (AVS) that strongly binds MeHg in soils. Results obtained from this study call for attention to the increased MeHg production and phytoavailability in paddy soils under elevated redox potentials due to water management, which might aggravate the MeHg production induced by S fertilization and thus enhance MeHg accumulation in rice.

Understanding the effects of long-term different fertilizer applications on methylmercury accumulation in rice (Oryza sativa L.) plants

The effects of long-term applications of fertilizer form and rate on methylmercury concentration in rice grain have been previously reported. However, there remains a large gap in understanding fertilizer treatments affect the transformation of mercury (Hg) in paddy soils and the translocation of Methylmercury (MeHg) in rice plants. Thus, we investigated the effects of four fertilizer treatments (NK, nitrogen and potassium; NPK, nitrogen, phosphorus and potassium; NPKM, nitrogen, phosphorus, potassium and swine manure; and NPKS, nitrogen, phosphorus, potassium and rice straw) on mercury (Hg) methylation in paddy soils and on MeHg translocation in rice plants after a 24-year period; specifically, we investigated differences in MeHg production in paddy soils and MeHg mass flow in rice plants. The results showed that Geobacteracea and Methanosarcinacea were the major Hg-methylating microorganisms and their relative abundances were significantly promoted by rice straw return, which led to the significantly higher MeHg concentration both in paddy soils and in the rice grain of NPKS treatment compared with control. In addition, the phosphate and organic fertilizers increased MeHg translocation from the roots to the leaves and grain more than the nitrogen and potassium fertilizers did. In detail, compared with the control, NK increased MeHg concentrations in the rice grain by approximately 1-fold; NPK and NPKM, by approximately 3-fold; and NPKS, by approximately 8-fold. Last, we suggest that policy-makers worldwide should comprehensively assess the high potential risk of MeHg in rice grain in large production areas where rice straw is returned.

Methylmercury cytotoxicity and possible mechanisms in human trophoblastic HTR-8/SVneo cells

Methylmercury (MeHg) exposure during pregnancy can lead to adverse outcomes, including miscarriage and intrauterine growth retardation. In this study, MeHg cytotoxicity and its mechanisms in HTR-8/SVneo cells were investigated. MeHg inhibited HTR-8/SVneo cell viability and severely disrupted the cellular submicrostructure, showing a time-dose effect relationship. After MeHg treatment, the reactive oxygen species levels, malondialdehyde content, and superoxide dismutase (SOD) and catalase activities in the HTR-8/SVneo cells increased significantly with increased MeHg concentration (P<0.05). Similarly, MeHg also induced HTR-8/SVneo cell apoptosis in a dose-dependent manner. The proportion of cells in G1 phase decreased with increasing MeHg concentration, while that in the S and G2/M phases gradually increased. Moreover, cell migration and invasion capacities gradually decreased with increasing MeHg concentration, showing a significant difference between the MeHg-treated and control groups. Genes related to oxidative stress (HSPA6, HSPA1A, Nrf2, SOD1, HO-1, NQO1, OSGIN1, and gPX1), cell cycle (P21 and CDC25A), apoptosis (CYCS and AIFM2), and migration and invasion (CXCL8, CXCL3, CLU, IL24, COL3A1, MAPT, and ITGA7) were differentially expressed in the MeHg-treated group, indicating MeHg toxicity and mechanism of action. This study will provide insights into the prevention and treatment of pregnancy-related diseases caused by MeHg.

Tissue-Specific Accumulation and Antioxidant Defenses in Flounder (Paralichthys olivaceus) Juveniles Experimentally Exposed to Methylmercury.

Methylmercury (MeHg) is the most toxic form of mercury and can accumulate in the cells of marine organisms, such as fish, causing adverse effects on various physiological functions. This study examined MeHg accumulation and its toxicological role in antioxidant defenses in tissues, including the liver, gills, and muscle of flounder (Paralichthys olivaceus) juveniles. After 30 d of MeHg exposure (0, 0.1, 1.0, 10.0, and 20.0µgL-1), the accumulation of MeHg in the three tissues correlated positively with the concentration of MeHg and exhibited tissue specificity in the order of liver > gills > muscle. Among the antioxidant markers, the activities of SOD (superoxide dismutase) and GST (glutathione S-transferase) as well as the content of glutathione (GSH) in the liver and gills were induced at 0.1-10.0µgL-1 but repressed at 20.0µgL-1. The activities of SOD and GST and the content of GSH in the muscle significantly increased with increasing MeHg concentration. Catalase (CAT) activity in the liver was induced at 0.1-1.0µgL-1 but inhibited at 10.0-20.0µgL-1, whereas exposure to MeHg did not remarkably affect CAT activity in the gills and muscle. The levels of lipid peroxidation (LPO) increased dose dependently, showing tissue specificity with the highest level in the liver, then the gills, followed by muscles. Overall, higher sensitivity to oxidative stress induced by MeHg was detected in the liver than the gills and muscle. These findings improve our understanding of the tissue-specific accumulation of heavy metals and their roles in antioxidant responses in marine fish subjected to MeHg exposure.

