MeHg Demethylation Research Articles

Mercury isotopes of key tissues document mercury metabolic processes in seabirds

Seabirds accumulate significant amounts of mercury (Hg) due to their long-life span together with their medium to high trophic position in marine food webs. Hg speciation and Hg isotopic analyses of total Hg in different tissues (pectoral muscles, liver, brain, kidneys, blood and feathers) were assessed to investigate their detoxification mechanisms. Three species with contrasted ecological characteristics were studied: the Antarctic prion (zooplankton feeder), the white-chinned petrel (pelagic generalist consumer) and the southern giant petrel (scavenger on seabirds and marine mammals). The difference of mass-dependent fractionation (MDF, δ202Hg) values between liver and muscles (up to 0.94 ‰) in all three seabirds strongly suggests hepatic demethylation of the isotopically lighter methylmercury (MeHg) and subsequent redistribution of the isotopically heavier fraction of MeHg towards the muscles. Similarly, higher δ202Hg values in feathers (up to 1.88 ‰) relative to muscles and higher proportion of MeHg in feathers (94–97%) than muscles (30–70%) likely indicate potential MeHg demethylation in muscle and preferential excretion of MeHg (isotopically heavier) in the growing feathers during moult. The extents of these key detoxification processes were strongly dependent on the species-specific detoxification strategies and levels of dietary MeHg exposure. We also found higher mass-independent fractionation (MIF, Δ199Hg) values in feathers relative to internal tissues, possibly due to different integration times of Hg exposure between permanently active organs and inert tissues as feathers. Hg isotope variations reported in this study show evidence of detoxification processes in seabirds and propose a powerful approach for deep investigation of the Hg metabolic processes in seabirds.

Mercury isotopes as tracers of ecology and metabolism in two sympatric shark species

In coastal ecosystems, top predators are exposed to a wide variety of nutrient and contaminant sources due to the diversity of trophic webs within inshore marine habitats. Mercury contamination could represent an additional threat to shark populations that are declining worldwide. Here we measured total mercury, carbon and nitrogen isotopes, as well as mercury isotopes, in two co-occurring shark species (the bull shark Carcharhinus leucas and the tiger shark Galeocerdo cuvier) and their potential prey from a coastal ecosystem of the western Indian Ocean (La Réunion Island). Our primary goals were to (i) determine the main trophic Hg sources for sharks and (ii) better characterize their diet composition and foraging habitat. Hg isotope signatures (Δ199Hg and δ202Hg) of shark prey suggested that bull sharks were exposed to methylmercury (MeHg) produced in offshore epipelagic waters, while tiger sharks were exposed to offshore mesopelagic MeHg with additional microbial transformation in slope sediments. Δ199Hg values efficiently traced the ecology of the two predators, demonstrating that bull sharks targeted coastal prey in shallow waters while tiger sharks were mainly foraging on mesopelagic species in the deeper waters of the island slope. Unexpectedly, we found a positive shift in δ202Hg (>1‰) between sharks and their prey, leading to high δ202Hg values in the two shark species (e.g. 1.91 ± 0.52‰ in bull sharks). This large shift in δ202Hg indicates that sharks may display strong MeHg demethylation abilities, possibly reflecting evolutionary pathways for mitigating their MeHg contamination.

Kinetics and metabolism of mercury in rats fed with mercury contaminated rice using mass balance and mercury isotope approach

