Uptake Of Methylmercury Research Articles

Fate and Developmental Effects of Dietary Uptake of Methylmercury in Silurana tropicalis Tadpoles

Adverse effects of methylmercury (MeHg) exposure during amphibian metamorphosis remain to be fully characterized. Most previous investigations determined effects of short-term exposure to elevated dose rates, without information on mercury (Hg) depuration and degradation pathways. Since metamorphosis is primarily controlled by thyroid hormones (TH), alterations in this process suggest a disruption of the TH endocrine axis. The aim of this research was to (1) characterize patterns of MeHg accumulation and depuration in tadpoles and (2) examine effects of MeHg accumulation on metamorphosis and the TH axis. Silurana tropicalis tadpoles were exposed to environmental levels of dietary MeHg until metamorphic climax. Whole-body MeHg and total Hg (THg) levels were measured, as well as the number of metamorphs, rate of metamorphosis, body size, and whole-body triiodothyronine (T3) levels at metamorphosis. Tadpoles exposed to a higher level of MeHg exhibited increased mortality and size, and reduced metamorphosis. At lower levels of MeHg, body burdens increased rapidly and eventually reached a plateau, whereas no plateau was reached at a higher level of MeHg exposure. T3 levels were not affected. Data indicate that at low and medium levels of exposure, depuration of MeHg may prevent toxicity in tadpoles. However, depuration mechanisms may be insufficient at high doses, producing disruption of metamorphosis and death. Although there were no marked effects of MeHg on whole-body T3 levels, further investigation of other components of the TH axis is warranted.

Modulation of methylmercury uptake by methionine: Prevention of mitochondrial dysfunction in rat liver slices by a mimicry mechanism

Methylmercury (MeHg) is an ubiquitous environmental pollutant which is transported into the mammalian cells when present as the methylmercury-cysteine conjugate (MeHg–Cys). With special emphasis on hepatic cells, due to their particular propensity to accumulate an appreciable amount of Hg after exposure to MeHg, this study was performed to evaluate the effects of methionine (Met) on Hg uptake, reactive species (RS) formation, oxygen consumption and mitochondrial function/cellular viability in both liver slices and mitochondria isolated from these slices, after exposure to MeHg or the MeHg–Cys complex. The liver slices were pre-treated with Met (250μM) 15min before being exposed to MeHg (25μM) or MeHg–Cys (25μM each) for 30min at 37°C. The treatment with MeHg caused a significant increase in the Hg concentration in both liver slices and mitochondria isolated from liver slices. Moreover, the Hg uptake was higher in the group exposed to the MeHg–Cys complex. In the DCF (dichlorofluorescein) assay, the exposure to MeHg and MeHg–Cys produced a significant increase in DFC reactive species (DFC-RS) formation only in the mitochondria isolated from liver slices. As observed with Hg uptake, DFC-RS levels were significantly higher in the mitochondria treated with the MeHg–Cys complex compared to MeHg alone. MeHg exposure also caused a marked decrease in the oxygen consumption of liver slices when compared to the control group, and this effect was more pronounced in the liver slices treated with the MeHg–Cys complex. Similarly, the loss of mitochondrial activity/cell viability was greater in liver slices exposed to the MeHg–Cys complex when compared to slices treated only with MeHg. In all studied parameters, Met pre-treatment was effective in preventing the MeHg- and/or MeHg–Cys-induced toxicity in both liver slices and mitochondria. Part of the protection afforded by Met against MeHg may be related to a direct interaction with MeHg or to the competition of Met with the complex formed between MeHg and endogenous cysteine. In summary, our results show that Met pre-treatment produces pronounced protection against the toxic effects induced by MeHg and/or the MeHg–Cys complex on mitochondrial function and cell viability. Consequently, this amino acid offers considerable promise as a potential agent for treating acute MeHg exposure.

Importance of Speciation in Understanding Mercury Bioaccumulation in Tilapia Controlled by Salinity and Dissolved Organic Matter

We explored the roles of mercury speciation on the bioaccumulation (both aqueous and dietary uptake and elimination) of inorganic mercury (Hg[II]) and methylmercury (MeHg) in tilapia (Oreochromis niloticus) by controlling the mercury binding to inorganic and organic ligands. For the aqueous uptake, we showed that the uptake rates of Hg(II) were significantly higher at 0 psu compared with those at 10 psu and 28 psu. Based on the mercury-Cl complexes distribution, we found a positive relationship between the Hg(II) aqueous uptake rate and the abundance of neutral HgCl(2). Such relationship was further confirmed by the uptake experiments conducted over a lower salinity range (0-6 psu), suggesting that HgCl(2)(0) was the predominant species taken up by tilapia. In the presence of dissolved organic carbon (DOC) from different sources (Suwannee River and natural local waters), mercury uptake rates all decreased dramatically over a wide range of salinity, especially for Hg(II), indicating the overwhelming influence of DOC as opposed to the single effect of salinity. Using the mercury-Cl-DOC model, we demonstrated for the first time that the inhibition of DOC was dependent on the Cl(-), which was less significant at middle salinity level for both mercury forms. In contrast to the complex influence of water conditions on dissolved uptake, we found no significant influence of acclimated salinity on the dietary assimilation and elimination of both mercury species in tilapia. Our results demonstrated the importance of speciation in understanding the mercury bioaccumulation in various natural systems and its broad biogeochemical cycling.

