Hg Bioavailability Research Articles

Effects of exogenous dissolved organic matter on the adsorption–desorption behaviors and bioavailabilities of Cd and Hg in a plant–soil system

Dissolved organic matter (DOM) is one of the most active soil components and plays critical direct and indirect roles in heavy metal migration, transformation, bioavailability, and toxicity in soils. In this study, isothermal adsorption/desorption experiments and pot experiments were performed and samples were physically characterized to study the effects of different sources of DOM on adsorption and desorption behavior and bioavailability of Cd and Hg in a plant–soil system. The results showed that microbial DOM promoted Cd and Hg adsorption in soil and decreased Cd and Hg bioavailability to pak choi (Brassica chinensis Linn.). In contrast, straw DOM and farmyard manure DOM decreased Cd and Hg adsorption and improved Cd and Hg migration and bioavailability. These results might be explained by the different types of DOM having different molecular weights and degrees of aromaticity. Cd was more readily desorbed by the soil and was more phytoavailable than Hg. We concluded that exogenous microbial DOM can inhibit Cd and Hg migration and bioavailability in soil but straw DOM and farmyard manure DOM can activate Cd and Hg in soil and promote Cd and Hg accumulation in plants. The results could help in developing rational agricultural fertilization regimes.

Seasonality of Hg dynamics in the Ebrié Lagoon (Côte d'Ivoire) ecosystem: influence of biogeochemical factors.

This study addresses the different biogeochemical parameters that control the dynamics of Hg, which is a less-studied metal in the Ebrié Lagoon. During two hydrological seasons, the dry season and the rainy season, we regularly sampled and analysed various compartments (e.g. sediments and fishes (Tilapia sp.)) of the lagoon. Thus, the physicochemical parameters were measured in situ (e.g. temperature, pH, salinity, redox potential and dissolved oxygen, total dissolved organic carbon, nitrates and sulphates), and the microbiological parameters (e.g. cultivable cells, total enzymatic activity and catabolic activity) were measured to establish the seasonal variations in the links between Hg and biogeochemical parameters through multivariate statistical analyses. The bioavailability of Hg from an unpolluted site was studied by comparing the ratios of fish and sediment. The results indicated that the seasons influenced the different biogeochemical factors, although for some factors, the variations were not significant. This influence was more pronounced in the dry season than in the rainy season. The impact of microbial activities and organic matter on Hg dynamics was observed in all seasons. However, other factors, such as pH, temperature, salinity, Eh and sulphates, influenced the dynamics of Hg only in the dry season.

Bioaccumulation of Hg in Rice Leaf Facilitates Selenium Bioaccumulation in Rice (Oryza sativa L.) Leaf in the Wanshan Mercury Mine.

Mercury (Hg) bioaccumulation in rice poses a health issue for rice consumers. In rice paddies, selenium (Se) can decrease the bioavailability of Hg through forming the less bioavailable Hg selenides (HgSe) in soil. Rice leaves can directly uptake a substantial amount of elemental Hg from the atmosphere, however, whether the bioaccumulation of Hg in rice leaves can affect the bioaccumulation of Se in rice plants is not known. Here, we conducted field and controlled studies to investigate the bioaccumulation of Hg and Se in the rice-soil system. In the field study, we observed a significantly positive correlation between Hg concentrations and BAFs of Se in rice leaves (r2 = 0.60, p < 0.01) collected from the Wanshan Mercury Mine, SW China, suggesting that the bioaccumulation of atmospheric Hg in rice leaves can facilitate the uptake of soil Se, perhaps through the formation of Hg-Se complex in rice leaves. This conclusion was supported by the controlled study, which observed significantly higher concentrations and BAFs of Se in rice leaf at a high atmospheric Hg site at WMM, compared to a low atmospheric Hg site in Guiyang, SW China.

Mercury forms in the benthic food web of a temperate coastal lagoon (southern Baltic Sea)

The study was conducted in the coastal zone of the southern Baltic. The research material consisted of macrozoobenthos and elements of its diet. The samples were analysed for Hg and its labile and stable forms, using the thermodesorption method. The results showed that the level of total Hg in zoobenthos was associated with dietary preferences and the share of bioavailable Hg in its food. The Hg fractionation in the macrofauna was conditioned by biological features (morphological structure) and environmental parameters (oxygenation, pH) which shape the mobility and assimilation of Hg. The absorption of the most toxic organic Hg in macrofauna was more effective in aerobic conditions, at low primary production and with the limited inflow of organic matter. The trophic transfer of Hg was favoured by the limited biomass of primary producers, and consequently of zoobenthos. An important factor influencing the biomagnification was also the share of labile Hg in macrozoobenthos.