Consumption of mercury (Hg) contaminated rice can be a major environmental health issue but the toxicokinetics is not well known. Hg isotopes have been shown to be good tracers in studying Hg exposure and metabolic processes. We established a Hg mass balance and Hg isotope model in rats fed with Hg contaminated rice (THg 51.3 ng/g; MeHg 25 ng/g) for 90 days to investigate Hg toxicokinetics. Overall 80% of feeding THg was recovered in rat body and excrement, while the excrement accounted for 55% of total observed THg in rats. Feces were the main route of Hg elimination in rats, while urinary excretion was negligible. However, only 32% of utilized MeHg was recovered in rats, indicating significant demethylation of MeHg in rat body. Positive net fractionations of δ202Hg (relative to the feeding rice) were observed in hair and blood samples (1.21‰ and 1.25‰, respectively), which have similar trend with the results obtained in human hair study, exhibiting higher δ202Hg values (2‰– 3‰) than consumed fish and rice. Most importantly, we observed negative net fractionations in feces (−0.44‰), which confirmed the missed Hg with negative δ202Hg signal. We concluded that mass balance and Hg isotope are useful tools for quantifying toxicokinetics of Hg. Demethylation of MeHg in the intestine were the important detoxification process in rat body characterizing with negative net Hg fractionations in feces.

Mercury stable isotopes in flying fish as a monitor of photochemical degradation of methylmercury in the Atlantic and Pacific Oceans

The photochemical degradation of the neurotoxin methylmercury (MeHg) in marine surface waters is of great interest because it reduces the amount of MeHg available for uptake and bioaccumulation in marine aquatic food webs. Studies have shown that the dominant cause of odd isotope mass independent fractionation (odd-MIF) of Hg in marine foodwebs is the photo-degradation of MeHg. Residual MeHg is then incorporated into low trophic level organisms and bio-accumulated without additional mass independent fractionation. Based on this understanding of Hg isotope fractionation we sought to use Hg isotope measurements of fish tissues containing Hg mostly as MeHg to assess the relative degree of photochemical decomposition across the world's oceans. In 19 samples of flying fish, the magnitude of odd-MIF varies by a factor of ~2. We estimate that 56 to 80% of MeHg was photo-degraded prior to entering the food web depending on location. The proportion of MeHg degradation does not correlate with latitude, solar radiation or estimates of the concentration of DOC or chlorophyll at the collection sites, but it does correlate with proxies for water clarity. The ratio of odd-MIF for 199Hg compared to 201Hg is constant in all flying fish sampled (1.20 ± 0.03) suggesting that there is a common mechanism for photo-degradation of MeHg in surface waters across oceans and hemispheres. The ratio of odd-MIF to mass dependent fractionation (MDF) is generally consistent with photochemical degradation but is more variable (2.76 ± 0.33), suggesting that there is negligible internal demethylation of MeHg in flying fish and that variable amounts of MDF occur at different locations in the ocean, likely driven by variable rates of methylation and demethylation of Hg in the water column.

Genomic Characterization of a Mercury Resistant Arthrobacter sp. H-02-3 Reveals the Presence of Heavy Metal and Antibiotic Resistance Determinants

Nuclear production and industrial activities led to widespread contamination of the Department of Energy (DOE) managed Savannah River Site (SRS), located in South Carolina, United States. The H-02 wetland system was constructed in 2007 for the treatment of industrial and storm water runoff from the SRS Tritium Facility. Albeit at low levels, mercury (Hg) has been detected in the soils of the H-02 wetland ecosystem. In anoxic sediments, Hg is typically methylated by anaerobic microbiota, forming the highly neurotoxic methylmercury (MeHg), which biomagnifies across food webs. However, in surficial oxic wetland soils, microbially mediated demethylation and/or volatilization processes can transform Hg2+ into the less toxic Hg0 form which is released into the atmosphere, thus circumventing MeHg formation. To obtain a deeper understanding on bacterial Hg volatilization, a robust Hg-resistant (HgR) bacteria, called as strain H-02-3 was isolated from the H-02 soils. A draft genome sequence of this strain was obtained at a coverage of 700×, which assembled in 44 contigs with an N50 of 171,569 bp. The genomic size of the strain H-02-3 was 4,708,612 bp with a total number of 4,240 genes; phylogenomic analysis revealed the strain as an Arthrobacter species. Comparative genomics revealed the presence of 1100 unique genes in strain H-02-3, representing 26.7% of the total genome; many identified previously as metal resistance genes (MRGs). Specific to Hg-cycling, the presence of mercuric ion reductase (merA), the organomercurial lyase (merB), and the mercuric resistance operon regulatory protein, were identified. By inference, it can be proposed that the organomercurial lyase facilitates the demethylation of MeHg into Hg2+ which is then reduced to Hg0 by MerA in strain H-02-3. Furthermore, gene prediction using resistome analysis of strain H-02-3 revealed the presence of several antibiotic resistance genes (ARGs), that statistically correlated with the presence of metal resistant genes (MRGs), suggesting co-occurrence patterns of MRGs and ARGs in the strain. Overall, this study delineates environmentally beneficial traits that likely facilitates survival of Arthrobacter sp. H-02-3 within the H-02 wetland soil. Finally, this study also highlights the largely ignored public health risk associated with the co-development of ARGs and MRGs in bacteria native to historically contaminated soils.

