Mercury Bioaccumulation in Estuarine Fishes: Novel Insights from Sulfur Stable Isotopes

A comprehensive sulfate and DOM framework to assess methylmercury formation and risk in subtropical wetlands

Wetlands play a vital role in contaminant cycling and uptake. Understanding how sulfate (SO42‒) 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 SO42‒ gradients in three freshwater Everglades wetlands to assess linkages between SO42‒, MeHg, dissolved organic matter (DOM), and inorganic sulfide (S(‒II)). Increasing SO42‒ concentrations increase S(‒II) and DOM concentrations and DOM aromaticity. MeHg concentration show a unimodal response to surface water SO42‒, which reflect high Hg(II) methylation at low-to-intermediate SO42‒concentration (2-12 mg/L) and low Hg(II) methylation at higher SO42‒concentrations ( > 12 mg/L). MeHg concentrations in surface waters correlate positively with MeHg concentrations in prey fish. The coherent biogeochemical relationships between SO42‒ 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 SO42‒.

Coastal foraging increases mercury concentrations in a breeding seabird: Insights from isotopes, biologging, and prey

Mercury concentrations can vary substantially across spatial and temporal scales. As mobile marine predators, seabirds offer a unique opportunity to directly link foraging tactics with mercury burdens, because temporal variation in mercury can be related to spatial variation in foraging. Breeding razorbills (Alca torda) forage in various habitats around colonies, which can affect their mercury burdens. Here, we explore how mercury concentrations in red blood cells are influenced by foraging tactics (movement and trophic ecology) using GPS tracking and stable isotope dietary analysis (δ15N, δ13C, and δ34S), as well as by environmental signals, assessed through prey observations using nest-based cameras in two years (2021 and 2022) in breeding razorbills in the Gulf of St-Lawrence, Canada. Total mercury levels varied significantly between years, and all individuals exceeded low-risk toxicity thresholds. In the higher mercury year, razorbills foraged closer to the coast and had lower δ13C and δ34S values, suggesting a shift in foraging tactics. Although prey species composition did not change between years, individuals in the high mercury year brought back smaller prey and more items per load. These findings suggest that elevated mercury concentrations in razorbills may be linked to foraging in more coastal areas. Thus, small-scale changes, such as a shift to inshore coastal foraging, may expose seabirds to cumulative freshwater/terrestrial inputs and potentially higher mercury concentrations in prey. By investigating ecotoxicological risks associated with foraging-related contamination using multiple simultaneous approaches, our study provides insights into how feeding tactics can drive mercury contamination in sympatric seabirds foraging in coastal environments.

Global mercury concentrations in biota: their use as a basis for a global biomonitoring framework

An important provision of the Minamata Convention on Mercury is to monitor and evaluate the effectiveness of the adopted measures and its implementation. Here, we describe for the first time currently available biotic mercury (Hg) data on a global scale to improve the understanding of global efforts to reduce the impact of Hg pollution on people and the environment. Data from the peer-reviewed literature were compiled in the Global Biotic Mercury Synthesis (GBMS) database (>550,000 data points). These data provide a foundation for establishing a biomonitoring framework needed to track Hg concentrations in biota globally. We describe Hg exposure in the taxa identified by the Minamata Convention: fish, sea turtles, birds, and marine mammals. Based on the GBMS database, Hg concentrations are presented at relevant geographic scales for continents and oceanic basins. We identify some effective regional templates for monitoring methylmercury (MeHg) availability in the environment, but overall illustrate that there is a general lack of regional biomonitoring initiatives around the world, especially in Africa, Australia, Indo-Pacific, Middle East, and South Atlantic and Pacific Oceans. Temporal trend data for Hg in biota are generally limited. Ecologically sensitive sites (where biota have above average MeHg tissue concentrations) have been identified throughout the world. Efforts to model and quantify ecosystem sensitivity locally, regionally, and globally could help establish effective and efficient biomonitoring programs. We present a framework for a global Hg biomonitoring network that includes a three-step continental and oceanic approach to integrate existing biomonitoring efforts and prioritize filling regional data gaps linked with key Hg sources. We describe a standardized approach that builds on an evidence-based evaluation to assess the Minamata Convention's progress to reduce the impact of global Hg pollution on people and the environment.