A physiologically based toxicokinetic model for methylmercury in female American kestrels

A physiologically based toxicokinetic (PBTK) model was developed to describe the uptake, distribution, and elimination of methylmercury (CH(3)Hg) in female American kestrels. The model consists of six tissue compartments corresponding to the brain, liver, kidney, gut, red blood cells, and remaining carcass. Additional compartments describe the elimination of CH(3)Hg to eggs and growing feathers. Dietary uptake of CH(3)Hg was modeled as a diffusion-limited process, and the distribution of CH(3)Hg among compartments was assumed to be mediated by the flow of blood plasma. To the extent possible, model parameters were developed using information from American kestrels. Additional parameters were based on measured values for closely related species and allometric relationships for birds. The model was calibrated using data from dietary dosing studies with American kestrels. Good agreement between model simulations and measured CH(3)Hg concentrations in blood and tissues during the loading phase of these studies was obtained by fitting model parameters that control dietary uptake of CH(3)Hg and possible hepatic demethylation. Modeled results tended to underestimate the observed effect of egg production on circulating levels of CH(3)Hg. In general, however, simulations were consistent with observed patterns of CH(3)Hg uptake and elimination in birds, including the dominant role of feather molt. This model could be used to extrapolate CH(3)Hg kinetics from American kestrels to other bird species by appropriate reassignment of parameter values. Alternatively, when combined with a bioenergetics-based description, the model could be used to simulate CH(3)Hg kinetics in a long-term environmental exposure.

Uptake and efflux of methylmercury in vitro: Comparison of transport mechanisms in C6, B35 and RBE4 cells

Methylmercury (MeHg) is a neurotoxicant which enters the brain and may cause permanent change. Thus, the properties of MeHg transport across cell membranes are a key factor in designing an appropriate model for MeHg neurotoxicity. This study uses cell cultures to examine the uptake and efflux mechanisms of methylmercury in C6 glioma, B35 neuroblastoma and rat brain endothelial (RBE4) cells. The cellular uptake and efflux of MeHg was investigated using 14C-labeled MeHg. The uptake of MeHg-chloride was temperature-independent while the uptake of MeHg- l-cysteine was temperature-dependent in all the three cell types. This indicates that uptake of MeHg-chloride is due to passive diffusion and uptake of MeHg- l-cysteine is due to a protein carrier. Substrates of the amino acid transport system L inhibited uptake of MeHg- l-cysteine in C6 and RBE4 cells, but not B35 cells, indicating a role for system L in MeHg-uptake in the former two. Probenecid, Na +-free medium, MeHg and several l-amino acids did not alter the efflux of MeHg from C6 and RBE4 cells. The amino acids l-cysteine and cystine however, increased the efflux. Both cysteine and cystine are important in the generation of glutathione (GSH), suggesting the involvement of GSH in MeHg efflux. HgCl 2 at low concentrations (0.5 and 1.0 μM) decreased the MeHg efflux and at high concentrations (5.0 and 10.0 μM) increased the efflux. This inhibiting effect of HgCl 2 at low concentrations is possibly due to binding to GSH while the effect of high HgCl 2 concentrations is attributed to disrupted membrane integrity, as measured by Trypan blue. This study demonstrates differing transport mechanisms of MeHg in the cell lines C6, B35 and RBE4.

Influence of plankton mercury dynamics and trophic pathways on mercury concentrations of top predator fish of a mining-impacted reservoir

Physical and biogeochemical characteristics of the aquatic environment that affect growth dynamics of phytoplankton and the zooplankton communities that depend on them may also affect uptake of methylmercury (MeHg) into the pelagic food web of oligotrophic reservoirs. We evaluated changes in the quality and quantity of suspended particulate material, zooplankton taxonomy, and MeHg concentrations coincident with seasonal changes in water storage of a mining-impacted reservoir in northern California, USA. MeHg concentrations in bulk zooplankton increased from 4 ng·g–1at low water to 77 ± 6.1 ng·g–1at high water and were positively correlated with cladoceran biomass (r = 0.66) and negatively correlated with rotifer biomass (r = –0.65). Stable isotope analysis revealed overall higher MeHg concentrations in the pelagic-based food web relative to the benthic-based food web. Statistically similar patterns of trophic enrichment of MeHg (slopes) for the pelagic and benthic food webs and slightly higher MeHg concentrations in zooplankton than in benthic invertebrates suggest that the difference in MeHg bioaccumulation among trophic pathways is set at the base of the food webs. These results suggest an important role for plankton dynamics in driving the MeHg content of zooplankton and ultimately MeHg bioaccumulation in top predators in pelagic-based food webs.