Mercury Accumulation and Effects in the Brain of the Atlantic Sharpnose Shark (Rhizoprionodon terraenovae).

Few published studies have examined whether the elevated concentrations of the nonessential toxic metal mercury (Hg) often observed in shark muscle also occur in the shark brain or whether Hg accumulation affects shark neurophysiology. Therefore, this study examined accumulation and distribution of Hg in the shark brain, as well as effects of Hg on oxidative stress in the shark central nervous system, with particular focus on the Atlantic sharpnose shark (Rhizoprionodon terraenovae). Sharks were collected along the southeastern U.S. coast throughout most of this species' U.S. geographical range. Total Hg (THg) concentrations were measured in and compared between shark muscle and brain, whereas known biomarkers of Hg-induced neurological effects, including glutathione depletion, lipid peroxidation, and concentrations of a protein marker of glial cell damage (S100b), were measured in shark cerebrospinal fluid. Brain THg concentrations were correlated with muscle THg levels but were significantly lower and did not exceed most published thresholds for neurological effects, suggesting limited potential for detrimental responses. Biomarker concentrations supported this premise, because these data were not correlated with brain THg levels. Hg speciation also was examined. Unlike muscle, methylmercury (MeHg) did not comprise a high percentage of THg in the brain, suggesting that differential uptake or loss of organic and inorganic Hg and/or demethylation of MeHg may occur in this organ. Although Hg accumulation in the shark brain generally fell below toxicity thresholds, higher THg levels were measured in the shark forebrain compared with the midbrain and hindbrain. Therefore, there is potential for selective effects on certain aspects of shark neurophysiology if brain Hg accumulation is increased.

Mercury speciation in meconium and associated factors

Meconium is formed early in gestation and it is normally not excreted until after birth. Thus it may provide a longer and cumulative record of exposure to mercury (Hg). The present study aims to speciate Hg in meconium samples (N = 488) from Slovenian and Croatian new-borns prenatally exposed to low levels of methyl-Hg (MeHg) from maternal seafood intake and to Hg0 from maternal dental amalgam fillings. We had complete data of total Hg (THg) and MeHg in meconium and THg in maternal hair (MH), while THg and MeHg in maternal blood (MB) were available only for Croatian mothers. Personal data namely maternal seafood intake, age, pre-pregnancy BMI, parity, smoking, estimated gestational age at birth, sex, and birth weight were available for the majority of participants, except the number of dental amalgams which was in most cases missing for Croatian mothers. The median THg concentration in meconium was 11.1 (range: 0.41–375.2) ng/g and inorganic Hg (Hg(II)) presented 98.8% (range: 82%–100%, CV: 2%) of THg. We observed significant correlation between meconium and MH Hg levels, with the highest correlation between hair THg and meconium MeHg. Correlation analysis including MB (available only for Croatian population) showed a significant positive correlation between THg in meconium and THg in MB (Rs = 0.642). Additionally, MeHg from MB was correlated with MeHg in meconium (Rs = 0.898), while the correlation between Hg(II) in MB and meconium was positive, but not significant. Maternal seafood intake was significantly correlated with meconium MeHg (Rs = 0.498) and Hg(II) (Rs = 0.201). Multiple linear regression (performed on the Slovenian population, N = 143) confirmed a positive association between meconium MeHg and seafood intake. Furthermore, meconium Hg(II) was positively associated with the number of maternal dental amalgam fillings, but linear regression models did not confirm correlation between seafood intake and meconium Hg(II) levels. We assume that Hg0 released from maternal dental amalgam fillings and MeHg from seafood intake were both transported through the placental barrier and portioned between different foetal compartments including meconium. Weak correlation between maternal seafood intake and Hg(II) levels in meconium suggests that there is certain evidence of MeHg demethylation. However, because this correlation was not confirmed by the multiple regression, MeHg demethylation during prenatal life cannot be neither confirmed nor excluded. Further investigations at higher level of exposure are needed to confirm this observations. We can conclude that meconium is a suitable biomarker for MeHg and Hg0 exposure during pregnancy. However, comparability of the results reported in meconium in different studies is hindered by a lack of standardized sampling protocols, storage, and analysis.