Procedures from samples to sulfur isotopic data: A review

RATIONALE

Sulfur isotopes have been widely used to solve some key scientific questions, especially in the last two decades with advanced instruments and analytical schemes. Different sulfur speciation and multiple isotopes analyzed in laboratories worldwide and in situ microanalysis have also been reported in many articles. However, methods of sampling to measurements are multifarious, and occasionally some inaccuracies are present in published papers. Vague methods may mislead newcomers to the field, puzzle readers, or lead to incorrect data-based correlations.

METHODS

We have reviewed multiple methods on sulfur isotopic analyses from the perspectives of sampling, laboratory work, and instrumental analysis in order to help reduce operational inhomogeneity and ensure the fidelity of sulfur isotopic data. We do not deem our proposed solutions as the ultimate standard methods but as a lead-in to the overall introduction and summary of the current methods used.

RESULTS

It has been shown that external contamination and transformation of different sulfur species should be avoided during the sampling, pretreatment, storage, and chemical treatment processes. Conversion rates and sulfur isotopic fractionations during sulfur extraction, purification, and conversion processes must be verified by researchers using standard or known samples. The unification of absence of isotopic fractionation is needed during all steps, and long-term monitoring of standard samples is recommended.

CONCLUSION

This review compiles more details on different methods in sampling, laboratory operation, and measurement of sulfur isotopes, which is beneficial for researchers' better practice in laboratories. Microanalyses and molecular studies are the frontier techniques that compare the bulk sample with the elemental analysis/continuous flow-gas source stable isotope ratio mass spectrometry method, but the latter is widely used. The development of sulfur isotopic measurements will lead to the innovation in scientific issues with sulfur proxies.

The Roles of Diet and Habitat Use in Pesticide Bioaccumulation by Juvenile Chinook Salmon: Insights from Stable Isotopes and Fatty Acid Biomarkers

Stable isotopes (SI) and fatty acid (FA) biomarkers can provide insights regarding trophic pathways and habitats associated with contaminant bioaccumulation. We assessed relationships between SI and FA biomarkers and published data on concentrations of two pesticides [dichlorodiphenyltrichloroethane and degradation products (DDX) and bifenthrin] in juvenile Chinook Salmon (Oncorhynchus tshawytscha) from the Sacramento River and Yolo Bypass floodplain in Northern California near Sacramento. We also conducted SI and FA analyses of zooplankton and macroinvertebrates to determine whether particular trophic pathways and habitats were associated with elevated pesticide concentrations in fish. Relationships between DDX and both sulfur (δ34S) and carbon (δ13C) SI ratios in salmon indicated that diet is a major exposure route for DDX, particularly for individuals with a benthic detrital energy base. Greater use of a benthic detrital energy base likely accounted for the higher frequency of salmon with DDX concentrations > 60 ng/g dw in the Yolo Bypass compared to the Sacramento River. Chironomid larvae and zooplankton were implicated as prey items likely responsible for trophic transfer of DDX to salmon. Sulfur SI ratios enabled identification of hatchery-origin fish that had likely spent insufficient time in the wild to substantially bioaccumulate DDX. Bifenthrin concentration was unrelated to SI or FA biomarkers in salmon, potentially due to aqueous uptake, biotransformation and elimination of the pesticide, or indistinct biomarker compositions among invertebrates with low and high bifenthrin concentrations. One FA [docosahexaenoic acid (DHA)] and DDX were negatively correlated in salmon, potentially due to a greater uptake of DDX from invertebrates with low DHA or effects of DDX on FA metabolism. Trophic biomarkers may be useful indicators of DDX accumulation and effects in juvenile Chinook Salmon in the Sacramento River Delta.

