Hg Bioaccumulation Research Articles

Mercury and its form in a dammed reservoir ecosystem during the charging phase.

Throughout continents, reservoirs tend to have elevated methylmercury (MeHg) concentration transformed from mercury (Hg/total Hg). This impact may be pronounced in the reservoir with less velocity of water during the charging period resulted in the deposition of sediments. In sediments on favorable conditions, methylation may be enhanced by the decomposition of flood organic material, which can release Hg and enhance microbial activity. However, much less is known about the transfer ratio of Hg and its form MeHg from sediment to biota in the hydrological reservoir during the dam charging phase. The objective of our study was to understand the interrelationship between total Hg and MeHg in two key components sediment and fish in the reservoir ecosystem. This study was performed at the Three Gorges Reservoir (TGR) located on upstream of the Yangtze River in China. At the TGR charging phase, during winter time, the water level was high due to blockade of water by Three Gorges Dam (TGD). Sediment and fish samples were collected in winter season for total Hg, MeHg, and several ancillary parameters. The results showed that total Hg in sediment samples of the winter season were ranged from 6.2 ± 0.001 to 193.3 ± 0.001 × 10-3mg/kg, with an average value of 53.76 ± 51.80 × 10-3mg/kg, and for MeHg was ranged from 12.1 ± 0.04 to 348.7 ± 0.16 × 10-2ng/g, with an average value of 98.96 ± 93.07 × 10-2ng/g. Total Hg and MeHg in fish samples of the winter season were from 42.48 ± 6.71 to 166 ± 52.56ng/g, with an average value of 76.22 ± 31.23ng/g, and from 21.09 ± 2.31 to 61.60 ± 13.30ng/g, with an average value of 37.89 ± 11.96ng/g. The relationship of total Hg and MeHg concentrations in fish to those of sediments from corresponding sites showed a negative relationship. This might include a strong association of total Hg with an inorganic component of sediment (e.g., bound to sulfides or coprecipitated with other metal oxides such as manganese and iron). The average concentration of fish MeHg found in this study, at rates greater than 1.72g/day, was estimated hazardous to human health. This study concludes sediment was acting as sequestrate for total Hg and MeHg in TGR. The bioaccumulation of total Hg and MeHg in fish was not controlled by sediment further investigation about pathological routes and dietary habits of fish needed to be identified for total Hg and MeHg study in TGR.

Influence of Thermal Pollution on the Physiological Conditions and Bioaccumulation of Metals, Metalloids, and Trace Metals in Whitefish (Coregonus lavaretus L.).

The Kola nuclear power plant, which discharges warm water into one of the bays of subarctic Lake Imandra, significantly changes fish habitats. The temperature gradient of the lake is between 2 and 8 °C, which makes it significantly different from the natural temperature of the lake water. The stenothermal cold-water native species (lake whitefish (Coregonus lavaretus L.)), living for more than 40 years under conditions of thermal pollution, has adapted to this stressor. Moreover, this population differs favorably from the population in the natural-temperature environment in terms of its physiological state. Firstly, the hemoglobin concentrations in the fish blood are in the range of the ecological optimum, and secondly, it has a higher somatic growth, as estimated by Fulton’s condition factor. One of its main adaptive mechanisms of ion regulation is an intense metabolism of Na due to the high respiratory activity of the whitefish in warmer water. An increased accumulation of Rb and excretion of Se, Mo, and Si are associated more or less with that feature. Under conditions of an increased water temperature, the main metabolic need is due to a deficiency of Se in fish. The intensive metabolism of selenoproteins may involve risks of toxic effects and the bioaccumulation of Hg, As, and Cu in cases of increased existing stressors or the appearance of new ones.

Warmer temperature increases mercury toxicity in a marine copepod.