Fur and faeces – Routes of mercury elimination in the Baltic grey seal (Halichoerus grypus grypus)

This study focused on evaluating the elimination of Hg by Baltic grey seals (Halichoerus grypus grypus) via faeces and pelage. In addition, we investigated the potential for ecosystem contamination via these routes. Faeces and fur were collected in 2014–2017 from captive adult grey seals and their pups. The concentrations of total mercury (THg) and methylmercury (MeHg) were measured in the samples. The amount of mercury eliminated in a labile form (the sum of the bioavailable Hg(II) and methylmercury) was also determined. An adult seal removed about 46% of mercury supplied with food via the faeces, of which only 17% was MeHg. Considering that mercury is mainly supplied to the body as MeHg, it can be assumed that mercury excreted along with faeces has undergone transformation inside the animal body. Despite the much higher THg and MeHg concentrations measured in fur, the incorporation of mercury into newly formed fur is a less effective method of Hg elimination removing just 4% of mercury entering the body via the alimentary route. The presence of mercury in lanugo is evidence of maternal transfer. First droppings of the pups were characterised by the highest content of MeHg and a low THg concentration. Then, despite the limited supply of mercury with food, and the rapid growth of the pup, the concentration of THg increased, suggesting that mercury started to be transformed into less toxic forms.It was estimated that faeces and fur expelled by seals could deliver about 800 g of mercury to the Baltic Sea. For both faeces and fur, most of the mercury (>95% for excrements and >85% for fur) was expelled in a labile form that can be quickly recycled.

Mitigation of methylmercury production in eutrophic waters by interfacial oxygen nanobubbles

In mercury (Hg)-polluted eutrophic waters, algal blooms are likely to aggravate methylmercury (MeHg) production by causing intensified hypoxia and enriching organic matter at the sediment-water interface. The technology of interfacial oxygen (O2) nanobubbles is proven to alleviate hypoxia and may have potential to mitigate the risks of MeHg formation. In this study, incubation column experiments were performed using sediment and overlying water samples collected from the Baihua Reservoir (China), which is currently suffering from co-contamination of Hg and eutrophication. The results indicated that after the application of O2 nanobubbles, the %MeHg (ratio of MeHg to total Hg) in the overlying water and surface sediment decreased by up to 76% and 56% respectively. In addition, the MeHg concentrations decreased from 0.54 ± 0.15 to 0.17 ± 0.01 ng L−1 in the overlying water and from 56.61 ± 9.23 to 25.48 ± 4.08 ng g−1 in the surface sediment. The decline could be attributed to the alleviation of anoxia and the decrease of labile organic matter and bioavailable Hg. In addition, hgcA gene abundances in the overlying water and surface sediment decreased by up to 69% and 44% after the addition of O2 nanobubbles, as is consistent with MeHg occurrence in such areas. Accordingly, this work proposed a promising strategy of using interfacial oxygen nanobubbles to alleviate the potentially enhanced MeHg production during algal bloom outbreaks in Hg-polluted eutrophic waters.

Mercury methylation and demethylation potentials in Arctic lake sediments

Mercury (Hg) transformations in sediments are key factors in the Hg exposure pathway for wildlife and humans yet are poorly characterized in Arctic lakes. As the Arctic is rapidly warming, it is important to understand how the rates of Hg methylation and demethylation (wich determine Hg bioavailability) change with temperature in lake sediments. Methylation and demethylation potentials were determined for littoral sediments (2.5 m water depth) in two deep and two shallow lakes in the Canadian Arctic using Hg stable isotope tracers at incubation temperatures of 4, 8, or 16 °C for 24 h. Compared to sediments from other regions, Hg methylation and demethylation potentials in these sediments are low. The maximum depth of the lake from which sediment was collected exerted a stronger influence over methylation potential than sediment Hg concentration or organic matter content; the shallowest lake had the highest Hg methylation potential. Sediments from the shallowest lake also demonstrated the greatest response to the temperature treatments, with significantly higher methylation potentials in the 8 and 16 °C treatments. Sediments from the deep lakes demonstrated greater demethylation potentials than shallow lakes. The methylmercury to total Hg ratio in sediments supported the measured transformation potentials as the lake with the greatest methylation potential had the highest ratio. This study supports previous works indicating that Hg methylation potential may increase as the Arctic warms, but demethylation potential does not respond to warming to the same degree, indicating that Hg methylation may predominate in warming Arctic sediments.