High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales (Globicephala melas)

Whales accumulate mercury (Hg), but do not seem to show immediate evidence of toxic effects. Analysis of different tissues (liver, kidney, muscle) and biofluids (blood, milk) from a pod of stranded long-finned pilot whales (Globicephala melas) showed accumulation of Hg as a function of age, with a significant decrease in the MeHg fraction. Isotopic analysis revealed remarkable differences between juvenile and adult whales. During the first period of life, Hg in the liver became isotopically lighter (δ202Hg decreased) with a strongly decreasing methylmercury (MeHg) fraction. We suggest this is due to preferential demethylation of MeHg with the lighter Hg isotopes and transport of MeHg to less sensitive organs, such as the muscles. Also changes in diet, with high MeHg intake in utero and during lactation, followed by increasing consumption of solid food contribute to this behavior. Interestingly, this trend in δ202Hg is reversed for livers of adult whales (increasing δ202Hg value), accompanied by a progressive decrease of δ202Hg in muscle at older ages. These total Hg (THg) isotopic trends suggest changes in the Hg metabolism of the long-finned pilot whales, development of (a) detoxification mechanism(s) (e.g., though the formation of HgSe particles), and Hg redistribution across the different organs.

Impacts of experimental alteration of water table regime and vascular plant community composition on peat mercury profiles and methylmercury production

Climate change is expected to alter the hydrology and vascular plant communities in peatland ecosystems. These changes may have as yet unexplored impacts on peat mercury (Hg) concentrations and net methylmercury (MeHg) production. In this study, peat was collected from PEATcosm, an outdoor, controlled mesocosm experiment where peatland water table regimes and vascular plant functional groups were manipulated over several years to simulate potential climate change effects. Potential Hg(II) methylation and MeHg demethylation rate constants were assessed using enriched stable isotope incubations at the end of the study in 2015, and ambient peat total Hg (THg) and MeHg concentration depth profiles were tracked annually from 2011 to 2014. Peat THg and MeHg concentrations and the proportion of THg methylated (%MeHg) increased significantly within the zone of water table fluctuation when water tables were lowered, but potential Hg(II) methylation rate constants were similar regardless of water table treatment. When sedges dominate over ericaceous shrubs, MeHg concentrations and %MeHg became significantly elevated within the sedge rooting zone. Increased desorption of Hg(II) and MeHg from the solid phase peat into pore water occurred with a lowered water table and predominant sedge cover, likely due to greater aerobic peat decomposition. Deeper, more variable water tables and a transition to sedge-dominated communities coincided with increased MeHg accumulation within the zone of water table fluctuation. Sustained high water tables promoted the net downward migration of Hg(II) and MeHg. The simultaneous decrease in Hg(II) and MeHg concentrations in the near-surface peat and accumulation deeper in the peat profile, combined with the trends in Hg(II) and MeHg partitioning to mobile pore waters, suggest that changes to peatland hydrology and vascular plant functional groups redistribute peat Hg(II) and MeHg via vertical hydrochemical transport mechanisms.