Ecology and environmental characteristics influence methylmercury bioaccumulation in coastal invertebrates

Quantifying mercury (Hg) concentrations in invertebrates is fundamental to determining risk for bioaccumulation in higher trophic level organisms in coastal food webs. Bioaccumulation is influenced by local mercury concentrations, site geochemistry, individual feeding ecologies, and trophic position. We sampled seven species of invertebrates from five coastal sites in the Minas Basin, Bay of Fundy, and determined body concentrations of methylmercury (MeHg), total mercury (THg), and stable isotopes of nitrogen (δ15N) and carbon (δ13C). To evaluate the effects of environmental chemistry on Hg production and bioaccumulation, bulk sediments from all sites were analysed for THg, %Loss on ignition (LOI) (carbon), and sulfur isotopes (δ34S), and concentrations of MeHg, Total Organic Carbon (TOC), sulfate, and sulfide were measured in porewaters. The mean concentration of MeHg in tissues for all invertebrates sampled was 10.03 ± 7.04 ng g-1). MeHg in porewater (mean = 0.22-1.59 ng L-1) was the strongest predictor of invertebrate MeHg, but sediment δ34S (-0.80-14.1‰) was also a relatively strong predictor. δ34S in tissues (measured in three species; Corophium volutator, Ilyanassa obsoleta, and Littorina littorea) were positively related to MeHg in invertebrates (r = 0.55, 0.22, and 0.71 respectively), and when used in combination with δ15N and δ13C values improved predictions of Hg concentrations in biota. Hg concentrations in the amphipod Corophium volutator (mean MeHg = 10.60 ± 1.90 ng g-1) were particularly well predicted using porewater and sediment chemistry, highlighting this species as a useful bioindicator of Hg contamination in sediments of the Bay of Fundy.

Mercury accumulation, biomagnification, and relationships to δ13C, δ15N and δ34S of fishes and marine mammals in a coastal Arctic marine food web

Combining mercury and stable isotope data sets of consumers facilitates the quantification of whether contaminant variation in predators is due to diet, habitat use and/or environmental factors. We investigated inter-species variation in total Hg (THg) concentrations, trophic magnification slope between δ15N and THg, and relationships of THg with δ13C and δ34S in 15 fish and four marine mammal species (249 individuals in total) in coastal Arctic waters. Median THg concentration in muscle varied between species ranging from 0.08 ± 0.04 μg g-1 dw in capelin to 3.10 ± 0.80 μg g-1 dw in beluga whales. Both δ15N (r2 = 0.26) and δ34S (r2 = 0.19) best explained variation in log-THg across consumers. Higher THg concentrations occurred in higher trophic level species that consumed more pelagic-associated prey than consumers that rely on the benthic microbial-based food web. Our study illustrates the importance of using a multi-isotopic approach that includes δ34S when investigating trophic Hg dynamics in coastal marine systems.

Revealing the environmental pollution of two estuaries through histopathological biomarkers in five fishes from different trophic guilds of northeastern Brazil

Estuaries in Brazil are mostly anthropically affected due to the discharge of industrial and domestic effluents. In two of them, the Santa Cruz Channel Estuary (ITAP) and Sirinhaém River Estuary (SIR), historically affected by mercury pollution and sugarcane industry in Northeast Brazil, we assessed environmental pollution using liver and gill histopathological biomarkers in fish from different trophic levels. Liver samples exhibited serious damages such as hepatic steatosis, necrosis, and infiltration. The gills showed moderate to severe changes, such as lifting of epithelial cells, lamellar aneurysm, and rupture of lamellar epithelium. Most of the changes in the liver and gills were reported for species Centropomus undecimalis and the Gobionellus stomatus, which were considered as good sentinels of pollution. The combination of biomarker methodologies was efficient in diagnosing the serious damage to the species, reinforcing the need for monitoring the health of the ecosystems evaluated.