Mercury cycling in surface water, pore water and sediments of Mugu Lagoon, CA, USA

Mugu Lagoon is an estuary in southern California, listed as impaired for mercury. In 2005, we examined mercury cycling at ten sites within at most four habitats. In surface water (unfiltered and filtered) and pore water, the concentration of total mercury was correlated with methylmercury levels ( R 2 = 0.29, 0.26, 0.27, respectively, p < 0.05), in contrast to sediments, where organic matter and reduced iron levels were most correlated with methylmercury content ( R 2 = 0.37, 0.26, respectively, p < 0.05). Interestingly, levels for percent methylmercury of total mercury in sediments were higher than typical values for estuarine sediments (average 5.4%, range 0.024–38%, n = 59), while pore water methylmercury K d values were also high (average 3.1, range 2.0–4.2 l kg −1, n = 39), and the estimated methylmercury flux from sediments was low (average 1.7, range 0.14–5.3 ng m −2 day −1, n = 19). Mercury levels in predatory fish tissue at Mugu are >0.3 ppm, suggesting biogeochemical controls on methylmercury mobility do not completely mitigate methylmercury uptake through the food web.

Methylmercury in arctic Alaskan mosquitoes: implications for impact of atmospheric mercury depletion events

Environmental context. Recent research suggests that gross mercury deposition in the Arctic is increased significantly as a result of springtime Atmospheric Mercury Depletion Events (AMDE). A primary environmental and human health concern is whether mercury deposited with these events leads to enhanced production and uptake of the toxic methylmercury species in polar ecosystems. Here, we present an initial assessment of potential impact from AMDE utilising mosquitoes as bioindicators of methylmercury accumulation in freshwater and terrestrial food webs within 200 km of the Arctic Ocean. Abstract. Atmospheric Mercury Depletion Events (AMDE) – phenomena in which elemental Hg is oxidised and stripped from the atmosphere over an 8–12-week period following polar sunrise – appear to increase Hg deposition to environs near the Arctic Ocean with a lesser impact inland. A key concern is whether such events lead to enhanced production and uptake of the toxic methylmercury (MeHg) species into arctic food webs. Here, we used mosquitoes, which are sensitive and site-specific bioindicators of Hg loadings, to assess the impact of AMDE on ecosystem MeHg contamination along a 200-km transect between the Arctic Ocean coast and foothills of the Brooks Range, where gross atmospheric Hg deposition appears to be ~20-fold less than that near the coast. This preliminary survey revealed little variation and no gradient in mosquito MeHg levels, which suggests comparable ecosystem impact. This may also point to significant cycling and reemission (e.g. via photoreduction) of Hg deposited during AMDE from the snow and ice pack to the atmosphere.

Accumulation of Inorganic and Methylmercury by Freshwater Phytoplankton in Two Contrasting Water Bodies

Phytoplankton concentrate mercury from their aqueous surroundings and represent the primary entry point for Hg in aquatic food webs. We used 203Hg to compare the uptake of inorganic mercury, Hg(II), and methylmercury, MeHg, in four phytoplankton species (a diatom, a chlorophyte, a cryptophyte, and a cyanobacterium) in two waters containing different concentrations of dissolved organic carbon (DOC). At steady state, volume concentration factors (VCFs) for Hg(II) in the four species were similar and ranged from 0.5 to 5 x 10(4) for both water types, whereas VCFs for MeHg exceeded those for Hg(II) and ranged from 1.3 to 14.6 x 10(5). The VCFs for MeHg in the three eukaryotic cells in the high DOC water were 2-2.6 times greater than those in the low DOC water, but the VCFs for the prokaryote were similar in both waters. Higher cell surface area to volume ratios correlated with increased MeHg concentrations but not with Hg(II). In both water types, VCFs of Hg(II) were similar for living and heat-killed cells, but the VCFs of MeHg were 1.5-5.0 times greater in living cells, suggesting an active uptake component for MeHg. Hg(II) and MeHg were entirely bound to cell surfaces of the dead cells, whereas 59-64% of the MeHg and 9-16% of the Hg(II) in living cells entered the cytoplasm.

