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Selenium (Se)-dependent enzymes (selenoenzymes) protect brain tissues against oxidative damage and perform other vital functions, but their synthesis requires a steady supply of Se. High methylmercury (CH₃Hg) exposures can severely diminish Se transport across the placenta and irreversibly inhibit fetal brain selenoenzymes. However, supplemental dietary Se preserves their activities and thus prevents pathological consequences. The modified Se health benefit value (HBVSₑ) is a risk assessment criterion based on the molar concentrations of CH₃Hg and Se present in a fish or seafood. It was developed to reflect the contrasting effects of maternal CH₃Hg and Se intakes on fetal brain selenoenzyme activities. However, the original equation was prone to divide-by-zero-type errors whereby the calculated values increased exponentially in samples with low CH₃Hg contents. The equation was refined to provide an improved index to better reflect the risks of CH₃Hg exposures and the benefits provided by dietary Se. The HBVSₑ provides a biochemically based perspective that confirms and supports the FDA/EPA advice for pregnant and breast-feeding women regarding seafoods that should be avoided vs. those that are beneficial to consume. Since Se can be highly variable between watersheds, further evaluation of freshwater fish is needed to identify locations where fish with negative HBVSₑ may arise and be consumed by vulnerable subpopulation groups.

Mercury (Hg) exposure poses substantial risks to human health. Investigating a longer chain from economic activities to human health can reveal the sources and critical processes of Hg-related health risks. Thus, we develop a more comprehensive assessment method which is applied to mainland China—the largest global Hg emitter. We present a map of Hg-related health risks in China and estimate that 0.14 points of per-foetus intelligence quotient (IQ) decrements and 7,360 deaths from fatal heart attacks are related to the intake of methylmercury in 2010. This study, for the first time, reveals the significant impacts of interprovincial trade on Hg-related health risks across the whole country. For instance, interprovincial trade induced by final consumption prevents 0.39 × 10 −2 points for per-foetus IQ decrements and 194 deaths from fatal heart attacks. These findings highlight the importance of policy decisions in different stages of economic supply chains to reduce Hg-related health risks. Mercury (Hg) is a global neurotoxic pollutant and has a long chain from economic activities to human health risks. Here the authors presented a map of Hg-related health risks in China and found significant impacts of interprovincial trade on health risks, such as the prevention of deaths from fatal heart attacks by the trade induced by final consumption.

The Minamata Convention needs policy-relevant insight The Minamata Convention on Mercury entered into force in August 2017, committing its currently 92 parties to take action to protect human health and the environment from anthropogenic emissions and releases of mercury. But how can we tell whether the convention is achieving its objective? Although the convention requires periodic effectiveness evaluation ( 1 ), scientific uncertainties challenge our ability to trace how mercury policies translate into reduced human and wildlife exposure and impacts. Mercury emissions to air and releases to land and water follow a complex path through the environment before accumulating as methylmercury in fish, mammals, and birds. As these environmental processes are both uncertain and variable, analyzing existing data alone does not currently provide a clear signal of whether policies are effective. A global-scale metric to assess the impact of mercury emissions policies would help parties assess progress toward the convention's goal. Here, I build on the example of the Montreal Protocol on Substances that Deplete the Ozone Layer to identify criteria for a mercury metric. I then summarize why existing mercury data are insufficient and present and discuss a proposed new metric based on mercury emissions to air. Finally, I identify key scientific uncertainties that challenge future effectiveness evaluation.

Background: Fish consumption is considered the primary pathway of methylmercury (MeHg) exposure for most people in the world. However, in the inland regions of China, most of the residents eat little fish, but they live in areas where a significant amount of mercury (Hg) is present in the environment. O b jectives: We assessed concentrations of total Hg and MeHg in samples of water, air, agricultural products, and other exposure media to determine the main exposure pathway of Hg in populations in inland China. Methods: We selected Guizhou Province for our study because it is highly contaminated with Hg and therefore is representative of other Hg-contaminated areas in China. We selected four study locations in Guizhou Province: three that represent typical environments with severe Hg pollution [due to Hg mining and smelting (Wanshan), traditional zinc smelting (recently closed; Weining), and heavy coal-based industry (Qingzhen)], and a village in a remote nature reserve (Leigong). R esults: The probable daily intake (PDI) of MeHg for an adult population based on 60 kg body weight (bw) was considerably higher in Wanshan than in the other three locations. With an average PDI of 0.096 μg/kg bw/day (range, 0.015–0.45 μg/kg bw/day), approximately 34% of the inhabitants in Wanshan exceeded the reference dose of 0.1 μg/kg bw/day established by the U.S. Environmental Protection Agency. The PDI of MeHg for residents in the three other locations were all well below 0.1 μg/kg bw/day (averages from 0.017 to 0.023 μg/kg bw/day, with a maximum of 0.095 μg/kg bw/day). In all four areas, rice consumption accounted for 94–96% of the PDI of MeHg. C onclusion: We found that rice consumption is by far the most important MeHg exposure route; however, most of the residents (except those in Hg-mining areas) have low PDIs of MeHg.