Tracing Sulfate Source and Transformation in the Groundwater of the Linhuan Coal Mining Area, Huaibei Coalfield, China

Mining activities cause surface sulfate enrichment, which has negative impacts on human health and ecosystems. These high concentrations of sulfate may enter groundwater through the unsaturated zone (UZ), threatening groundwater quality. Therefore, we combined hydrochemical and dual isotopic analyses of sulfate in surface water, soil water and groundwater with evaluations of the UZ to identify the groundwater sulfate source and transformation in the coal mining area. Soil profile samples were collected near gangue heaps (UZ-1, UZ-2) and the mean sulfate concentrations of the UZ-1 profile and UZ-2 profile were 35.4 mg/L and 69.63 mg/L, respectively. The shallow groundwater sulfate was mainly from dissolution of evaporite, sulfide oxidation and sewage. Different sulfate contaminated areas showed different characteristics of sulfate sources. The sulfate source to groundwater near the coal gangue heaps was sulfide oxidation. The groundwater sulfate near the gangue heaps and industrial park compound contamination area was mainly derived from industrial and domestic sewage and sulfide oxidation. In addition, the role of bacterial sulfate reduction (BSR) in the groundwater was not obvious. This research result is of great significance for promoting the safe mining of coal resources and sustainable utilization of groundwater in the Huaibei coal mining area and other coal mining areas in China.

Sources, trophodynamics, contamination and risk assessment of toxic metals in a coastal ecosystem by using a receptor model and Monte Carlo simulation

Heavy metal (HM) pollution in coastal ecosystems have posed threats to organisms and human worldwide. This study comprehensively investigated the concentrations, sources, trophodynamics, contamination, and risks of six HMs in the coastal ecosystem of Jiaozhou Bay, northern China, by stable isotope analysis, positive matrix factorization (PMF), and Monte Carlo simulation. Overall, Co, Cu, Ni, Pb, and Zn were significantly bio-diluted in the food web, while Cr was significantly biomagnified with a trophic magnification factor of 1.23. In addition, trophodynamics of the six HMs was different among fish, mollusk, and crustacean. Furthermore, detailed transfer pathways of six HMs in the food web including eight trophic levels were different from one another. Bioaccumulation order of the six HMs was Cu > Zn > Co, Cr, Ni, and Pb. Zinc concentrations were the highest in seawater, sediments, and organisms. Anthropogenic sources contributed to 71% for Zn, 31% for Cu and Pb, and 27% for Co, Cr, and Ni in the sediment, which was moderately contaminated with moderate ecological risk. However, the human health risk of HMs from eating seafood was relatively low. To protect the Jiaozhou Bay ecosystem, HM contamination should be further controlled in future.

Influence of Feeding Ecology on Legacy Organochlorine Contaminants in Freshwater Fishes of Lake Erie

Persistent organic pollutants (POPs) in biota are influenced by ecological, physiological, and physicochemical properties; however, there is a need for a better understanding about the interplay of these parameters on POP dynamics and fate. To address this, POPs in three Lake Erie freshwater fishes (freshwater drum, Aplodinotus grunniens; walleye, Sander vitreus; and white perch, Morone americana) with different feeding ecologies were assessed using life history characteristics and three stable isotopes (δ¹³ C, δ¹⁵ N, and δ³⁴ S). Lipid normalized POP concentrations were in the range of past studies and were generally similar among the three species when all ages were combined. Principal component analysis (PCA) found the two significant PCs (explaining 59% and 10% of the variation), with all POPs loading significantly onto PC1, which indicated a common source of contamination, likely legacy sediment loads. Loadings on both PCs were correlated with POP log K_OW . Age, habitat use (δ¹³ C and δ³⁴ S), trophic position (δ¹⁵ N) and interactions between age and δ¹⁵ N, age and species, and δ¹⁵ N and δ³⁴ S were significant predictors of POP concentration based on PC1 scores, whereas δ¹³ C and species were significant predictors of PC2 scores. The similar concentrations among the species, yet variation related to the ecology (age and trophic position) across individuals demonstrates the complexity of contaminant dynamics in freshwater fish in a large lake system and the need to consider variation across individuals within species. Environ Toxicol Chem 2021;40:3421-3433. © 2021 SETAC.