Marine biota have been co-challenged with ocean warming and mercury (Hg) pollution over many generations because of human activities; however, the molecular mechanisms to explain their combined effects are not well understood. In this study, a marine planktonic copepod Pseudodiaptomus annandalei was acutely exposed to different temperature (22 and 25°C) and Hg (0 and 118μg/L) treatments in a 24-h cross-factored experiment. Hg accumulation and its subcellular fractions were determined in the copepods after exposure. The expression of the genes of superoxide dismutase (SOD), glutathione peroxidase (GPx), metallothionein1 (mt1), heat shock protein 70 (hsp70), hsp90, hexokinase (hk), and pyruvate kinase (pk) was also analyzed. Both the Hg treatment alone and the combined exposure of warmer temperature plus Hg pollution remarkably facilitated Hg bioaccumulation in the exposed copepods. Compared with the Hg treatment alone, the combined exposure increased total Hg accumulation and also the amount of Hg stored in the metal-sensitive fractions (MSF), suggesting elevated Hg toxicity in P. annandalei under a warmer environment, given that the MSF is directly related to metal toxicity. The warmer temperature significantly up-regulated the mRNA levels of mt1, hsp70, hsp90, and hk, indicating the copepods suffered from thermal stress. With exposure to Hg, the mRNA level of SOD increased strikingly but the transcript levels of hsp90, hk, and pk decreased significantly, indicating that Hg induced toxic events (e.g., oxidative damage and energy depletion). Particularly, in contrast to the Hg treatment alone, the combined exposure significantly down-regulated the mRNA levels of SOD and GPx but up-regulated the mRNA levels of mt1, hsp70, hsp90, hk, and pk. Collectively, the results of this study indicate that ocean warming will potentially boost Hg toxicity in the marine copepod P. annandalei, which is information that will increase the accuracy of the projections of marine ecosystem responses to the joint effects of climate change stressors and metal pollution on the future ocean.

Inorganic mercury and dietary safe feed additives enriched diet impacts on growth, immunity, tissue bioaccumulation, and disease resistance in Nile tilapia (Oreochromis niloticus)

Little is known about the impacts of dietary exposure to inorganic mercury (Hg) for a long duration on the health indicators, growth, and disease resistance in Oreochromis niloticus. Accordingly, the current study was designed to assess the effects of Hg contaminated diets on blood biochemistry, growth, chemical composition, Hg bioaccumulation in the tissues, histopathology of liver and head kidneys, and disease resistance to Aeromonas hydrophila of O. niloticus. Also, the efficiency of citronella oil, geranium oil (GO), curcumin (CUR), Bacillus toyonensis (BT), and Bacillus subtilis (BS) as dietary supplements on reversing the negative impacts of Hg were assessed. A total of 240 tilapia fingerlings were assigned to eight dietary treatments fed on the basal diet (G1), G1 diet contaminated with 50 ppm Hg (G2), whereas the other groups fed the G2 diet and enriched with 400 mg CO (G3), 400 mg GO (G4), 200 mg CUR (G5), 7 × 107 cells BT (G6), 7 × 107 cells BS (G7), and 7 × 107 BT + BS/ kg diet (G8) for 16 weeks. The obtained results showed that fish fed on the G2 diet had significantly impaired growth performance indicators, blood parameters, and resistance to bacterial infection compared with fish in the control group. Additionally, distinct pathological perturbations in liver and head kidneys were observed. In contrast, fish groups G3 to G8 had a significant enhancement in the growth performance, Hg bioaccumulation in fish tissues, blood biochemistry, and resistance against A. hydrophila infection compared with fish in the G2 group. Maximum improvement was recorded in G5, G6, and G8. Conclusively, from both health and an economic point of view, these results suggested that several benefits might be gained by adding these additives, especially CUR, BT, and BT + BS, on growth enhancement and ameliorating Hg negative impacts in O. niloticus.

Mercury distribution and transfer in sediment-mangrove system in urban mangroves of fast-developing coastal region, Southern China

In urban mangroves affected by surrounding urbanization and anthropogenic activities, the fate of mercury (Hg) in sediment-mangrove system remains unclear. In this study, four urban mangroves with different urban functional zonings in fast-developing Shenzhen of China were selected: Shajing mangrove (SJM) and Xixiang mangrove (XXM) featured with industry district, Futian mangrove (FTM) and Baguang mangrove (BGM) featured with central business district and ecological preserve, respectively. The distributions of Hg in sediment, iron plaque (IP), and Kandelia obovata were determined. The mean concentrations of total Hg in mangrove sediments were SJM > XXM, FTM > BGM. Higher ecological risk was also detected in SJM based on geo-accumulation index, potential ecological risk factor, and contamination factor. The speciation analysis of Hg in non-rhizosphere sediments showed that residual, oxidizable, and volatile Hg acted as dominant fractions in urban mangroves, with volatile Hg to be important for Hg bioaccumulation in K. obovata. IP acted as a natural barrier to limit transfer of Hg from sediment to K. obovate due to lower Hg concentrations in IP compared to that in sediment. In terms of the total and bioavailable Hg, translocation factors (TF) of Hg in mangroves featured with different urban functions were: TFtotal: BGM (0.23) ≈ FTM (0.23) > SJM (0.15) ≈ XXM (0.14); TFavail: BGM (0.81) ≈ FTM (0.74) > SJM (0.40) ≈ XXM (0.37). The reduced TFs of Hg were important for protecting K. obovata from heavy Hg pollution in urban mangroves featured with industry district.