Mobility of mercury in soil and its transport into the sea.

Mercury (Hg) is deposited temporarily in soil and can be remobilised into rivers and seas. Given that rivers are a significant part of the mercury budget in the southern Baltic region (inland sea located in northern Europe) and meteorological changes (e.g. intense rain, drought) are observed more frequently, it is important to recognize the factors affecting the cycling of bioavailable Hg forms. The aim of this study was to identify the processes influencing the changes of labile and stabile mercury proportion in soil and the potential impact on the outflow of labile Hg into fluvial systems. For this purpose, soil samples, river sediments, and river water were collected from the Reda River (southern Baltic Sea catchment area) during the 2015 hydrologic year. The material was analysed for total and particulate mercury content and Hg forms, by a thermo-desorption method. The analysis showed that due to changes of meteorological and hydrological conditions Hg can enter rivers and then be introduced into the marine environment in various forms. On the one hand due to high precipitation events washing out of labile (i.e. bond with halogenides, MeHg, HgSO4), Hg forms into the river can be enhanced which affects increasing of availability of the most dangerous Hg form in the water systems. On the other hand the same event can cause the limitation of bioavailable mercury forms by a conversion of labile Hg into the most stable one (HgSO4 ➔ HgS) under anaerobic conditions.

Silica nanoparticles alleviate mercury toxicity via immobilization and inactivation of Hg(ii) in soybean (Glycine max)

Nano-SiO2 alleviates Hg phytotoxicity through reducing Hg bioavailability and inhibiting uptake, translocation and accumulation of Hg in plant tissues.

Use of hardwood and sulfurized-hardwood biochars as amendments to floodplain soil from South River, VA, USA: Impacts of drying-rewetting on Hg removal

Periodic flooding and drying conditions in floodplains affect the mobility and bioavailability of Hg in aquatic sediments and surrounding soils. Sulfurized materials have been recently proposed as Hg sorbents due to their high affinity to bind Hg, while sulfurizing organic matter may enhance methylmercury (MeHg) production, offsetting the beneficial aspects of these materials. This study evaluated hardwood biochar (OAK) and sulfurized-hardwood biochar (MOAK) as soil amendments for controlling Hg release in a contaminated floodplain soil under conditions representative of periodic flooding and drying in microcosm experiments in three stages: (1) wet biochar amended-systems with river water in an anoxic environment up to 200 d; (2) dry selected reaction vessels in an oxic environment for 90 d; (3) rewet such vessels with river water in an anoxic environment for 90 d. In Stage 1, greater Hg removal (17–98% for unfiltered total Hg (THg) and 47–99% for 0.45-μm THg) and lower MeHg concentrations (<20 ng L−1) were observed in MOAK-amended systems (10%MOAKs). In Stage 3, release of Hg in 10%MOAKs was eight-fold lower than in soil controls (SedCTRs), while increases in aqueous (up to 21 ng L−1) and solid (up to 88 ng g−1) MeHg concentrations were observed. The increases in MeHg corresponded to elevated aqueous concentrations of Mn, Fe, SO42−, and HS− in Stage 3. Results of S K-edge X-ray absorption near edge structure (XANES) analysis suggest oxidation of S in Stage 2 and formation of polysulfur in Stage 3. Results of pyrosequencing analysis indicate sulfate-reducing bacteria (SRB) became abundant in Stage 3 in 10%MOAKs. The shifts in biogeochemical conditions in 10%MOAKs in Stage 3 may increase the bioavailability of Hg to methylating bacteria. The results suggest limited impacts on Hg removal during drying and rewetting, while changes in biogeochemical conditions may affect MeHg production in sulfurized biochar-amended systems.