Mercury Accumulation and Elimination in Different Tissues of Zebrafish (Danio rerio) Exposed to a Mercury-Supplemented Diet

In this study, we evaluated the bioaccumulation of mercury in zebrafish (Danio rerio) exposed to mercury-contaminated food for 21 days and the depuration of mercury for a subsequent post-exposure period of 28 days. Four tissues (muscle, liver, gills, and skin) were analyzed for mercury content. Overall, data indicated that Hg accumulation in the liver is faster than in other tissues. Furthermore, the liver is the tissue with the highest accumulation rate per day (0.021 µg Hg g−1 day−1), followed by muscle, skin, and gills. Conversely, the Hg depuration rates in different tissues showed the following order: gills > skin > muscle > liver. The bioaccumulation factor values of liver and muscle increased linearly during the uptake period. The ratios between mercury concentration in liver and muscle during the experiment also increased during the uptake period and remained higher than 1 during the elimination period, suggesting that Danio rerio needed more than 4 weeks of depuration. Finally, the distribution of Hg in the water column during the accumulation period is Hg particulate > Hg dissolved, and during the depuration period it is the opposite, mercury particulate < mercury dissolved. In conclusion, this study contributes to a better understanding of the differences in Hg dynamics during the accumulation and depuration stages in a model fish, also emphasizing the alterations on Hg available in the water column.

Typical herbicide residues, trophic transfer, bioconcentration, and health risk of marine organisms

Atrazine, a potent herbicide for weeds removal during the growing season, has been widely used in China. It is known to be distributed in aquatic ecosystems with a long half-life, thus presenting a potential risk to species and consumers. This study analyzed the concentrations of degraded atrazine residues in marine organisms (N = 129) including 3 species of mollusks, 2 species of crustaceans, and 15 species of fish from a semi-enclosed bay, Jiaozhou Bay (JZB), adjacent to the Northwest Pacific Ocean in China. The corresponding trophic magnification factors (TMF), bioaccumulation factors (BCFs), and subsequent risks to final consumers were also determined. The results showed an average atrazine concentration of (0.301 ± 0.03) ng g^-1 and (0.305 ± 0.04) ng g^-1 in fish and invertebrates, respectively. The BCFs were (5.23 ± 1.75) L kg^-1 and (5.81 ± 1.31) L kg^-1 for fish and invertebrates, respectively. Atrazine was significantly bio-diluted in JZB through the sampled marine organisms with increasing trophic levels, with a TMF value below 1 (P < 0.01). An analysis of the species sensitivity distribution (SSD) predicted that<0.02% of species were exposed to a dissolved concentration of atrazine (57.88 ng L^-1) that would lead to detrimental effects, while risk quotients predicted low long-term risks for species in the bay. Finally, people with a diet limited to species from JZB were found to face no associated health risk due to a significantly small daily intake and target hazard quotient of atrazine. The corresponding non-carcinogenic effect showed no significant risk from seafood consumption.