Linking cadmium and mercury accumulation to nutritional intake in common dolphins (Delphinus delphis) from Patagonia, Argentina

Bioaccumulation of Hg and Cd from food is a complex ecological process that has been oversimplified in the past. Common dolphins (Delphinus delphis) provide a powerful model to biomonitor metal concentrations in marine environments worldwide. We combined proportions-based nutritional geometry with metal analysis, stomach content analysis and the proximate composition of prey, to yield novel insights into the accumulation of Hg and Cd. Our analysis showed an age-related accumulation trend for Cd and Hg in kidney and liver, with highest concentrations found at 18 years of age. When viewed through the lens of nutritional ecology, Argentine anchovy (58.1 Mass %) and South American long-finned squid (22.7 Mass %), provided most of the dietary intake of protein (P) and lipids (L) (P:L ratio = 2.6:1.0) and also represented the main source for Cd and Hg levels accumulated in their bodies. This study presents unprecedented evidence on metal accumulation in relation to age and nutritional intake in a marine predator.

Hermetia illucens exhibits bioaccumulative potential for 15 different elements – Implications for feed and food production

A new branch of the insect-based food and feed industry is intensively developing all over the world, the best proof of which is the recent change in legislation at the European level allowing the use of insect protein in the production of feed for aquaculture animals. Previous publications have proven that some heavy metals can be bioaccumulated in fly H. illucens (black soldier fly), even when the insect is raised on optimal feed with an acceptable heavy metal content. The purpose of this study was to determine the bioaccumulation potential of H. illucens in relation to micro- and macroelements, toxic elements and for the first time, selected non-essential elements from optimal feed. Our results showed that bioaccumulation of Ba, Bi, Cu, Fe, Hg, Mg, Mo, Se and Zn occurred in all stages of insect development and in puparia, while bioaccumulation of Al, As, Co, K, Pb and Si was not found. The highest bioaccumulation factors were obtained for Ca and Mn in puparia - 38 and 21 respectively. In addition, Ca, Cd, Ga, Mn, P and S were bioaccumulated only in some developmental stages of the insect. The results are discussed in the context of the safety of feed and food production from H. illucens.

Mercury in Juvenile Solea senegalensis: Linking Bioaccumulation, Seafood Safety, and Neuro-Oxidative Responses under Climate Change-Related Stressors

Mercury (Hg) is globally recognized as a persistent chemical contaminant that accumulates in marine biota, thus constituting an ecological hazard, as well as a health risk to seafood consumers. Climate change-related stressors may influence the bioaccumulation, detoxification, and toxicity of chemical contaminants, such as Hg. Yet, the potential interactions between environmental stressors and contaminants, as well as their impacts on marine organisms and seafood safety, are still unclear. Hence, the aim of this work was to assess the bioaccumulation of Hg and neuro-oxidative responses on the commercial flat fish species Solea senegalensis (muscle, liver, and brain) co-exposed to dietary Hg in its most toxic form (i.e., MeHg), seawater warming (ΔT°C = +4 °C), and acidification (pCO2 = +1000 µatm, equivalent to ΔpH = −0.4 units). In general, fish liver exhibited the highest Hg concentration, followed by brain and muscle. Warming enhanced Hg bioaccumulation, whereas acidification decreased this element’s levels. Neuro-oxidative responses to stressors were affected by both climate change-related stressors and Hg dietary exposure. Hazard quotient (HQ) estimations evidenced that human exposure to Hg through the consumption of fish species may be aggravated in tomorrow’s ocean, thus raising concerns from the seafood safety perspective.