Increased Methylmercury Accumulation in Rice after Straw Amendment.

Consumption of rice has been shown to be an important route of dietary exposure to methylmercury (MeHg, a neurotoxin) for Asians having a low fish but high rice diet. Therefore, factors that increase MeHg production and bioaccumulation in soil-rice systems, could enhance the risk of MeHg exposure. On the basis of a national-scale survey in China (64 sites in 12 provinces) and rice cultivation experiments, we report that straw amendment, a globally prevalent farming practice, could increase MeHg concentrations in paddy soils (11-1043%) and rice grains (95%). By carrying out a series of batch incubation, seedling uptake and sand culture experiments, we demonstrate that these increases could be attributed to (1) enhanced abundances/activities of microbial methylators and the transformation of refractory HgS to organic matter-complexed Hg, facilitating microbial Hg methylation in soils; (2) enhanced MeHg mobility, and increased root lengths (35-41%) and tip numbers (60-105%), increasing MeHg uptake by rice roots; and (3) enhanced MeHg translocation to rice grains from other tissues. Results of this study emphasize fresh organic matter-enhanced MeHg production and bioaccumulation, and highlight the increased risk of MeHg after straw amendment and thus the need for new policies concerning straw management.

Nutrients mediate the effects of temperature on methylmercury concentrations in freshwater zooplankton

Methylmercury (MeHg) bioaccumulation in freshwater aquatic systems is impacted by anthropogenic stressors, including climate change and nutrient enrichment. The goal of this study was to determine how warmer water temperatures and excess nutrients would alter zooplankton communities and phytoplankton concentrations, and whether those changes would in turn increase or decrease MeHg concentrations in freshwater zooplankton. To test this, we employed a 2 × 2 factorial experimental design with nutrient and temperature treatments. Mesocosms were filled with ambient water and plankton from Cottage Grove Reservoir, Oregon, U.S.A., a waterbody that has experienced decades of elevated MeHg concentrations and corresponding fish consumption advisories due to run-off from Black Butte Mine tailings, located within the watershed. Treatment combinations of warmer temperature (increased by 0.7 °C), nutrient addition (a single pulse of 10× ambient concentrations of nitrogen and phosphorous), control, and a combination of temperature and nutrients were applied to mesocosms. The individual treatments altered phytoplankton densities and community structure, but alone the effects on MeHg concentrations were muted. Importantly, we found a significant interactive effect of nutrients and temperature: the nutrient addition appeared to buffer against increased MeHg concentrations associated with elevated temperature. However, there was variability in this response, which seems to be related to the abundance of Daphnia and edible phytoplankton. Nutrients at low temperature were associated with marginal increases (1.1×) in zooplankton MeHg. Our findings suggest that global change drivers that influence community composition and ecosystem energetics of both zooplankton and phytoplankton can alter MeHg pathways through food webs.

Organic carbon content drives methylmercury levels in the water column and in estuarine food webs across latitudes in the Northeast United States

Estuaries are dynamic ecosystems which vary widely in loading of the contaminant methylmercury (MeHg), and in environmental factors which control MeHg exposure to the estuarine foodweb. Inputs of organic carbon and rates of primary production are important influences on MeHg loading and bioaccumulation, and are predicted to increase with changes in climate and land use pressures. To further understand these influences on MeHg levels in estuarine biota, we used a field study approach in sites across different temperature regions, and with varying organic carbon levels. In paired comparisons of sites with high vs. low organic carbon, fish had lower MeHg bioaccumulation factors (normalized to water concentrations) in high carbon sites, particularly subsites with large coastal wetlands and large variability in dissolved organic carbon levels in the water column. Across sites, MeHg level in the water column was strongly tied to dissolved organic carbon, and was the major driver of MeHg concentrations in fish and invertebrates. Higher primary productivity (chlorophyll-a) was associated with increased MeHg partitioning to suspended particulates, but not to the biota. These findings suggest that increased inputs of MeHg and loss of wetlands associated with climate change and anthropogenic land use pressure will increase MeHg concentrations in estuarine food webs.