Patterns in forage fish mercury concentrations across Northeast US estuaries

Biogeochemical conditions and landscape can have strong influences on mercury bioaccumulation in fish, but these effects across regional scales and between sites with and without point sources of contamination are not well understood. Normal means clustering, a type of unsupervised machine learning, was used to analyze relationships between forage fish (Fundulus heteroclitus and Menidia menidia) mercury (Hg) concentrations and sediment and water column Hg and methylmercury (MeHg) concentrations, ancillary variables, and land classifications within the sub-watershed. The analysis utilized data from 38 sites in 8 estuarine systems in the Northeast US, collected over five years. A large range of mercury concentrations and land use proportions were observed across sites. The cluster correlations indicated that for Fundulus, benthic and pelagic Hg and MeHg concentrations were most related to tissue concentrations, while Menidia Hg was most related to water column MeHg, reflecting differing feeding modes between the species. For both species, dissolved MeHg was most related to tissue concentrations, with sediment Hg concentrations influential at contaminated sites. The models considering only uncontaminated sites showed reduced influence of bulk sediment MeHg for both species, but Fundulus retained sediment drivers at some sites, with dissolved MeHg still highly correlated for both. Dissolved organic carbon (DOC), chlorophyll, land use, and other ancillary variables were of lesser importance in driving bioaccumulation, though DOC was strongly related within some clusters, likely in relation to dissolved Hg. Land use, though not of primary importance, showed relationships opposite to those observed in freshwater, with development positively correlated and forests and agriculture negatively correlated with tissue concentrations across clusters and species. Clusters were composed of sites from geographically distinct systems, indicating the greater importance of small scale drivers of MeHg formation and uptake into the food web over system or region-wide influences.

Mercury Methylation Genes Identified across Diverse Anaerobic Microbial Guilds in a Eutrophic Sulfate-Enriched Lake

Mercury (Hg) methylation is a microbially mediated process that converts inorganic Hg into bioaccumulative, neurotoxic methylmercury (MeHg). The metabolic activity of methylating organisms is highly dependent on biogeochemical conditions, which subsequently influences MeHg production. However, our understanding of the ecophysiology of methylators in natural ecosystems is still limited. Here, we identified potential locations of MeHg production in the anoxic, sulfidic hypolimnion of a freshwater lake. At these sites, we used shotgun metagenomics to characterize microorganisms with the Hg-methylation gene hgcA. Putative methylators were dominated by hgcA sequences divergent from those in well-studied, confirmed methylators. Using genome-resolved metagenomics, we identified organisms with hgcA (hgcA+) within the Bacteroidetes and the recently described Kiritimatiellaeota phyla. We identified hgcA+ genomes derived from sulfate-reducing bacteria, but these accounted for only 22% of hgcA+ genome coverage. The most abundant hgcA+ genomes were from fermenters, accounting for over half of the hgcA gene coverage. Many of these organisms also mediate hydrolysis of polysaccharides, likely from cyanobacterial blooms. This work highlights the distribution of the Hg-methylation genes across microbial metabolic guilds and indicate that primary degradation of polysaccharides and fermentation may play an important but unrecognized role in MeHg production in the anoxic hypolimnion of freshwater lakes.

Trophodynamics of arsenic for different species in coastal regions of the Northwest Pacific Ocean: In situ evidence and a meta-analysis

China has been the major fishery producer in the Northwest Pacific Ocean for decades and the seafood safety deserves continuous concern. In this study, 188 organism and 27 sediment samples were collected from the Jiaozhou Bay, a typical semi-enclosed bay adjacent to the Northwest Pacific Ocean, to study the arsenic (As) pollution level and trophodynamics in the coastal regions of China combined with a meta-analysis. Results showed that arsenic was the most abundant in crustaceans with the average of 28.84 ± 4.95 mg/kg in dry weight, in comparison with molluscs (18.68 ± 2.51 mg/kg) and fish (9.31 ± 1.45 mg/kg). Additionally, based on a meta-analysis, arsenic in coastal organisms generally decreased from north to south in China. With increasing values of δ¹⁵N, arsenic was significantly biomagnified in the molluscs but bio-diluted in the groups of crustaceans and fish. When all the species were taken into consideration, overall bio-dilution of As was observed through the simplified food chain in the Jiaozhou Bay. Based on the target hazard quotient (THQ), the health risk of consuming seafood from the Jiaozhou Bay was not significant except for several kinds of crustaceans. The smaller THQs indicated lower health risk of eating molluscs and fish than crustaceans. Besides, urban households tended to undertake much higher risk than rural households. Based on our results, it is recommended for urban citizens to reduce the frequency of consuming crustaceans and give preference to fish when choosing seafood.