Testing the Use of Standardized Laboratory Tests to Infer Hg Bioaccumulation in Indigenous Benthic Organisms of Lake Maggiore (NW Italy)

The chronic toxicity of mercury essentially derives from its strong tendency to biomagnify along food webs. For this reason, the European regulatory framework establishes an environmental quality standard for Hg based on the total Hg concentration in prey fish to protect top predators. A considerable part of the Hg burden of prey fish can come from the ingestion of benthic organisms that, in the presence of contaminated sediments, may remobilize substantial amounts of Hg towards the pelagic food webs. The present study evaluated whether Hg accumulation in assemblages of indigenous chironomids and oligochaetes could be predicted using standardized laboratory bioaccumulation tests with Chironomus riparius and Lumbriculus variegatus. Indigenous chironomids and oligochaetes were recovered at different sites in a lake suffering from legacy Hg pollution and analyzed for total Hg content. Sediment aliquots from the same sites were used to assess Hg bioaccumulation using laboratory-reared C. riparius and L. variegatus. Mercury concentrations in indigenous versus laboratory organisms showed a good correlation (p < 0.05; Spearman correlation test) only in the case of C. riparius versus indigenous chironomids, suggesting the possibility of using linear regressions to predict Hg accumulation by these benthic invertebrates. Further research needs and caveats as to the applicability of the present results to other aquatic systems are identified and discussed.

Mercury Isotope Fractionation during the Photochemical Reduction of Hg(II) Coordinated with Organic Ligands.

The photochemical reduction of Hg(II) is an important pathway in the environmental Hg cycle because it competes with Hg methylation and potentially limits the formation of bioaccumulative methylmercury. Hg stable isotope systematics have proven to be an effective tool for investigating the transport, transformation, and bioaccumulation of Hg. The dominant cause of mass independent isotope fractionation (MIF) of Hg in nature is the photochemical reduction of various species of Hg(II). However, it is difficult to fully interpret Hg stable isotope signatures due to the lack of mechanistic information about which Hg compounds are susceptible to MIF and why. This study investigates Hg isotope fractionation during the photochemical reduction of Hg(II) complexed to organic ligands, which are representative of the available binding sites in natural dissolved organic matter. The photochemical reduction of Hg(II) in the presence of cysteine resulted in both negative and positive MIF in residual Hg(II), where the sign depended on pH and dissolved oxygen level. In the presence of serine, either nuclear volume or magnetic isotope effects were observed depending on the wavelength of light and the extent of Hg(II) complexation by serine. In the presence of ethylenediamine, MIF was negative. Our Hg stable isotope results suggest that MDF and MIF are induced at different steps in the overall photochemical reduction reaction and that MIF does not depend on the rate-determining step but instead depends on photophysical aspects of the reaction such as intersystem crossing and hyperfine coupling. The behavior of Hg isotopes reported here will allow for a better understanding of the underlying reaction mechanisms controlling the Hg isotope signatures recorded in natural samples.

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.

Influence of Macrophyte and Gut Microbiota on Mercury Contamination in Fish: A Microcosms Study

The freshwater lakes of southwestern France are subject to the development of invasive macrophytes which are associated with mercury (Hg) contamination of the food web. The aim of this study was to determine the bioavailability of methylmercury (MeHg) produced by plant roots in aquatic ecosystems. A microcosm experiment was performed using isotopically enriched inorganic Hg at environmental concentrations (1 µg 199IHg·L−1). For all conditions, total Hg in fish as well as Hg species associated with different compartments (water, sediments, plant roots, fish) were analyzed by gas chromatography-inductively coupled plasma-mass spectrometry (GC-ICP-MS). In addition, sediment, plants, and fish gut microbiota were studied by MiSEQ sequencing. Some strains were isolated and tested for their ability to methylate Hg. The results revealed 199MeHg production in plant roots and the presence of this form in fish (tissues and gut), highlighting a MeHg trophic transfer. Moreover, methylator bacteria were identified from the gut contents of the fish when they were in the presence of plants. Some of them were related to bacteria found in the plant roots. On the basis of these results, the transfer of MeHg and bacteria from plants to fish is highlighted; in addition, Hg methylation is strongly suspected in the fish gut, potentially increasing the Hg bioaccumulation.