Risk tradeoffs associated with traditional food advisories for Labrador Inuit

The traditional Inuit diet includes wild birds, fish and marine mammals, which can contain high concentrations of the neurotoxicant methylmercury (MeHg). Hydroelectric development may increase MeHg concentrations in traditional foods. Consumption advisories are often used to mitigate such risks and can result in reduced intake of traditional foods. Data from a dietary survey, MeHg exposure assessment and risk analysis for individuals in three Inuit communities in Labrador, Canada (n = 1145) in 2014 indicate reducing traditional food intake is likely to exacerbate deficiencies in n-3 polyunsaturated fatty acids and vitamins B12 and B2. Traditional foods accounted for < 5% of per-capita calories but up to 70% of nutrients consumed. Although consumption advisories could lower neurodevelopmental risks associated with an increase in MeHg exposure (90th-percentile ∆IQ = − 0.12 vs. − 0.34), they may lead to greater risks of cardiovascular mortality (90th-percentile increase: + 58% to + 116% vs. + 25%) and cancer mortality (90th-percentile increase + 2% to + 4% vs. no increase). Conversely, greater consumption of locally caught salmon mostly unaffected by hydroelectric flooding would lower all these risks (90th-percentile ∆IQ = + 0.4; cardiovascular risk: − 45%; cancer risk: − 1.4%). We thus conclude that continued consumption of traditional foods is essential for Inuit health in these communities.

Biochar and nitrate reduce risk of methylmercury in soils under straw amendment

There is growing evidence that incorporating crop straw into soils, which is being widely encouraged in many parts of the world, could increase net methylmercury (MeHg) production in soils and MeHg accumulation in crops. We explored the possibility of mitigating the risk of increased MeHg levels under straw amendment by transforming straw into biochar (BC). Greenhouse and batch experiments were conducted, in which soil MeHg concentrations, MeHg phytoavailability and accumulation in rice, dynamics of sulfate, nitrate and abundances of sulfate reducing bacteria (SRB) were compared in ‘Control’ (Hg contaminated soil), ‘Straw’ (soil with 1% rice straw), ‘Straw+BC’ (soil with 1% straw and 1% biochar), and ‘Straw+BC+N’ (soil with 1% straw, 1% biochar and 0.12% nitrate). Our results indicate that straw amendment increased MeHg concentrations in soils (28–136% higher) and rice plants (26% higher in grains, ‘Straw’ versus ‘Control’), while co-application of biochar with straw reduced grain MeHg levels (60% lower, ‘Straw+BC’ versus ‘Straw’). This could be mainly attributed to the reduced MeHg availability to rice plants (phytoavailability, extraction rates of MeHg by ammonium thiosulfate) under biochar amendment (64–99% lower, ‘Straw+BC’ versus ‘Straw’). However, biochar amendment enhanced soil MeHg levels (5–75% higher, ‘Straw+BC’ versus ‘Control’). Interestingly, nitrate addition helped reduce soil MeHg concentrations (11–41% lower, ‘Straw+BC+N’ versus ‘Straw+BC’) by facilitating nitrate reduction while inhibiting SRB activities. Subsequently, addition of nitrate with biochar, compared with biochar alone, further reduced grain MeHg levels by 34%. Therefore, straw biochar together with nitrate could possibly be effective in mitigating the risk of MeHg under straw amendment. Furthermore, the results evidence the impacts of straw management on the risk posed by MeHg in soils and emphasize the necessity to carefully consider the straw management policy in Hg-contaminated areas.

