Organic MeHg Research Articles

Methylmercury biomagnification in aquatic food webs of Poyang Lake, China: Insights from amino acid signatures

As the dominant mercury species in fish, methylmercury (MeHg) biomagnifies during its trophic transfer through aquatic food webs. MeHg is known to bind to cysteine, forming the complex of MeHg-cysteine. However, relationship between MeHg and cysteine in large-scale food webs has not been explored and contrasted with MeHg biomagnification models. Here, we quantified the compound-specific nitrogen isotopic analysis of amino acids (CSIA-AA), MeHg, and amino acid composition in aquatic organisms of Poyang Lake, the largest freshwater lake in China. The trophic positions (TPAA) of organisms ranged from 1.0 ± 0.1–3.7 ± 0.2 based on CSIA-AA approach. The trophic magnification factor (TMF) of MeHg, derived from the regression slope of Log-transformed MeHg in organisms upon their TPAA for the entire food web was 9.5 ± 0.5. Significantly positive regression between MeHg and cysteine (R2 = 0.64, p < 0.01) was documented, suggesting MeHg-cysteine complex may potentially play a critical role in the bioaccumulation of MeHg. Furthermore, TMFs of MeHg calculated with and without cysteine normalization compared well (7.7–8.7) when excluding primary producers. Our results implied that MeHg may biomagnify as the complex of MeHg-cysteine and contribute to our understanding of MeHg trophic transfer at the molecular level.

Effects of geography and species variation on selenium and mercury molar ratios in Northeast Atlantic marine fish communities

Methylmercury (MeHg) is a potent neurotoxin that bioaccumulates in seafood. Co-occurrence of selenium (Se) may affect the bioavailability and toxicity of MeHg in organisms. Here we report the concentrations of total mercury (Hg) and Se in 17 teleost fish species (n = 8459) sampled during 2006–2015 from the North East Atlantic Ocean (NEAO) and evaluate species variation and effects of geography. Mean Hg concentration ranged from 0.04 mg kg−1 ww in Atlantic mackerel (Scomber scombrus) and blue whiting (Micromesistius poutassou) to 0.72 mg kg−1 ww in blue ling (Molva dypterygia). Se concentrations were less variable and ranged from 0.27 mg kg−1 ww in Atlantic cod (Gadus morhua) to 0.56 mg kg−1 ww in redfish (Sebastes spp.). The mean Se:Hg molar ratio ranged from 1.9 in blue ling to 43.3 in mackerel. Pelagic species had the lowest Hg concentrations and the highest Se:Hg ratios, whereas demersal species had the highest Hg concentrations and the lowest Se:Hg ratios. Se and Hg concentrations were positively correlated in 13 of the 17 species. Hg concentrations increased from the North to South in contrast to the Se:Hg molar ratio which exhibited the opposite trend. Fish from fjord and coastal areas had higher concentrations of Hg and lower Se:Hg molar ratios compared to fish sampled offshore. All species had average Se:Hg molar ratios >1 and Hg concentrations were largely below the EU maximum level of 0.5 mg kg−1 ww with few exceptions including the deep water species tusk (Brosme brosme) and blue ling sampled from fjord and coastal habitats. Our results show that two fillet servings of tusk, blue ling or Atlantic halibut (Hippoglossus hippoglossus) exceeded the tolerable weekly intake of MeHg although the surplus Se may possibly ameliorate the toxic effects of MeHg. However, some individuals with selenium deficiencies may exhibit greater sensitivity to MeHg.

Spatial and temporal trophic transfer dynamics of mercury and methylmercury into zooplankton and phytoplankton of Long Island Sound

AbstractThe foundation of methylmercury (MeHg) bioaccumulation in aquatic systems is uptake and trophic transfer into phytoplankton and zooplankton. Phytoplankton and zooplankton samples were collected seasonally from Long Island Sound (LIS) and its adjacent shelf waters, located along the northeast coast of the United States. Phytoplankton samples were size fractioned into 0.2–5 μm, 5–20 μm, and seston of &gt; 20 μm. Average phytoplankton MeHg concentrations were 0.01–1.5 pmol g−1 (wet weight), and MeHg bioconcentration factors (logBCF) ranged from 2.5 to 5.5. There was higher MeHg concentration and logBCF values with increasing phytoplankton size. Zooplankton samples were size fractioned into 0.2–0.5 mm, 0.5–1.0 mm, 1.0–2.0 mm, and &gt; 2.0 mm classes and analyzed for Hg and MeHg as well as carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) isotopes. The %MeHg in organisms was highest in the &gt; 2.0 mm size class for coastal LIS and shelf species, indicating higher MeHg bioaccumulation with increasing zooplankton sizes. Eastern LIS zooplankton populations yielded higher overall Hg accumulation than western LIS populations, in alignment with phytoplankton fractions. Seasonal differences were evident for plankton %MeHg, although only ELIS displayed an increase in summertime %MeHg throughout the fractions. The results suggest the importance of biodilution in determining plankton MeHg concentrations. The δ13C values infer that zooplankton were opportunistically feeding on food containing variable levels of Hg and MeHg. The δ15N isotopes indicated enrichment characteristic of anthropogenic input in populations at the western end of LIS. The δ34S isotopes signified that organisms were primarily feeding on pelagic plankton.