The assessment and remediation of mercury contaminated sites: A review of current approaches

Remediation of mercury (Hg) contaminated sites has long relied on traditional approaches, such as removal and containment/capping. Here we review contemporary practices in the assessment and remediation of industrial-scale Hg contaminated sites and discuss recent advances. Significant improvements have been made in site assessment, including the use of XRF to rapidly identify the spatial extent of contamination, Hg stable isotope fractionation to identify sources and transformation processes, and solid-phase characterization (XAFS) to evaluate Hg forms. The understanding of Hg bioavailability for methylation has been improved by methods such as sequential chemical extractions and porewater measurements, including the use of diffuse gradient in thin-film (DGT) samplers. These approaches have shown varying success in identifying bioavailable Hg fractions and further study and field applications are needed. The downstream accumulation of methylmercury (MeHg) in biota is a concern at many contaminated sites. Identifying the variables limiting/controlling MeHg production—such as bioavailable inorganic Hg, organic carbon, and/or terminal electron acceptors (e.g. sulfate, iron) is critical. Mercury can be released from contaminated sites to the air and water, both of which are influenced by meteorological and hydrological conditions. Mercury mobilized from contaminated sites is predominantly bound to particles, highly correlated with total sediment solids (TSS), and elevated during stormflow. Remediation techniques to address Hg contamination can include the removal or containment of Hg contaminated materials, the application of amendments to reduce mobility and bioavailability, landscape/waterbody manipulations to reduce MeHg production, and food web manipulations through stocking or extirpation to reduce MeHg accumulated in desired species. These approaches often rely on knowledge of the Hg forms/speciation at the site, and utilize physical, chemical, thermal and biological methods to achieve remediation goals. Overall, the complexity of Hg cycling allows many different opportunities to reduce/mitigate impacts, which creates flexibility in determining suitable and logistically feasible remedies.

Mercury concentrations in fish and invertebrates of the Finger Lakes in central New York, USA.

Deleterious health effects in humans and wildlife are associated with the consumption of fish contaminated by mercury (Hg). This study was conducted to assess Hg concentrations in biota of the Finger Lakes (New York, USA), a region where fisheries are important for the economy but where no assessment of the drivers of food web Hg dynamics exists to date. Additionally, this region is of interest for the study of Hg bioaccumulation because of the importance of agricultural land cover, which can affect lake trophic status and thus the bioavailability of methyl Hg (MeHg). The study objectives were to (1) assess if fish Hg concentrations were of concern to humans and wildlife, (2) determine if differences in biota Hg concentrations exist among lakes, and (3) assess models developed for New York State as predictors of present day Finger Lakes fish Hg concentrations. Exploratory analyses were also conducted to assess predictors of fish Hg concentrations using lower trophic level MeHg concentrations, water quality, and lake and land cover characteristics. Fish concentrations were above the EPA criterion (300 ng/g wet weight, ww) in 24% of fish, but only Walleye (Sander vitreus) from Owasco Lake exceeded New York State Department of Health consumption guidelines (1000 ng/g ww). The threshold indicating biological changes within fish (500 ng/g ww) was exceeded in 11% of the individuals sampled. Significant differences were found among lakes for all fish species except Largemouth Bass (Micropterus salmoides). Notably, Lake Trout (Salvelinus namaycush) had significantly lower Hg concentrations in Cayuga Lake compared to other Finger Lakes. This trend was not mirrored in the lower food web, as benthic invertebrates had higher MeHg concentrations in Cayuga Lake. Using models developed for New York State in 2003-2005, observed concentrations in 90% of fish were different (±200 ng/g ww) than expected. Findings from this study suggest Hg dynamics in the Finger Lakes require consideration of fish age, growth rates, and food web structure to accurately predict fish Hg concentrations among lakes.

