Feiyue Wang

Mercury-selenium compounds and their toxicological significance: Toward a molecular understanding of the mercury-selenium antagonism†

The interaction between mercury (Hg) and selenium (Se) is one of the best known examples of biological antagonism, yet the underlying mechanism remains unclear. This review focuses on the possible pathways leading to the Hg-Se antagonism, with an emphasis on the potential Hg-Se compounds that are responsible for the antagonism at the molecular level (i.e., bis[methylmercuric]selenide, methylmercury selenocysteinate, selenoprotein P-bound HgSe clusters, and the biominerals HgSe(x)S(1-x)). The presence of these compounds in biological systems has been suggested by direct or indirect evidence, and their chemical properties support their potentially key roles in alleviating the toxicity of Hg and Se (at high Hg and Se exposures, respectively) and deficiency of Se (at low Se exposures). Direct analytical evidences are needed, however, to confirm their in vivo presence and metabolic pathways, as well as to identify the roles of other potential Hg-Se compounds. Further studies are also warranted for the determination of thermodynamic properties of these compounds under physiological conditions toward a better understanding of the Hg-Se antagonism in biota, particularly under real world exposure scenarios.

Major element chemistry of the Changjiang (Yangtze River)

The chemistry of major elements (Ca, Mg, Na, K, HCO3, SO4, Cl, and Si) in the river water of the Changjiang (Yangtze River) was studied, based on continuously monitored data at 191 stations in the drainage basin for the period 1958–1990. The results show that the total dissolved solid (TDS) concentration of the Changjiang varies over an order of magnitude throughout the basin (49.7–518.1 mg/l), with a medium TDS concentration of 205.9 mg/l, about three times the global average. In contrast, the TDS at a given main-channel station varies only slightly in different seasons with a variation factor less than 2.0, despite a substantial water dilution in the summer flood season. The major element chemistry of the Changjiang is mainly controlled by rock weathering, with the anion HCO3 and the cation Ca dominating the major ion composition, due to the abundance of carbonate rocks in the basin. A persistently increasing trend has been observed in the concentrations of SO4 and, to a lesser extent, Cl in the Changjiang, a signature of considerable anthropogenic impacts (e.g., acid deposition). Flux calculations at Datong (the most downstream main-channel station without tidal influence) indicate that the Changjiang transports ca. 154×106 tons/year of TDS to the sea, second only to the Amazon in the world.

Conducting ecological risk assessments of inorganic metals and metalloids: current status

Ecological risk assessment (ERA) of inorganic metals and metalloids (metals) must be specific to these substances and cannot be generic because most metals are naturally occurring, some are essential, speciation affects bioavailability, and bioavailability is determined by both external environmental conditions and organism physiological/biological characteristics. Key information required for ERA of metals includes: emissions, pathways, and movements in the environment (Do metals accumulate in biota above background concentrations?); the relationship between internal dose and/or external concentration (Are these metals bioreactive?); and the incidence and severity of any effects (Are bioreactive metals likely to result in adverse or, in the case of essential metals, beneficial effects?) — ground-truthed in contaminated areas by field observations. Specific requirements for metals ERA are delineated for each ERA component (Hazard Identification, Exposure Analysis, Effects Analysis, Risk Characterization), updating Chapman and Wang (2000). In addition, key specific information required for ERA is delineated by major information category (conceptual diagrams, bioavailability, predicted environmental concentration [PEC], predicted no effect concentration [PNEC], tolerance, application [uncertainty] factors, risk characterization) relative to three different tiered, iterative levels of ERA: Problem Formulation, Screening Level ERA (SLERA), and Detailed Level ERA (DLERA). Although data gaps remain, a great deal of progress has been made in the last three years, forming the basis for substantial improvements to ERA for metals.