Patterns of mercury exposure and relationships with isotopes and markers of oxidative status in chicks of a Mediterranean seabird

The Mediterranean basin is a hotspot of mercury (Hg) contamination owing to intense anthropogenic emissions, volcanic activity and oligotrophic conditions. Little work has been done to assess the sources of Hg exposure for seabirds and, particularly, the physiological consequences of Hg bioaccumulation. In this study, we (i) describe the individual and temporal variation in blood concentration of total Hg (THg) over three breeding seasons, (ii) identify the factors that affect the THg exposure and (iii) determine the individual- and population-level connections between THg and blood-based markers of oxidative status in chicks of Scopoli’s shearwaters (Calonectris diomedea) breeding on the island of Linosa in the southern Mediterranean. We carried out the work on chicks near fledging because they are fed with prey captured near the colony, thus their Hg levels reflect local contamination. The concentration of THg in erythrocytes varied from 0.23 to 4.29 μg g−1 dw. Chicks that were fed upon higher trophic level prey (i.e., higher δ15N values) had higher THg levels. Individual variation in THg concentrations was not explained by parental identity, sex nor δ13C values. There was significant variation in THg among chicks born from the same mother in different years. We found significant correlations between THg and markers of oxidative status; however, these correlations were no longer significant when we took into account the annual variation in mean values of all metrics. Males with higher values of body condition index had higher blood THg, while THg and body condition index were not correlated in females. Our data indicate that THg levels were moderate to high if compared to other seabirds. However, there is little evidence for harmful short-term detrimental effects owing to THg exposure.

Oxidative stress ecology in brook trout (Salvelinus fontinalis) from a high-mountain lake (Cottian Alps).

High-mountain lakes are pristine ecosystems characterized by extreme environmental conditions. The atmospheric transport of pollutants from lowlands may add further stress to organisms inhabiting these environments. We investigated the environmental stress pressure on brook trout (Salvelinus fontinalis) from a high-mountain lake in the Cottian Alps (Piedmont, northwest Italy). To do this, males and females of brook trout were sampled from Balma Lake in summer (August) and autumn (October) 2017 in order to assess the influence of trace elements accumulation and environmental parameters (physicochemical parameters and nutrient characteristics of water) on oxidative stress biomarkers. Bioaccumulation of Al, As, Cd, Cr, Cu, Fe, Hg, Pb, Ni, Se, and Zn and metallothionein levels were measured in muscle tissue of males and females. Liver, gills, kidney, and spleen tissue samples were analyzed for superoxide dismutase, catalase, total glutathione peroxidase, selenium-dependent glutathione peroxidase, glutathione reductase, and glutathione S-transferase activity. Analysis of environmental parameters showed changes in biomarker levels with seasonal variations. Water temperature was significantly higher in summer than autumn (Wilcoxon test; p=.0078), while pH was significantly higher in autumn than in summer (Wilcoxon test; p=.0078). Sex-related differences in oxidative stress biomarkers in tissues were unremarkable, whereas seasonal variability of oxidative stress biomarkers was observed, with major differences occurred for liver in summer and for gills, kidney, spleen and muscle in autumn. Positive correlations between environmental parameters and biomarkers were noted. Major fluctuations in water temperature, pH, Cu, Pb and Hg produced changes in biomarker levels; however, increased food intake during the ice-free season was probably the main factor that influenced changes in oxidative stress biomarker levels in brook trout in this extreme ecosystem.

Mercury in fish marketed in the Amazon Triple Frontier and Health Risk Assessment

Fish has great socioeconomic, cultural, and nutritional importance for Amazonian populations. Despite all health benefits, fish can accumulate great amounts of mercury (Hg). The entry of Hg in aquatic trophic chains is an issue of concern to animal and human health. Higher risks of human exposure are strongly related to fish consumption. Upper Solimões population has one of the highest fish consumption rates of the Amazon. This study aimed to access the concentration of total Hg (THg) in muscle, liver, and gills of 17 species of fishes marketed in the Upper Solimões Region and Health Risk Assessment. Higher concentrations were observed in Carnivores/Piscivores. The highest THg concentration was found in liver of Cichla ocellaris (4.549 μg/g) and the lowest in gills of Hoplosternum littorale (0.002 μg/g). Most species had higher THg concentrations in muscle>liver>gills, in the Ebb period, and liver>muscle>gills, in the Flood period. Hoplias malabaricus, Pseudoplatystoma fasciatum, Plagioscion squamosissimus, Ageneiosus inermis, and C. ocellaris presented average THg concentrations above the safe limit stablished by WHO. THg levels in C. ocellaris, H. malabaricus, P. squamosissimus, P. fasciatum, and Semaprochilodus insignis were higher than those found in fish of heavily impacted areas. Signs of bioaccumulation and biomagnifications of Hg can already be observed in this region. The Western Amazon Region urgently needs government actions to inhibit Hg release in aquatic ecosystems and to advise this population on the safe amount of fish to be eaten according to species and period of the year.