Wetlands play a vital role in contaminant cycling and uptake. Understanding how sulfate (SO 4 2‒ ) influences the conversion of inorganic mercury (Hg(II)) to toxic methylmercury (MeHg) is critical for predicting wetland responses to land use and climate change. Here, we sampled surface and pore waters across SO 4 2‒ gradients in three freshwater Everglades wetlands to assess linkages between SO 4 2‒ , MeHg, dissolved organic matter (DOM), and inorganic sulfide (S(‒II)). Increasing SO 4 2‒ concentrations increase S(‒II) and DOM concentrations and DOM aromaticity. MeHg concentration show a unimodal response to surface water SO 4 2‒ , which reflect high Hg(II) methylation at low-to-intermediate SO 4 2‒ concentration (2-12 mg/L) and low Hg(II) methylation at higher SO 4 2‒ concentrations ( > 12 mg/L). MeHg concentrations in surface waters correlate positively with MeHg concentrations in prey fish. The coherent biogeochemical relationships between SO 4 2‒ and MeHg concentrations and biologic uptake improve MeHg risk assessment for aquatic food webs and are globally relevant due to anthropogenic and climate-driven increases in SO 4 2‒ . Agricultural runoff adds sulfate to Florida wetlands, which increases production of toxic methylmercury at moderate sulfate concentrations that is assimilated in fish. The study helps predict wetland responses to management and climate variables.

Neurotoxic methylmercury (MeHg) formed from inorganic divalent mercury (Hg II ) accumulates in aquatic biota and remains at high levels worldwide. It is poorly understood to what extent different geochemical Hg pools contribute to these levels. Here we report quantitative data on MeHg formation and bioaccumulation, in mesocosm water-sediment model ecosystems, using five Hg II and MeHg isotope tracers simulating recent Hg inputs to the water phase and Hg stored in sediment as bound to natural organic matter or as metacinnabar. Calculations for an estuarine ecosystem suggest that the chemical speciation of Hg II solid/adsorbed phases control the sediment Hg pool’s contribution to MeHg, but that input of MeHg from terrestrial and atmospheric sources bioaccumulates to a substantially greater extent than MeHg formed in situ in sediment. Our findings emphasize the importance of MeHg loadings from catchment runoff to MeHg content in estuarine biota and we suggest that this contribution has been underestimated. Methylmercury in aquatic biota constitutes a severe environmental issue globally, but models struggle to provide accurate quantification. Here, the authors combine experimental approaches to assess contributions from different pools and use the data to interpret the mercury cycle in an estuarine setting.

Methylmercury (MeHg) is the most biologically available and toxic form of mercury, and can act as a powerful teratogen, neurotoxin and endocrine disruptor in vertebrates. However, mechanisms of endocrine impairment and net effects on demography of biota are poorly understood. Here, we report that experimental exposure of an aquatic bird over 3 years to environmentally relevant dietary MeHg concentrations (0.05–0.3 ppm wet weight) resulted in dose-related increases in male–male pairing behaviour (to 55% of males), and decreases in egg productivity (to 30%). Dosed males showed decreased rates of key courtship behaviours, and were approached less by courting females in comparison to control males. Within dosed groups, homosexual males showed a similar reduction when compared with dosed heterosexual males. We found an average 35 per cent decrease in fledgling production in high-dose birds over the study duration. These results are of interest because (i) MeHg exposure is experimentally tied to demographically important reproductive deficits, (ii) these effects were found at low, chronic exposure levels commonly experienced by wildlife, and (iii) effects on reproductive behaviour and sexual preference mediated by endocrine disruption represent a novel and probably under-reported mechanism by which contaminants may influence wild populations of birds.

Mercuric Hg(II) species form complexes with natural dissolved organic matter (DOM) such as humic acid (HA), and this binding is known to affect the chemical and biological transformation and cycling of mercury in aquatic environments. Dissolved elemental mercury, Hg(0), is also widely observed in sediments and water. However, reactions between Hg(0) and DOM have rarely been studied in anoxic environments. Here, under anoxic dark conditions we show strong interactions between reduced HA and Hg(0) through thiolate ligand-induced oxidative complexation with an estimated binding capacity of approximately 3.5 μmol Hg/g HA and a partitioning coefficient >10⁶ mL/g. We further demonstrate that Hg(II) can be effectively reduced to Hg(0) in the presence of as little as 0.2 mg/L reduced HA, whereas production of Hg(0) is inhibited by complexation as HA concentration increases. This dual role played by DOM in the reduction and complexation of mercury is likely widespread in anoxic sediments and water and can be expected to significantly influence the mercury species transformations and biological uptake that leads to the formation of toxic methylmercury.

Marine food webs are the most important link between the global contaminant, methylmercury (MeHg), and human exposure through consumption of seafood. Warming temperatures may increase human exposure to MeHg, a potent neurotoxin, by increasing MeHg production as well as bioaccumulation and trophic transfer through marine food webs. Studies of the effects of temperature on MeHg bioaccumulation are rare and no study has specifically related temperature to MeHg fate by linking laboratory experiments with natural field manipulations in coastal ecosystems. We performed laboratory and field experiments on MeHg accumulation under varying temperature regimes using the killifish, Fundulus heteroclitus. Temperature treatments were established in salt pools on a coastal salt marsh using a natural temperature gradient where killifish fed on natural food sources. Temperatures were manipulated across a wider range in laboratory experiments with killifish exposed to MeHg enriched food. In both laboratory microcosms and field mesocosms, MeHg concentrations in killifish significantly increased at elevated temperatures. Moreover, in field experiments, other ancillary variables (salinity, MeHg in sediment, etc.) did not relate to MeHg bioaccumulation. Modeling of laboratory experimental results suggested increases in metabolic rate as a driving factor. The elevated temperatures we tested are consistent with predicted trends in climate warming, and indicate that in the absence of confounding factors, warmer sea surface temperatures could result in greater in bioaccumulation of MeHg in fish, and consequently, increased human exposure.