A mass balance inventory of mercury in the Arctic Ocean

Environmental context. Mercury (Hg) occurs at high concentrations in Arctic marine wildlife, posing a possible health risk to northern peoples who use these animals for food. We find that although the dramatic Hg increases in Arctic Ocean animals since pre-industrial times can be explained by sustained small annual inputs, recent rapid increases probably cannot, because of the existing large oceanic Hg reservoir (the ‘flywheel’ effect). Climate change is a possible alternative force underpinning recent trends. Abstract. The present mercury (Hg) mass balance was developed to gain insights into the sources, sinks and processes regulating biological Hg trends in the Arctic Ocean. Annual total Hg inputs (mainly wet deposition, coastal erosion, seawater import, and ‘excess’ deposition due to atmospheric Hg depletion events) are nearly in balance with outputs (mainly shelf sedimentation and seawater export), with a net 0.3% year–1 increase in total mass. Marine biota represent a small fraction of the ocean’s existing total Hg and methyl-Hg (MeHg) inventories. The inertia associated with these large non-biological reservoirs means that ‘bottom-up’ processes (control of bioavailable Hg concentrations by mass inputs or Hg speciation) are probably incapable of explaining recent biotic Hg trends, contrary to prevailing opinion. Instead, varying rates of bioaccumulation and trophic transfer from the abiotic MeHg reservoir may be key, and are susceptible to ecological, climatic and biogeochemical influences. Deep and sustained cuts to global anthropogenic Hg emissions are required to return biotic Hg levels to their natural state. However, because of mass inertia and the less dominant role of atmospheric inputs, the decline of seawater and biotic Hg concentrations in the Arctic Ocean will be more gradual than the rate of emission reduction and slower than in other oceans and freshwaters. Climate warming has likely already influenced Arctic Hg dynamics, with shrinking sea-ice cover one of the defining variables. Future warming will probably force more Hg out of the ocean’s euphotic zone through greater evasion to air and faster Hg sedimentation driven by higher primary productivity; these losses will be countered by enhanced inputs from coastal erosion and rivers.

Increasing Contaminant Burdens in an Arctic Fish, Burbot (Lota lota), in a Warming Climate

The temporal patterns of mercury (Hg), polychlorinated biphenyls (PCBs), and other contaminants in Arctic aquatic biota are usually attributed to changing atmospheric sources. However, climate variability and change is another means of altering contaminant fate and bioavailability. We show here that the concentrations of Hg and PCBs in Mackenzie River burbot ( Lota lota ), a top predator fish and important staple food for northern Canadian communities, have increased significantly over the last 25 years despite falling or stable atmospheric concentrations, suggesting that environmental processes subsequent to atmospheric transport are responsible. Using a dated sediment core from a tributary lake near the Mackenzie River sampling site, we show that variations in Hg concentrations downcore are strongly associated with labile, algal-derived organic matter (OM). Strong temporal correlations between increasing primary productivity and biotic Hg and PCBs as reflected by burbot suggest that warming temperatures and reduced ice cover may lead to increased exposure to these contaminants in high trophic level Arctic freshwater biota.

How does climate change influence arctic mercury

Recent studies have shown that climate change is already having significant impacts on many aspects of transport pathways, speciation and cycling of mercury within Arctic ecosystems. For example, the extensive loss of sea-ice in the Arctic Ocean and the concurrent shift from greater proportions of perennial to annual types have been shown to promote changes in primary productivity, shift foodweb structures, alter mercury methylation and demethylation rates, and influence mercury distribution and transport across the ocean-sea-ice-atmosphere interface (bottom-up processes). In addition, changes in animal social behavior associated with changing sea-ice regimes can affect dietary exposure to mercury (top-down processes). In this review, we address these and other possible ramifications of climate variability on mercury cycling, processes and exposure by applying recent literature to the following nine questions; 1) What impact has climate change had on Arctic physical characteristics and processes? 2) How do rising temperatures affect atmospheric mercury chemistry? 3) Will a decrease in sea-ice coverage have an impact on the amount of atmospheric mercury deposited to or emitted from the Arctic Ocean, and if so, how? 4) Does climate affect air-surface mercury flux, and riverine mercury fluxes, in Arctic freshwater and terrestrial systems, and if so, how? 5) How does climate change affect mercury methylation/demethylation in different compartments in the Arctic Ocean and freshwater systems? 6) How will climate change alter the structure and dynamics of freshwater food webs, and thereby affect the bioaccumulation of mercury? 7) How will climate change alter the structure and dynamics of marine food webs, and thereby affect the bioaccumulation of marine mercury? 8) What are the likely mercury emissions from melting glaciers and thawing permafrost under climate change scenarios? and 9) What can be learned from current mass balance inventories of mercury in the Arctic? The review finishes with several conclusions and recommendations.