Intracellular Demethylation of Methylmercury to Inorganic Mercury by Organomercurial Lyase (MerB) Strengthens Cytotoxicity.

Some methylmercury (MeHg) is converted to inorganic mercury (Hg2+) after incorporation into human and animal tissues, where it can remain for a long time. To determine the overall toxicity of MeHg in tissues, studies should evaluate low concentrations of Hg2+. Although demethylation is involved, the participating enzymes or underlying mechanisms are unknown; in addition, the low cell membrane permeability of Hg2+ makes these analyses challenging. We established model cell lines to assess toxicities of low concentrations of Hg2+ using bacterial organomercury lyase (MerB). We engineered MerB-expressing HEK293 and HeLa cell lines that catalyze MeHg demethylation. These cells were significantly more sensitive to MeHg exposure compared to the parental cells. MeHg treatment remarkably induced metallothioneins (MTs) and hemeoxygenase-1 (HMOX-1) mRNAs and modest expression of superoxide dismutase 1, whereas catalase and glutathione peroxidase 1 mRNAs were not up-regulated. merB knockdown using small interfering RNA supported the induction of MT and HMOX-1 mRNA by MerB enzymatic activity. Pretreatment with Trolox, a water-soluble vitamin E analog, did not inhibit MeHg-induced elevation of MT-Ix and HMOX-1 mRNAs in MerB-expressing cells, suggesting that Hg2+ works independently of reactive oxygen species generation. Similar results were obtained in cells expressing MerB, suggesting that high MTs and HMOX-1 induction and cytotoxicity are common cellular responses to low intracellular Hg2+ concentrations. This is the first study to establish cell lines that demethylate intracellular MeHg to Hg2+ using bacterial MerB for overcoming the low membrane permeability of Hg2+ and exploring the intracellular responses and toxicities of low Hg2+ concentrations.

Biotic and Abiotic Degradation of Methylmercury in Aquatic Ecosystems: A Review.

Mercury (Hg) methylation and demethylation is supposed to simultaneously exist in the environment and form a cycle, which determines the net production of methylmercury (MeHg). Exploring the mechanisms of MeHg formation and degradation, and its final fate in the natural environment is essential to understanding the biogeochemical cycle of Hg. However, MeHg demethylation has been less studied in the past years comparing with Hg methylation, particularly in anaerobic microorganisms whose demethylation role has been under-evaluated. This review described the current state of knowledge on biotic (microorganisms) and abiotic demethylation (photodegradation, chemical degradation) of MeHg. The decomposition of MeHg performed by microorganisms has been identified as two different pathways, reductive demethylation (RD) and oxidative demethylation (OD). Anaerobic and aerobic microorganisms involved in the process of RD and OD, influencing factors as well as research background and histories are systematically described in this review. It is predicted that the photodegradation mechanism, as well as anaerobic microorganisms involved in MeHg formation and degradation cycle will be the focus of future research.

Intestinal Methylation and Demethylation of Mercury.

Mercury (Hg) is a global pollutant, which is linked with different diseases. The methylation of Hg and demethylation of methylmercury (MeHg) in the environment were extensively studied and summarized; however, the transformation of Hg in the intestine is less presented. In this review, the research progress and the perspectives on the intestinal transformation of Hg were discussed. Studies found that MeHg could be formed when exposed to inorganic Hg by the gut microbiota in aquatic organisms, terrestrial invertebrates, and mammals, etc. hgcAB genes could be used as indicators for predicting Hg methylation potential. In vitro studies using fecal specimen, intestinal contents, and the isolated intestinal microbes confirmed the intestinal demethylation of MeHg. The investigation on the effects of Hg exposure to the abundance and diversity of intestinal microbes and their metabolites could shed light on the mechanism of the toxicity of Hg, especially the neurotoxicity of MeHg, which deserves further study.