Mercury bioaccumulation in freshwater fishes of the Chesapeake Bay watershed

Chemical contaminants are a threat to the Chesapeake Bay watershed, with mercury (Hg) among the most prevalent causes of impairment. Despite this, large-scale patterns of Hg concentrations, and the potential risks to fish, wildlife, and humans across the watershed, are poorly understood. We compiled fish Hg data from state monitoring programs and recent research efforts to address this knowledge gap and provide a comprehensive assessment of fish Hg concentrations in the watershed's freshwater habitats. The resulting dataset consisted of nearly 8000 total Hg (THg) concentrations from 600 locations. Across the watershed, fish THg concentrations spanned a 44-fold range, with mean concentrations varying by 2.6- and 8.8-fold among major sub-watersheds and individual 8-digit hydrological units, respectively. Although, mean THg concentrations tended to be moderate, fish frequently exceeded benchmarks for potential adverse health effects, with 45, 48, and 36% of all samples exceeding benchmarks for human, avian piscivore, and fish risk, respectively. Importantly, the percentage of fish exceeding these benchmarks was not uniform among species or locations. The variation in fish THg concentrations among species and sites highlights the roles of waterbody, landscape, and ecological processes in shaping broad patterns in Hg risk across the watershed. We outline an integrated Hg monitoring program that could identify key factors influencing Hg concentrations across the watershed and facilitate the implementation of management strategies to mitigate the risks posed by Hg.

Chemical tracers guide identification of the location and source of persistent organic pollutants in juvenile Chinook salmon (Oncorhynchus tshawytscha), migrating seaward through an estuary with multiple contaminant inputs

Understanding the spatial extent, magnitude, and source of contaminant exposure in biota is necessary to formulate appropriate conservation measures to reduce or remediate contaminant exposure. However, obtaining such information for migratory animals is challenging. Juvenile Chinook salmon (Oncorhynchus tshawytscha), a threatened species throughout the US Pacific Northwest, are exposed to persistent organic pollutants (POPs), including polybrominated diphenyl ether (PBDE) flame retardants and polychlorinated biphenyls (PCBs), in many developed rivers and estuaries. This study used three types of complementary chemical tracer data (contaminant concentrations, POP fingerprints, and stable isotopes), to determine the location and source of contaminant exposure for natural- and hatchery-origin Chinook salmon migrating seaward through a developed watershed with multiple contaminant sources. Concentration data revealed that salmon were exposed to and accumulated predominantly PBDEs and PCBs in the lower mainstem region of the river, with higher PBDEs in natural- than hatchery-origin fish but similar PCBs in both groups, associated with differences in contaminant inputs and/or habitat use. The POP fingerprints of the natural-origin-fish captured from this region were also distinct from other region and origin sample groups, with much higher proportions of PBDEs in the total POP concentration, indicating a different contaminant source or habitat use than the hatchery-origin fish. Stable isotopes, independent tracers of food sources and habitat use, revealed that natural-origin fish from this region also had depleted δ¹⁵N signatures compared to other sample groups, associated with exposure to nutrient-rich wastewater. The PBDE-enhanced POP fingerprints in these salmon were correlated with the degree of depletion in nitrogen stable isotopes of the fish, suggesting a common wastewater source for both the PBDEs and the nitrogen. Identification of the location and source of contaminant exposure allows environmental managers to establish conservation measures to control contaminant inputs, necessary steps to improve the health of Chinook salmon and enhance their marine survival.