Issues in Ecological Risk Assessment of Inorganic Metals and Metalloids

Ecological risk assessment (ERA) is a process that evaluates the potential for adverse ecological effects occurring as a result of exposure to contaminants or other stressors. ERA begins with hazard identification/problem formulation, progresses to effects and exposure assessment, and finishes with risk characterization (an estimate of the incidence and severity of any adverse effects likely to occur). Risk management initially sets the boundaries of the ERA and then uses its results for decision-making. Key information required for an ERA includes: the emissions, pathways and rates of movement of contaminants in the environment; and, information on the relationship between contaminant concentrations and the incidence and (or) severity of adverse effects. Because of specific properties and characteristics of metals in general and of certain metals in particular, a generalized ERA process applicable to organic substances is inappropriate for metals. First, metals are naturally occurring and can arise, sometimes in very high concentrations, from non-anthropogenic sources; organisms can and do adapt to a wide range of metal concentrations. Second, certain metals (e.g., copper, zinc) are essential for biotic health, which means there is an effect threshold for both deficiency and excess, and that standard body burden indices such as bioaccumulation factors (BCFs) can be misleading. Third, metals can occur in the environment in a variety of forms that are more or less available to biota but adverse biological effects can only occur if metals are or may become bioavailable. Fourth, whereas the bioavailability and hence the possibility of toxicity of persistent organic substances are mainly dependent on their intrinsic properties (i.e., lipophilicity), those of metals are generally controlled by external environmental conditions. Examples include pH and ligands, which affect the metal speciation and coexisting cations (e.g., H+, Ca2+) which compete with the metal ions. ERAs involving metals must include the above four major considerations; other considerations vary depending on whether the ERA is for a site, a region, or is global in scope.

Relation of sediment characteristics to trace metal concentrations: a statistical study

Twenty six geographically and hydrologically diverse sediment samples were collected from major rivers in eastern China and analyzed for sediment characteristics (grain size distribution, iron, manganese and their oxyhydroxides, aluminum, total organic carbon, surface area and surface site density) and Cd, Cu, Pb and Zn concentrations. The relation of sediment characteristics to trace metal concentrations was statistically studied by Kendall rank and partial rank correlation analysis. Although a variety of sediment characteristics showed apparent correlations with trace metal concentrations, most of them were caused by the interrelation among the characteristics themselves. The partial correlation analysis indicated that trace metal concentrations of the relatively uncontaminated sediments can be better interpreted by the sediment surface area (an integrated characteristic) and aluminum content (an indication of the local geochemical processes).

The relation between amyotrophic lateral sclerosis and inorganic selenium in drinking water: a population-based case-control study

BackgroundA community in northern Italy was previously reported to have an excess incidence of amyotrophic lateral sclerosis among residents exposed to high levels of inorganic selenium in their drinking water.MethodsTo assess the extent to which such association persisted in the decade following its initial observation, we conducted a population-based case-control study encompassing forty-one newly-diagnosed cases of amyotrophic lateral sclerosis and eighty-two age- and sex-matched controls. We measured long-term intake of inorganic selenium along with other potentially neurotoxic trace elements.ResultsWe found that consumption of drinking water containing ≥ 1 μg/l of inorganic selenium was associated with a relative risk for amyotrophic lateral sclerosis of 5.4 (95% confidence interval 1.1-26) after adjustment for confounding factors. Greater amounts of cumulative inorganic selenium intake were associated with progressively increasing effects, with a relative risk of 2.1 (95% confidence interval 0.5-9.1) for intermediate levels of cumulative intake and 6.4 (95% confidence interval 1.3-31) for high intake.ConclusionBased on these results, coupled with other epidemiologic data and with findings from animal studies that show specific toxicity of the trace element on motor neurons, we hypothesize that dietary intake of inorganic selenium through drinking water increases the risk for amyotrophic lateral sclerosis.

Chemical Demethylation of Methylmercury by Selenoamino Acids

A new chemical demethylation pathway for methylmercury under physiologically and environmentally relevant conditions is reported. The pathway involves the reaction between methylmercury and a selenoamino acid (L-selenocysteine, L-selenoglutathione, D,L-selenopenicillamine, or L-selenomethionine) via the formation of bis(methylmercuric)selenide and dimethylmercury as intermediates. The final degradation product is HgSe(s).