Total mercury and methylmercury concentrations in native and invasive fish species in Shadegan International Wetland, Iran, and health risk assessment.

Human exposure to mercury (Hg) mainly occurs through consumption of aquatics, especially fish. In aquatic systems, the bioaccumulation of Hg across trophic levels could be altered by invasive species through changing community composition. The present study is aimed at measuring total mercury (T-Hg) and methylmercury (MeHg) concentrations in non-native (redbelly tilapia (Tilapia zillii)) and native (Benni (Mesopotamichthys sharpeyi) and common carp (Cyprinus carpio)) fish species throughout Shadegan International Wetland and comparing health risk of their mercury contents to the local population. The concentrations were measured using a direct mercury analyzer (DMA 80). The average values of T-Hg and MeHg for native fishes were 19.8 and 10.49μg/kg. These concentrations for the invasive fish were 28 and 14.62μg/kg respectively. Despite having less length and weight than the native fish species, tilapia showed significantly higher T-Hg content, yet the lowest concentration of MeHg was observed in common carp with larger body length and weight. Concerning mercury health risk to consumers, tilapia demonstrated the highest estimated weekly intake (EWI) and percentages of tolerable weekly intake (%TWI) for both T-Hg and MeHg, while the highest hazard quotient (HQ) values were obtained for tilapia and Benni. Taken together, the mercury concentrations in the two native and non-native fishes were acceptable according to the international safety guidelines although the local people shall be warned for consumption of tilapia. Furthermore, the low calculated value of tissue residue criterion (TRC) for the wetland fishes sounds a warning.

Food sources are more important than biomagnification on mercury bioaccumulation in marine fishes

Marine animals often accumulate various harmful substances through the foods they ingest. The bioaccumulation levels of these harmful substances are affected by the degrees of pollution in the food and of biomagnification; however, which of these sources is more important is not well-investigated for mercury (Hg) bioaccumulation. Here we addressed this issue in fishes that inhabit the waters around Minamata Bay, located off the west coast of Kyushu Island in Kumamoto Prefecture, Japan. The total Hg concentration (hereafter [THg]) and carbon and nitrogen stable isotope ratios (δ13C and δ15N) were analyzed in the muscle tissue of 10 fish species, of which more than five individuals were caught by gillnet. Except one species, each was separated into two trophic groups with respective lower and higher δ13C values ranging from −17‰ to −16‰ and −15‰ to −14‰, which suggested that the fishes depended more on either phytoplankton- and microphytobenthos-derived foods (i.e., pelagic and benthic trophic pathways), respectively. Linear mixed effects models showed that the Hg levels were significantly associated with both δ15N and the differences in the trophic groups. [THg] increased with δ15N (i.e., indicative of higher trophic levels), but the slopes did not differ between the two trophic groups. [THg] was significantly higher in the group with higher δ13C values than in those with lower δ13C values. The effect size from marginal R squared (R2) values showed that the variation in [THg] was strongly ascribed to the trophic group difference rather than δ15N. These results suggest that the substantial Hg bioaccumulation in the fishes of Minamata Bay is mainly an effect of ingesting the microphytobenthos-derived foods that contain Hg, and that the subsequent biomagnification is secondary.