Methylmercury Bioaccumulation in an Urban Estuary: Delaware River, USA

Spatial variation in mercury (Hg) and methylmercury (MeHg) bioaccumulation in urban coastal watersheds reflects complex interactions between Hg sources, land use, and environmental gradients. We examined MeHg concentrations in fauna from the Delaware River estuary, and related these measurements to environmental parameters and human impacts on the waterway. The sampling sites followed a north to south gradient of increasing salinity, decreasing urban influence, and increasing marsh cover. Although mean total Hg in surface sediments (top 4cm) peaked in the urban estuarine turbidity maximum and generally decreased downstream, surface sediment MeHg concentrations showed no spatial patterns consistent with the examined environmental gradients, indicating urban influence on Hg loading to the sediment but not subsequent methylation. Surface water particulate MeHg concentration showed a positive correlation with marsh cover whereas dissolved MeHg concentrations were slightly elevated in the estuarine turbidity maximum region. Spatial patterns of MeHg bioaccumulation in resident fauna varied across taxa. Small fish showed increased MeHg concentrations in the more urban/industrial sites upstream, with concentrations generally decreasing farther downstream. Invertebrates either showed no clear spatial patterns in MeHg concentrations (blue crabs, fiddler crabs) or increasing concentrations further downstream (grass shrimp). Best-supported linear mixed models relating tissue concentration to environmental variables reflected these complex patterns, with species specific model results dominated by random site effects with a combination of particulate MeHg and landscape variables influencing bioaccumulation in some species. The data strengthen accumulating evidence that bioaccumulation in estuaries can be decoupled from sediment MeHg concentration, and that drivers of MeHg production and fate may vary within a small region.

Assessment of neuroanatomical and behavioural effects of in ovo methylmercury exposure in zebra finches (Taeniopygia guttata)

Methylmercury (MeHg) readily crosses the blood brain barrier and is a known neuro-toxicant. MeHg accumulation in the brain causes histopathological alterations, neurobehavioral changes, and impairments to cognitive motor functions in mammalian models. However, in birds the neurotoxic effects of MeHg on the developing pre-hatching brain and consequent behavioral alterations in adult birds have not received much attention. Moreover, passerine birds are poorly represented in MeHg neurotoxicology studies in comparison to other avian orders. Hence in this study, we used the egg injection method to investigate the long term effects of in ovo MeHg exposure on brain histopathology and courtship behavior in a model songbird species, the zebra finch (Taeniopygia guttata). Egg treatment groups included: a low MeHg dose of 0.2μg Hg g−1 egg, a high MeHg dose of 3.2μg Hg g−1 egg, and a vehicle control (water). No adverse effects of in ovo MeHg treatment were detected on courtship song quality or on mating behavior in experimental males at sexually maturity which would suggest that observable neurobehavioral effects of MeHg exposure may depend on the timing of exposure during offspring development. However, neuroanatomical analysis indicated an increase in telencephalon volume with increased MeHg concentrations which may suggest a prolonged inflammatory response in this region of the brain.

Impact of Beaver Pond Colonization History on Methylmercury Concentrations in Surface Water.

Elevated concentrations of methylmercury (MeHg) in freshwater ecosystems are of major environmental concern in large parts of the northern hemisphere. Beaver ponds have been identified as a potentially important source of MeHg. The role of beavers might be especially pronounced in large parts of Europe, where beaver populations have expanded rapidly following near-extirpation. This study evaluates the role of the age and colonization history (encompassing patterns of use and reuse) of ponds constructed by the Eurasian beaver Castor fiber in regulating MeHg concentrations in Swedish streams. In 12 beaver systems located in three regions, we quantified MeHg concentrations together with other relevant parameters on five occasions per year in 2012-2013. Five were pioneer systems, inundated for the first time since beaver extirpation, and seven were recolonized, with dams reconstructed by newly recolonizing beavers. MeHg concentrations in pioneer but not in recolonized beaver systems were up to 3.5 fold higher downstream than upstream of the ponds, and varied between seasons and years. Our results show that pioneer inundation by beavers can increase MeHg concentrations in streams, but that this effect is negligible when dams are reconstructed on previously used ponds. We therefore expect that the recovery and expansion of beavers in the boreal system will only have a transitional effect on MeHg in the environment.