Amanita muscaria: bio-concentration and bio-indicative potential for metallic elements

We examined the bio-concentration of and the bio-indicative potential for 16 metallic elements (Ag, Al, Ba, Ca, Cd, Co, Cu, Fe, Hg, K, Mg, Mn, Na, Rb, Sr and Zn) by Amanita muscaria (L.) Lam. using validated methods including inductively coupled plasma optical emission spectroscopy (ICP-OES) with an ultrasonic cross flow nebulizer and cold-vapor atomic absorption spectroscopy (CV-AAS). Mushrooms and top soil samples were collected from the large forest complexes of the Gostyninsko-Wloclawskie Forests, the Warcianska-Notecka Forests and the Lubraniec site in the central region, as well as from the Poniatowa site in the Chodel river valley in the south-eastern region of Poland. For topsoil both the leachable and the “pseudo-total” fractions of minerals have been determined. A. muscaria is a good bio-concentrator of toxic Hg, and highly effective for labile forms of Cd, K and Rb, while it bio-excludes Ag, Al, Ba, Fe, Mn and Sr. The bioavailability of several elements, e.g. Ag, Ba, Cd, Hg and Sr, as demonstrated by the bioconcentration factor (BCF) values (the quotient of the element content in mushrooms to the content in soil, calculated on dry weight basis), and their content in fruiting bodies differed between the sites; however, this was not the case for the soils. This forest fungus is omitted by mushroomers, while due to a seasonal abundance, large size of fruiting bodies and high bioconcentration potential of Cd (medians of BCF for the leachable fraction up to > 1000 for caps and > 500 for stipes) and Hg (up to 27 for caps and to 21 for stipes) play a role in transfer of those toxic elements to forest biota feeding on it. Particularly important can be Cd, the median concentrations of which in caps were in the range 13–22 mg kg−1 dry matter for the sites, while medians for Hg were up to 0.58 mg kg−1 dm, and for Ag up to 0.87 mg kg−1 dm. As a source of protein for animal life, it extracts from soil and transfers to fruiting bodies further to consumers also essential minerals, which occurred (median values for caps per site in mg kg−1 dm) in the range: K (35,000–42,000), Mg (810–1000), Ca (76–100), Co (0.24–0.39), Cu (26–58), Fe (82–240), Mn (12–43), Na (37–110) and Zn (120–220). Elements without a specific biological role were: Al (130–620 mg kg−1 dm), Rb (260–910 mg kg−1 dm), Ba (0.52–2.9 mg kg−1 dm) and Sr (0.27–0.64 mg kg−1 dm). The caps usually showed the elements in greater concentration than stipes, and exception was Na that at average was threefold greater in stipes, and for Al, Ba, Ca, Co and Sr, distribution between both morphological structures was near unity.

Relationships between mercury concentrations in fur and stomach contents of river otter (Lontra canadensis) and mink (Neovison vison) in Northern Alberta Canada and their applications as proxies for environmental factors determining mercury bioavailability

The fur of piscivorous animals such as river otter (Lontra canadensis) and mink (Neovison vison) has been proposed to be used as a biomarker medium to assess mercury (Hg) exposure, but the relationship with dietary and environmental Hg exposure has not been fully characterized. The objective of this study was to investigate the relationship between fur total mercury (THg) and stomach content THg in river otter and mink, and their relationships with environmental factors. THg concentrations were measured in fur and stomach contents of river otter (n = 35) and mink (n = 30) collected from northern Alberta, Canada between 2014 and 2017. The fur THg concentration (mean ± standard deviation) was 6.36 ± 4.12 μg/g fur weight and 5.25 ± 3.50 μg/g fur weight and the average stomach content THg was 0.95 ± 0.56 μg/g dry weight and 0.71 ± 0.54 μg/g dry weight in river otter and mink respectively. There was a positive relationship between the log fur THg and log stomach contents THg for both species (p < 0.05). There was a positive relationship between the log THg of stomach contents and the percent of deciduous forest and a negative relationship with soil pH. There was a positive relationship between the log THg of fur and the total area burned by forest fire and a negative relationship with the percentage of wetlands. These results provide field evidence that fur can be used to reflect dietary Hg exposure and to identify sources and environmental factors that affect the bioavailable Hg in the habitats of these wildlife species.

Characteristics, speciation, and bioavailability of mercury and methylmercury impacted by an abandoned coal gangue in southwestern China.