Mercury speciation, bioavailability and risk assessment on soil–rice systems from a watershed impacted by abandoned Hg mine-waste tailings

Mercury (Hg) is a global pollutant and can be accumulated in the food chain, posing exposure risks to humans. In this study, rice plants and corresponding rhizosphere soil samples were collected from a watershed of the Wawu River Basin that is heavily impacted by historic Hg mining and retorting activities. Total mercury (THg) and methylmercury (MeHg) in rice grains, as well as other tissues and soil samples, were measured. Five soil Hg fractions, as well as soil parameters, were also determined. The results show that the average concentrations of THg and MeHg in rice grains were 14 ± 7.0 μg kg−1 and 7.2 ± 4.0 μg kg−1. Soil organic-bound (Hg-o) and strong complex-bound (Hg-s) were the main Hg fractions, accounting for 44% of the total. To estimate the Hg–ligand interaction in the soils, soil -N/Hg (R = 0.451, p < 0.05), -S/Hg (R = 0.372, p < 0.1), and -OM/Hg ratio (R = 0.320, p < 0.5) with MeHgsoil were observed with significant positive correlations, indicating that the formation of Hg–OM, Hg–N–OM or Hg–S–OM complexes could prevent Hg(II) from methylation in soils. The significant positive correlations of the -N/Hg ratio, -S/Hg ratio and -OM/Hg ratio with MeHg in rice tissues suggested that Hg methylation and MeHg demethylation occurred throughout the rice paddy ecosystem. The estimated MeHg daily intake (EDI) was 0.075 ± 0.041 μg kg−1 bw d−1 and was lower than the RfD level of 0.1 μg kg−1 bw d−1 recommended by the US EPA. However, approximately 29% of the hazardous index (HI) of MeHg in grain exceeded 1, posing a potential threat to local populations, particularly pregnant women and children.

Immobilized Organo-Mercurial Lyase on Zeolite Using a Solid Binding Peptide

Methylmercury (MeHg) compounds can form naturally, are highly toxic, and of concern because of their tendency to bio-accumulate. Certain bacteria have evolved mechanisms that can tolerate MeHg by first demethylating MeHg compounds, before further processing. Drawing inspiration from this demethylation mechanism controlled by a single organo-mercurial lyase in a protonolysis reaction, this research uses a recombinant gene that produces this lyase plus an additional polypeptide that selectively binds to zeolite particles, effectively tethering the enzyme to the solid substrate. This work is part of a broader attempt to create a fixed bed reactor for de-methylation of MeHg. Enzyme immobilization was achieved using a solid binding peptide (SBP) with high affinity for faujasite zeolite (FZ), the choice of binding substrate in the present work. The lyase is coded for by the merB gene, and a sequence with highly conserved active site homology was obtained from E.coli plasmid R8361b. The SBP plus merB sequence was designed such that the SBP was positioned either on the N or C-terminal of the construct. The DNA was synthesized commercially, and expressed in E.coli (BL21DE3 Star) using pET100® vector. Sanger sequencing was used to confirm construct in transformed cells using standard T7 oligos. Expression was lactose induced, and SDS-PAGE electrophoresis was used to confirm protein production and size. LC-MS/MS and sequence bio-analytics confirmed peptide sequence. Silica binding assays using SDS-PAGE confirmed binding of the enzyme to the silica substrate. Enzyme functionality results using a non-methylated mercuric compound were inconclusive, however the enzyme has not been assessed using MeHg compounds at this stage.

Long-Term Feeding of Elemental Sulfur Alters Microbial Community Structure and Eliminates Mercury Methylation Potential in Sulfate-Reducing Bacteria Abundant Activated Sludge.