Toxicity of organic and inorganic mercury species in differentiated human neurons and human astrocytes

Organic mercury (Hg) species exert their toxicity primarily in the central nervous system. The food relevant Hg species methylmercury (MeHg) has been frequently studied regarding its neurotoxic effects in vitro and in vivo. Neurotoxicity of thiomersal, which is used as a preservative in medical preparations, is to date less characterised. Due to dealkylation of organic Hg or oxidation of elemental Hg, inorganic Hg is present in the brain albeit these species are not able to readily cross the blood brain barrier. This study compared for the first time toxic effects of organic MeHg chloride (MeHgCl) and thiomersal as well as inorganic mercury chloride (HgCl2) in differentiated human neurons (LUHMES) and human astrocytes (CCF-STTG1). The three Hg species differ in their degree and mechanism of toxicity in those two types of brain cells. Generally, neurons are more susceptible to Hg species induced cytotoxicity as compared to astrocytes. This might be due to the massive cellular mercury uptake in the differentiated neurons. The organic compounds exerted stronger cytotoxic effects as compared to inorganic HgCl2. In contrast to HgCl2 exposure, organic Hg compounds seem to induce the apoptotic cascade in neurons following low-level exposure. No indicators for apoptosis were identified for both inorganic and organic mercury species in astrocytes. Our studies clearly demonstrate species-specific toxic mechanisms. A mixed exposure towards all Hg species in the brain can be assumed. Thus, prospectively coexposure studies as well as cocultures of neurons and astrocytes could provide additional information in the investigation of Hg induced neurotoxicity.

Variation in terrestrial and aquatic sources of methylmercury in stream predators as revealed by stable mercury isotopes.

Mercury (Hg) is widely distributed in the environment, and its organic form, methylmercury (MeHg), can extensively bioaccumulate and biomagnify in aquatic and terrestrial food webs. Concentrations of MeHg in organisms are highly variable, and the sources in natural food webs are often not well understood. This study examined stable isotope ratios of MeHg (mass-dependent fractionation, as δ(202)HgMeHg; and mass-independent fractionation, as Δ(199)HgMeHg) in benthic invertebrates, juvenile steelhead trout (Oncorhynchus mykiss), and water striders (Gerris remigis) along a stream productivity gradient, as well as carnivorous terrestrial invertebrates, in a forested watershed at the headwater of South Fork Eel River in northern California. Throughout the sampling sites, δ(202)HgMeHg (after correction due to the effect of MeHg photodegradation) was significantly different between benthic (median = -1.40‰; range, -2.34 to -0.78‰; total number of samples = 29) and terrestrial invertebrates (median = +0.51‰; range, -0.37 to +1.40‰; total number of samples = 9), but no major difference between these two groups was found for Δ(199)HgMeHg. Steelhead trout (52 individual fishes) have MeHg of predominantly aquatic origins, with a few exceptions at the upstream locations (e.g., 1 fish collected in a tributary had a purely terrestrial MeHg source and 4 fishes had mixed aquatic and terrestrial MeHg sources). Water striders (seven pooled samples) derive MeHg largely from terrestrial sources throughout headwater sections. These data suggest that direct terrestrial subsidy (e.g., terrestrial invertebrates falling into water) can be important for some stream predators in headwater streams and could represent an important means of transfer of terrestrially derived MeHg (e.g., in situ methylation within forests, atmospheric sources) to aquatic ecosystems. Moreover, these findings show that terrestrial subsidies can enhance MeHg bioaccumulation of consumers in headwater streams where aqueous MeHg levels are very low.

Computational Studies of the Coordination Stereochemistry, Bonding, and Metal Selectivity of Mercury

Knowledge of the bonding and selectivity of organic mercury, [H3C-Hg]+ (MeHg+), and inorganic Hg2+ for protein and DNA functional groups is important for understanding the mechanism of heavy metal poisoning. Herein, we elucidate (1) the differences between inorganic Hg2+ and organic MeHg+ in their interactions with different ligands of biological interest, (2) the protein and DNA functional groups that Hg2+ and MeHg+ target in aqueous solution, and (3) the likelihood of "soft" Hg2+ displacing the "borderline" Zn2+ bound to "harder" nitrogen/oxygen-containing side chains such as His and Asp/Glu. The results reveal that, relative to Hg2+, the lower positive charge on MeHg+ results in a longer and weaker bond with a given ligand, in accord with the observed kinetic lability of MeHg+ complexes. They also indicate that negatively charged or polar amino acid side chains containing S-/O-/S/N donors could coordinate to both organic MeHg+ and inorganic Hg2+. In addition, Gua and Cyt could also coordinate to MeHg+ and disrupt Gua...Cyt base pairing. A key novel finding is that Hg2+ is a far better electron acceptor than Zn2+, and can thus accept more negative charge from the Zn ligands than the native Zn2+, thus enhancing Hg-ligand interactions and enabling Hg2+ to displace the native cofactor from zinc essential enzymes and "structural" Zn proteins. The results herein support several possible mechanisms for Hg poisoning. Ways that mercury poisoning could be prevented in cells are discussed.