During coal mining activities, a lot of coal gangue is produced, which usually contains high mercury (Hg) concentrations as well as the acid mine drainage (AMD) generator of pyrite. In the present study, the total mercury (THg) and methylmercury (MeHg) in gangue, water, sediment, paddy soil, and rice samples, collected from abandoned coal mining areas, were analyzed. Results showed that the THg concentrations ranged from 0.37 to 35mg/kg (11 ± 8.4mg/kg) and 0.15 to 19mg/kg (2.0 ± 3.9mg/kg) in gangue and sediments, respectively. For paddy soils, the THg concentrations and MeHg varied from 0.16 to 0.91mg/kg and 0.71 to 11ng/g, respectively. Rice samples exhibited wide concentration ranges of THg (3.0-22ng/g) and MeHg (0.71-8.9ng/g). Sequential extraction of Hg revealed that the nitric acid-extractable state Hg (F4) was the dominant Hg species in gangue and sediment, while humic acids state Hg (F3) was the dominant form in paddy soil. Compared with gangue, higher percentages of F3 and the residual state Hg (F5) in both sediment and soil samples implied the transformation of F4 to F3 and F5 during transportation. Soil n-HAs (the difference between the total organic carbon and humic acids) were positively correlated with both THg and MeHg in soil and rice, indicating that n-HAs enhance Hg bioavailability under acidic conditions. Further studies should be conducted to reveal the factors influencing the transformation of different Hg fractions, providing ideas on decreasing the bioavailability of Hg in coal mining areas.

Comparative study of the effects of different chelating ligands on the absorption and transport of mercury in maize (Zea mays L.)

Mercury (Hg) pollution seriously threatens food safety and has attracted global attention. Phytoextraction, due to its low cost, applicability, and environmental friendliness, is considered a new technology for clean-up of heavy metal contamination in the environment. However, the low bioavailability of Hg in polluted areas greatly limits the applicability of phytoextraction. Here, we compared the effects of six common chelating ligands on the absorption and transport of Hg in maize (Zea mays L.), which has a high biomass and short growth cycle. The results showed that the root length and biomass of maize seedlings of the groups treated with the six chelating ligands (EDTA, iodide, ammonium, thiosulfate, thiocyanate, and thiocarbamide) did not change compared with those of the non-treated groups. Co-exposure to Hg and each chelating ligand markedly alleviated the inhibitory effect induced by Hg. Iodide treatment resulted in the lowest root Hg content and highest translocation factor (TF) value, while ammonium treatment gave rise to the highest shoot Hg concentration and lowest TF. Compared with other chelating ligands, thiosulfate exhibited the maximum alleviation of Hg toxicity and achieved the highest concentration of Hg in the roots and aerial parts. Moreover, the TF and Hg accumulation in the thiosulfate and Hg co-exposed group were much higher than those in the group exposed to Hg alone. This finding suggests that, among these common chelating ligands, thiosulfate compounds have great potential for Hg phytoextraction, while the others can immobilize Hg in polluted areas.

The effects of selenium biofortification on mercury bioavailability and toxicity in the lettuce-slug food chain

The effects of foliar Se biofortification (Se+) of the lettuce on the transfer and toxicity of Hg from soil contaminated with HgCl2 (H) and soil collected near the former Hg smelter in Idrija (I), to terrestrial food chain are explored, with Spanish slug as a primary consumer. Foliar application of Se significantly increased Se content in the lettuce, with no detected toxic effects. Mercury exerted toxic effects on plants, decreasing plant biomass, photochemical efficiency of the photosystem II (Fv/Fm) and the total chlorophyll content. Selenium biofortification (Se+ test group) had no effect on Hg bioaccumulation in plants. In slugs, different responses were observed in H and I groups; the I/Se+ subgroup was the most strongly affected by Hg toxicity, exhibiting lower biomass, feeding and growth rate and a higher hepatopancreas/ muscle Hg translocation, pointing to a higher Hg mobility in comparison to H group. Selenium increased Hg bioavailability for slugs, but with opposite physiological responses: alleviating stress in H/Se+ and inducing it in I/Se+ group, indicating different mechanisms of Hg-Se interactions in the food chain under HgCl2 and Idrija soil exposures that can be mainly attributed to different Hg speciation and ligand environment in the soil.

The distributions, contamination status, and health risk assessments of mercury and arsenic in the soils from the Yellow River Delta of China.