This study reported a novel observation that the long-term cultivation of sulfur-reducing bacteria (S0RB) from a sulfate-reducing bacteria (SRB)-abundant seeding sludge with elemental sulfur feeding significantly shaped the microbial community structure and eliminated the mercury methylation potential in the S0RB-enriched sludge. In this study, the enrichments of SRB and S0RB from activated sludge were obtained through long-term cultivations. Subsequently, the batch tests showed that approximately 5000 μg/L Hg (II) was completely removed from the solution by both the SRB-enriched and S0RB-enriched sludge. Extremely low or no MeHg production was observed in the S0RB-enriched sludge (less than the limit of detection, 0.01 μg/L), while 1.49 μg/L MeHg accumulated in the SRB-enriched sludge. Other batch tests using the sludge samples from a replication of the cultivation showed that the methylation capability of the S0RB-enriching sludge gradually diminished to a negligible level over a 6 month cultivation time. However, some mercury-methylation-related bacteria were present in the enrichment of S0RB such as Geobacter. The absence of MeHg in the S0RB-enriched sludge may be attributed to the dissolved organic matter (DOM) instead of the sulfur- and sulfate-reduction pathway or MeHg demethylation when exposed to Hg (II). The cultivated S0RB could be used for mercury-contaminated wastewater treatment without MeHg concern.

Mechanisms of Methyl Mercury Net Degradation in Alder Swamps: The Role of Methanogens and Abiotic Processes

Wetlands are common net producers of the neurotoxin monomethylmercury (MeHg) and are largely responsible for MeHg bioaccumulation in aquatic food-webs. However, not all wetlands net produce MeHg; notable exceptions are black alder (Alnus glutinosa) swamps, which net degrade MeHg. Here we report the mechanisms of MeHg demethylation in one such swamp (EHT), shown to be a sink for MeHg during four consecutive years. The potential demethylation rate constant (kd) in soil incubations was ∼3 times higher in the downstream (EHT-D: kd ∼ 0.14 d–1) as compared to the upstream part of the swamp (EHT-U: kd ∼ 0.05 d–1). This difference concurred with increased stream and soil pH, and a change in plant community composition. Electron acceptor and inhibitor addition experiments revealed that abiotic demethylation dominated at EHT-U while an additional and equally large contribution from biotic degradation was observed at EHT-D, explaining the increase in MeHg degradation. Biotic demethylation (EHT-D) was primarily due t...

Mercury speciation and subcellular distribution in experimentally dosed and wild birds.

Many bird species are exposed to methylmercury (MeHg) at levels shown to cause sublethal effects. Although MeHg sensitivity and assimilation can vary among species and developmental stages, the underlying reasons (such as MeHg toxicokinetics) are poorly understood. We investigated Hg distribution at the tissue and cellular levels in birds by examining Hg speciation in blood, brain, and liver and Hg subcellular distribution in liver. We used MeHg egg injection of white leghorn chicken (Gallus gallus domesticus), sampled at 3 early developmental stages, and embryonic ring-billed gulls (Larus delawarensis) exposed to maternally deposited MeHg. The percentage of MeHg (relative to total Hg [THg]) in blood, brain, and liver ranged from 94 to 121%, indicating little MeHg demethylation. A liver subcellular partitioning procedure was used to determine how THg was distributed between potentially sensitive and detoxified compartments. The distributions of THg among subcellular fractions were similar among chicken time points, and between embryonic chicken and ring-billed gulls. A greater proportion of THg was associated with metal-sensitive fractions than detoxified fractions. Within the sensitive compartment, THg was found predominately in heat-denatured proteins (∼42-46%), followed by mitochondria (∼15-18%). A low rate of MeHg demethylation and high proportion of THg in metal-sensitive subcellular fractions further indicates that embryonic and hatchling time points are Hg-sensitive developmental stages, although further work is needed across a range of additional species and life stages. Environ Toxicol Chem 2017;36:3289-3298. © 2017 SETAC.