Five decades of declining methylmercury concentrations in boreal foodwebs suggest pivotal role for sulphate deposition

Widespread declines in mercury (Hg) in fish in pristine lakes in Fennoscandia since the 1970s are unexplained. Interactions between climate, atmospheric deposition, and elemental cycling of carbon (C), sulphur (S) and Hg are complex and affect Hg bioaccumulation. A parallel significant decline in methyl-Hg (MeHg) concentrations in aquatic macroinvertebrates (Chironomidae) was found between 1976–78 and 2004–15 in an intensely studied, pristine boreal lake (Langtjern, boreal Fennoscandia). Monitoring at Langtjern demonstrated a four-fold decrease in aqueous sulphate concentrations (SO4, 50-year record), significant lake browning (30-year records), increasing sediment Hg concentrations (50-year record), warming (45-year record) and increased runoff (40-year record). Contrasting Hg trends in biota (downward) and sediment (upward) indicated a disconnect between lake Hg loading and foodweb Hg bioaccumulation. We suggest that reduced SO4-deposition has 1) constrained substrate availability for SO4-reducing methylating bacteria (causing reduced foodweb MeHg exposure despite increased Hg loading to the lake), and 2), increased the binding affinity between aqueous organic matter and Hg species (leading to reduced MeHg bioavailability). The downward MeHg trend at the base of the foodweb at Langtjern is mirrored at higher trophic levels by strong declines in perch (Perca fluviatilis) and pike (Esox lucius) Hg concentrations in boreal Fennoscandia. A plausible explanation is that declining SO4-deposition, rather than climate change or reduced atmospheric Hg, is currently driving reduced MeHg contamination in northern freshwater foodwebs.

Mercury bioaccumulation in zooplankton and its relationship with eutrophication in the waters in the karst region of Guizhou Province, Southwest China.

Zooplankton play an important role in the transfer of mercury (Hg) from the lower to upper trophic positions in the food chain. In this study, total mercury (THg) and methylmercury (MeHg) levels were measured in three size fractions of zooplankton collected from three reservoirs (Hongfeng, Baihua, and Aha Reservoir) and one wetland in karst areas to understand mercury accumulation in zooplankton from alkaline environments. The results showed that the alkaline waters had lower zooplankton MeHg levels (0.1 to 66.8ngg-1) than most of the acidic waters reported. However, the zooplankton THg levels (6.3 to 494.9ngg-1) were comparable. The macro-zooplankton (> 500μm) had significantly higher THg and MeHg levels than meso-zooplankton (116 to 500μm) in the three reservoirs at all seasons, which showed biomagnification of mercury in the food chain. The correlation between Hg in water and zooplankton and Hg in zooplankton of different sizes indicated that THg bioaccumulation in zooplankton was related to the THg levels in water; however, MeHg bioaccumulation in zooplankton was controlled by many other factors, such as their feeding and living habits. In the three reservoirs, the THg and MeHg concentrations in zooplankton decreased with increasing eutrophication. However, compared with the three reservoirs, Caohai Wetland, with large amounts of aquatic plants, had a much lower trophic level and higher MeHg content in water but much lower zooplankton MeHg levels and bioaccumulation factors (BAFs). The large amounts of plant residue might dilute mercury in the food chain, revealing that high primary production could result in lower Hg bioaccumulation, rather than only being influenced by nutrient levels.

Bioaccumulations and potential human health risks assessment of heavy metals in ppk-expressing transgenic rice

In order to reduce the phosphorus (P) resource consume, the polyphosphatekinase (ppk)-expressing transgenic rice (ETR) with high utilization efficiency of P fertilizer had been constructed. However, synthesis polyphosphates (polyP) mediated byppkin the plants have the ability of chelating heavy metals, so the potential hazards of the new elite rice variety have raised concerns. In the study, we planted ETR and wild-type Nipponbare (WT) in paddy fields in southern China. After harvest, the concentrations of eight heavy metals in rice tissues were measured, and health risks assessments were performed. The field experiment showed that the ppkexpressions were detected in the roots and straws of ETR plants but did not increase the concentrations of As, Cd, Cr, Ni and Pb in rice tissues. The Hg concentration in the ETRD root was 1.70-fold higher than that in WT, but the abundant Hg bioaccumulation in ETRD only occurred in the root. The bioaccumulation factors (BAFs) of all the detected heavy metals in the ETRS were no different from WT except for Cu and Zn. The results of human health risks assessment of heavy metals in brown rice showed that the non-carcinogenic risks of Cu or Zn in ETRD were higher than that in WT, while there was no difference in the total noncarcinogenic risk of the eight heavy metals in ETR. The carcinogenic risks of heavy metals in ETR were also comparable to that in WT. The results of this study indicated that the ppk expression in rice did not increase human health risks of heavy metals by consuming brown rice, which would provide a safety guarantee for agricultural and environmental applications of ETR not only with single-copy line but also with double-copy line.