The effects of hydrologic fluctuation and sulfate regeneration on mercury cycling in an experimental peatland

AbstractA series of severe droughts during the course of a long‐term, atmospheric sulfate‐deposition experiment in a boreal peatland in northern Minnesota created a unique opportunity to study how methylmercury (MeHg) production responds to drying and rewetting events in peatlands under variable levels of sulfate loading. Peat oxidation during extended dry periods mobilized sulfate, MeHg, and total mercury (HgT) to peatland pore waters during rewetting events. Pore water sulfate concentrations were inversely related to antecedent moisture conditions and proportional to past and current levels of atmospheric sulfate deposition. Severe drying events caused oxidative release of MeHg to pore waters and resulted in increased net MeHg production likely because available sulfate stimulated the activity of sulfate‐reducing bacteria, an important group of Hg‐methylating bacteria in peatlands. Rewetting events led to increased MeHg concentrations across the peatland, but concentrations were highest in peat receiving elevated atmospheric sulfate deposition. Dissolved HgT concentrations also increased in peatland pore waters following drought but were not affected by sulfate loading and did not appear to be directly controlled by dissolved organic carbon mobilization to peatland pore waters. Peatlands are often considered to be sinks for sulfate and HgT in the landscape and sources of MeHg. Hydrologic fluctuations not only serve to release previously sequestered sulfate and HgT from peatlands but may also increase the strength of peatlands as sources of MeHg to downstream aquatic systems, particularly in regions that have experienced elevated levels of atmospheric sulfate deposition.

Increased methylmercury toxicity related to obesity in diabetic KK‐Ay mice

We examined the toxic effects of methylmercury (MeHg) in KK-Ay type 2 diabetic mice to clarify how metabolic changes associated with type 2 diabetes mellitus affect MeHg toxicity. MeHg (5 mg Hg kg (-1) day(-1) p.o.) was given to 4-week-old male KK-Ay and C57BL/6J (BL/6) mice three times per week for 6 weeks. Average body weights (BW) of vehicle-treated BL/6 and KK-Ay mice were 16.3 and 16.4 g respectively on the first day, and 24.8 and 42.3 g respectively on the last day of the experiment. MeHg-treated KK-Ay mice began to lose weight about 5 weeks after MeHg administration. Six of seven MeHg-treated KK-Ay mice showed hind-limb clasping in the final stage of the experiment. The mean blood mercury level of MeHg-treated KK-Ay mice reached a maximum of 9.8 µg ml(-1) , whereas that of the MeHg-treated BL/6 mice was 2.8 µg ml(-1) after 10 days of treatment. The average total mercury concentrations in the cerebrum and epididymal fat pad were 7.4 and 0.57 µg g(-1) , respectively, for BL/6 mice and 27 and 1.6 µg g(-1) , respectively, for KK-Ay mice. In MeHg-treated KK-Ay mice with neurological symptoms, CD204-positive macrophages were observed in the brain, kidney and spleen, indicating CD204 could be a marker for injured tissues. BW loss and significant pathological changes were not observed in other groups of mice. These results indicate that body fat gain in type 2 diabetes mellitus and low mercury accumulation in adipose tissue increased MeHg concentrations in organs and enhanced toxicity in KK-Ay mice at the same dose of MeHg per BW.

Whole-lake nitrate addition for control of methylmercury in mercury-contaminated Onondaga Lake, NY

Methylmercury (MeHg) strongly bioaccumulates in aquatic food webs resulting in exposure to humans and wildlife through consumption of fish. Production of MeHg is promoted by anaerobic conditions and the supply of inorganic Hg (Hg2+), sulfate (SO42−), and labile organic carbon. The anaerobic sediments of stratified lakes are particularly active zones for methylation of Hg2+ and can be an important source of MeHg to the water column during summer anoxia and fall turnover. Nitrate (NO3–) addition has recently been proposed as a novel approach for the control of MeHg accumulation in the hypolimnia of Hg-contaminated lakes. In 2011, a whole-lake NO3– addition pilot test was conducted in Hg-contaminated Onondaga Lake, NY with the objective of limiting release of MeHg from the pelagic sediments to the hypolimnion through maintenance of NO3––N concentrations >1mgN/L. A liquid calcium-nitrate solution was added to the hypolimnion as a neutrally buoyant plume approximately three times per week during the summer stratification interval. Maximum hypolimnetic concentrations of MeHg and soluble reactive phosphorus (SRP) decreased 94% and 95% from 2009 levels, suggesting increased sorption to Fe and Mn oxyhydroxides in surficial sediments as the regulating mechanism. Increased MeHg concentrations in the upper waters during fall turnover, which had been a generally recurring pattern, did not occur in 2011, resulting in decreased exposure of aquatic organisms to MeHg. Over the 1992–2011 interval, the hypolimnetic NO3– supply explained 85% and 95% of the interannual variations in hypolimnetic accumulations of SRP and MeHg, respectively.