The surface soils were collected from four areas in the Yellow River Delta (YRD), including three functional areas in the natural reserve of the YRD (the core area, buffer area, and experimental area) and a neighboring area of the natural reserve. The total concentrations, speciations, contamination status, and health risk assessments of the mercury (Hg) and arsenic (As) in surface soils of the YRD were investigated. The average Hg concentration was about three times that of the background value, while As was just slightly higher than the background. Hg levels in the sites of experimental area were significantly higher than those in the core area and buffer area, which was consistent with the human activity intensities of the three functional areas. However, no significant differences of As levels were observed across different areas. According to the sequential extraction experiments, only less than 5% of Hg and As were associated with the exchangeable fraction, while over 80% of them were found in the residual fraction, indicating low mobility and bioavailability of both Hg and As. The soil contamination status assessments suggested a "good state," and the health risk assessments indicated a "low risk" of Hg and As in the soils of YRD.

Influence of food (ciliate and phytoplankton) on the trophic transfer of inorganic and methyl-mercury in the Pacific cupped oyster Crassostrea gigas

Diet is an important route of mercury (Hg) uptake in marine organisms. Trophic transfer of Hg throughout the food webs may be influenced by various factors, including diet and Hg speciation. Bivalves such as oysters are widely used as bioindicators of trace element pollution such as Hg. Nevertheless, our current knowledge regarding their ability to accumulate Hg from their diet is mainly based on experiments performed using phytoplankton. In their natural environment, oysters feed on a variety of food items including ciliates, detritus, in addition to phytoplankton. The present study aimed at examining the influence of diet composition on the trophic transfer of inorganic Hg (iHg) and methylmercury (MeHg) in the Pacific cupped oyster Crassostrea gigas. The pulse-chase feeding method was used with two radiolabeled food items: a heterotrophic protist (Uronema marinum) and a phytoplanktonic diatom (Thalassiosira pseudonana). Depuration of dietary Hg in the oysters was followed for 50 d. Kinetic parameters including assimilation efficiency (AE) and efflux rate constant (ke) were calculated. Our results showed that oysters fed on ciliates assimilated 96 ± 1% and 31 ± 2% of the ingested MeHg and iHg, respectively whereas these elements were similarly assimilated in the oysters fed on phytoplankton (78 ± 3% and 86 ± 4% for MeHg and iHg, respectively). Mercury assimilation in oyster is thus diet dependent (significant differences in AE, p < 0.05), metal species-dependent and likely resulting from variations in Hg bioavailability in the two food items tested and a gut passage time-dependent of the ingested matrix.

Operationally defined mercury (Hg) species can delineate Hg bioaccumulation in mangrove sediment systems: A case study

This study investigated the linkage between mercury (Hg) speciation in the surficial sediments from a mangrove ecosystem of the Zuari Estuary, west coast of India, with Hg bioaccumulation in gastropods collected from the same area. Multiple operationally defined protocols and methods were used for determination of Hg speciation study in the mangrove sediments. Moderately low concentrations of Hg were observed in the sediments, ranging from 37.3 ± 1.9 to 79.6 ± 4.0 µg/kg. Geochemical fractionation showed that a significant part of sedimentary Hg was present within the structure of the sediment (residual fraction) and not bioavailable. Non-residual Hg was primarily associated with oxidizable (sedimentary organic matter (SOM) or sulfide) binding phase of the sediments, and ranged from 9.2 ± 0.3 to 78.5 ± 3.9 µg/kg. Concentration of methylmercury (MeHg) (a neurotoxin) in the sediments varied from 1.7 ± 0.1 to 4.4 ± 0.1 µg/kg. l-Cysteine, a suitable complexing ligand, extractable Hg concentration in the sediments ranged from 4.3 ± 0.1 to 15.9 ± 0.3 µg/kg. Statistical analysis suggested that MeHg was adsorbed on Fe/Mn oxyhydroxide phases in the sediments. l-Cysteine was found to extract sedimentary MeHg and thermodynamically less stable Hg-SOM complexes from the sediments. Concentrations of bioaccumulated Hg in soft tissues of the gastropod, Pirenella cingulata, ranged from 57.6 ± 4.4 to 224.4 ± 7.2 µg/kg. Positive correlations existed between the concentration of bioaccumulated Hg in the gastropods and the concentrations of Hg associated with the oxidizable phase, sedimentary MeHg and l-Cysteine extracted Hg in the sediments. This study indicated that operationally defined Hg species can be useful in estimating bioavailable Hg to obligatory deposit feeder in tropical mangrove systems.