Photomicrobial Visible Light-Induced Magnetic Mass Independent Fractionation of Mercury in a Marine Microalga

Methylmercury (MeHg), a highly neurotoxic substance, accumulates in aquatic food webs, and is enriched in odd isotopes (i.e., 199Hg and 201Hg), purportedly as a result of abiotic photodegradation in surface waters. Here, we highlight the potential role of phytoplankton in the mass independent fractionation (MIF) of MeHg in marine food-webs by providing evidence of (1) degradation of intracellular MeHg and reduction of intracellular inorganic mercury (Hg(II)) in the marine microalga, Isochrysis galbana; (2) a large, positive MIF (Δ199Hgreactant – Δ199Hgproduct ∼ 5–10‰) during intracellular degradation of MeHg in cells exposed to visible light with no UVB, consistent with the accumulation of odd isotope-enriched MeHg in marine food-webs; and (3) a negative MIF (−1‰) during intracellular reduction of Hg(II) in the presence of UVB light. If representative of the photochemical reactivity of MeHg in marine phytoplankton, our results indicate that algal cell-mediated demethylation of MeHg by visible light could ...

In Vivo Mercury Demethylation in a Marine Fish (Acanthopagrus schlegeli).

Mercury (Hg) in fish has attracted public attention for decades, and methylmercury (MeHg) is the predominant form in fish. However, the in vivo MeHg demethylation and its influence on Hg level in fish have not been well-addressed. The present study investigated the in vivo demethylation process in a marine fish (black seabream, Acanthopagrus schlegeli) under dietary MeHg exposure and depuration and quantified the biotransformation and interorgan transportation of MeHg by developing a physiologically based pharmacokinetic (PBPK) model. After exposure, we observed a 2-fold increase of the whole-body inorganic Hg (IHg), indicating the existence of an in vivo demethylation process. The results strongly suggested that the intestine played a predominant role in MeHg demethylation with a significant rate (6.6 ± 1.7 day-1) during exposure, whereas the hepatic demethylation appeared to be an extremely slow (0.011 ± 0.001 day-1) process and could hardly affect the whole-fish Hg level. Moreover, demethylation in the intestine served as an important pathway for MeHg detoxification. Our study also pointed out that in vivo MeHg demethylation could influence Hg level and speciation in fish although food is the major pathway for Hg accumulation. Enhancing in vivo MeHg biotransformation (especially in the intestine) could be a potential key solution in minimizing Hg contamination in fish. The related factors involved in intestinal demethylation deserve more attention in the future.

The influence of sample drying and storage conditions on methylmercury determination in soils and sediments

The separate influences of drying and storage conditions on methylmercury (MeHg) concentrations in soil and sediment samples were investigated. Concentrations of MeHg and total Hg were determined in various soil and sediment samples that had been stored or dried under differing conditions. The influence of drying conditions (oven-drying (40 °C) versus freeze-drying) on MeHg concentrations in marine sediments, river sediments, soils, and paddy field soils was investigated (n = 43). The ratio of the MeHg concentration in oven-dried sub-samples divided by the concentration in freeze-dried sub-samples ranged from 0 to 336%. In order to confirm the production of MeHg during storage in some samples, Hg2+ was added at 15 mg kg−1 to a paddy soil, and the sample was then stored at 30 °C. The concentrations of MeHg at 1-h, 1-day, 4-days and 7-days after Hg2+ spiking were 2.0 ± 0.1, 13.8 ± 1.0, 36.0 ± 5.0, and 24.9 ± 1.6 μg kg−1 (n = 3), respectively. The concentration of MeHg at 4-days after Hg spiking and sterilizing (121 °C, 30 min) was 1.8 μg kg−1, similar to the original value. These results indicate that bacterial Hg methylation and MeHg demethylation occurred within days in the soil. In addition, tests of the stability of MeHg in wet and dry samples during storage were also performed. Overall, our results indicate that the best way to preserve MeHg in soil and sediment samples is to freeze the samples immediately after collection, followed subsequently by freeze-drying, grinding, homogenization, and storage of the dry material in cool, dark conditions until analysis.