Methylmercury exposure in wildlife: A review of the ecological and physiological processes affecting contaminant concentrations and their interpretation

Exposure to methylmercury (MeHg) can result in detrimental health effects in wildlife. With advances in ecological indicators and analytical techniques for measurement of MeHg in a variety of tissues, numerous processes have been identified that can influence MeHg concentrations in wildlife. This review presents a synthesis of theoretical principals and applied information for measuring MeHg exposure and interpreting MeHg concentrations in wildlife. Mercury concentrations in wildlife are the net result of ecological processes influencing dietary exposure combined with physiological processes that regulate assimilation, transformation, and elimination. Therefore, consideration of both physiological and ecological processes should be integrated when formulating biomonitoring strategies. Ecological indicators, particularly stable isotopes of carbon, nitrogen, and sulfur, compound-specific stable isotopes, and fatty acids, can be effective tools to evaluate dietary MeHg exposure. Animal species differ in their physiological capacity for MeHg elimination, and animal tissues can be inert or physiologically active, act as sites of storage, transformation, or excretion of MeHg, and vary in the timing of MeHg exposure they represent. Biological influences such as age, sex, maternal transfer, and growth or fasting are also relevant for interpretation of tissue MeHg concentrations. Wildlife tissues that represent current or near-term bioaccumulation and in which MeHg is the predominant mercury species (such as blood and eggs) are most effective for biomonitoring ecosystems and understanding landscape drivers of MeHg exposure. Further research is suggested to critically evaluate the use of keratinized external tissues to measure MeHg bioaccumulation, particularly for less-well studied wildlife such as reptiles and terrestrial mammals. Suggested methods are provided to effectively use wildlife for quantifying patterns and drivers of MeHg bioaccumulation over time and space, as well as for assessing the potential risk and toxicological effects of MeHg on wildlife.

Microbial Mercury Methylation in Aquatic Environments: A Critical Review of Published Field and Laboratory Studies

Methylmercury (MeHg) is an environmental contaminant of concern because it biomagnifies in aquatic food webs and poses a health hazard to aquatic biota, piscivorous wildlife and humans. The dominant source of MeHg to freshwater systems is the methylation of inorganic Hg (IHg) by anaerobic microorganisms; and it is widely agreed that in situ rates of Hg methylation depend on two general factors: the activity of Hg methylators and their uptake of IHg. A large body of research has focused on the biogeochemical processes that regulate these two factors in nature; and studies conducted within the past ten years have made substantial progress in identifying the genetic basis for intracellular methylation and defining the processes that govern the cellular uptake of IHg. Current evidence indicates that all Hg methylating anaerobes possess the gene pair hgcAB that encodes proteins essential for Hg methylation. These genes are found in a large variety of anaerobes, including iron reducers and methanogens; but sulfate reduction is the metabolic process most often reported to show strong links to MeHg production. The uptake of Hg substrate prior to methylation may occur by passive or active transport, or by a combination of both. Competitive inhibition of Hg uptake by Zn speaks in favor of active transport and suggests that essential metal transporters are involved. Shortly after its formation, MeHg is typically released from cells, but the efflux mechanisms are unknown. Although methylation facilitates Hg depuration from the cell, evidence suggests that the hgcAB genes are not induced or favored by Hg contamination. Instead, high MeHg production can be linked to high Hg bioavailability as a result of the formation of Hg(SH)₂, HgS nanoparticles, and Hg-thiol complexes. It is also possible that sulfidic conditions require strong essential metal uptake systems that inadvertently bring Hg into the cytoplasm of Hg methylating microbes. In comparison with freshwaters, Hg methylation in open ocean waters appears less restricted to anoxic environments. It does seem to occur mainly in oxygen deficient zones (ODZs), and possibly within anaerobic microzones of settling organic matter, but MeHg (CH₃Hg⁺) and Me₂Hg ((CH₃)₂Hg) have been shown to form also in surface water samples from the euphotic zone. Future studies may disclose whether several different pathways lead to Hg methylation in marine waters and explain why Me₂Hg is a significant Hg species in oceans but seemingly not in most freshwaters.