Speciation of aqueous methylmercury influences uptake by a freshwater alga (Selenastrum capricornutum)

Uptake of methylmercury (MeHg) by the alga Selenastrum capricornutum was measured in freshwater batch culture bioassays. The concentration of MeHg in the alga increased rapidly (within 15 min), reached a maximum by 6 h, and then declined because of growth dilution. The alga's rapid growth rate (doubling time, approximately 10 h) contributed to the importance of growth dilution. Conditional first-order rate constants were calculated for uptake (k1 = 6.95 x 10(-9) L/cell/h) and growth (kG = 0.07/h). A competitive synthetic ligand, disodium ethylenediaminetetra-acetate, formed strong complexes with MeHg and reduced MeHg uptake, consistent with the biotic ligand model. A conditional equilibrium formation constant (K) for the MeHg-algae complex was estimated to be approximately 10(16) and was used to model the influence of natural ligands on MeHg bioavailability. Model results suggested MeHg would be most bioavailable at concentrations of dissolved organic matter (DOM) less than 10 mg/L and increasingly unavailable at higher DOM concentrations for the specific humic acid modeled. Similarly, at molar concentrations of sulfide (and, possibly, metal-sulfide clusters) equal to approximately half the MeHg concentration, MeHg was predicted to be unavailable to algae because of the formation of strong 2:1 MeHg-sulfide complexes.

Modelling Human Exposure of Methylmercury from Fish Consumption

Abstract Mercury and its compounds are widely distributed in the environment and the principal cause of methylmercury accumulation in humans is fish consumption. The rate of methylmercury accumulation depends on many factors including the amount, size, type and frequency of fish consumed, as well as contamination levels in the aquatic habitat. The ability to predict accurately human exposure to methylmercury through fish consumption is essential to the setting of public consumption guidelines. This paper describes the development of an innovative method of estimating human exposure to methylmercury through sport fish consumption by mathematical modelling. Through a judicious combination of fish methylmercury bioaccumulation models and survey information on human fish-eating habits, the model allows for a scientifically based estimation of the average daily exposure to methylmercury from fish consumption. It provides a practical tool to estimate the methylmercury uptake from a fish diet as governed by the diet frequency, fish species and fish size. The efficacy of the model is demonstrated by application to six common Lake Ontario fish species. Results showed that the human methylmercury exposure from fish consumption is a serious issue, as demonstrated by the exceedance of the tolerable daily intake levels in many instances. It was also found that the level of human methylmercury uptake depends heavily on the species of fish consumed; among the six species studied, walleye carries the highest risk, followed by yellow perch, while rainbow trout seems to be the safest with the lowest bioaccumulation levels.

Methylmercury uptake and distribution kinetics in sheepshead minnows, Cyprinodon variegatus, after exposure to Ch3Hg-spiked food.

The distribution kinetics of methylmercury (CH3Hg[II]) was determined in sheepshead minnows (Cyprinodon variegatus) after a single dose of different CH3Hg(II)-spiked food to determine what factors influence the bioavailability, uptake, and redistribution of CH3Hg(II) to various organs of C. variegatus. The kinetics of CH3Hg(II) distribution was measured in the different organs during a period of 0.1 to 35 d after dosage. The CH3Hg(II) distribution kinetics in the different tissues was modeled using a simple multicompartmental pharmacokinetic model, which assumed that blood was the conduit linking the CH3Hg(II) exchange between the different organs. The CH3Hg(II) was taken up into the intestinal tissue within hours after feeding, followed by a slow release to the blood and the other organs of the body. Exchange between the blood and the visceral organs was relatively slow, with maximum CH3Hg(II) uptake in the liver and gill occurring at 1.5 d following dietary exposure. Subsequently, the majority of the CH3Hg(II) was channeled from the viscera to the rest of the body with a substantial lag time after feeding. However, the rate of transfer between tissues in the studies reported here were faster than those measured by others for larger fish.

Influences of different selenium species on the uptake and assimilation of Hg(II) and methylmercury by diatoms and green mussels

We examined the influences of different concentrations and species of Se (selenite, selenate, and seleno -l-methionine) in the ambient environment on the accumulation of inorganic Hg(II) and methylmercury (MeHg) by the diatom Thalassiosira pseudonana and the green mussel Perna viridis. At the experimental concentrations tested (<500 μg l −1), selenite and selenate did not significantly affect the uptake of either mercury species by the diatoms and the green mussels. The assimilation efficiency of Hg(II) and MeHg by the mussels from ingested diatoms was also independent of the inorganic Se loadings in the food particles. In contrast, selenomethionine significantly inhibited the uptake of MeHg and enhanced the uptake of Hg(II) by the diatoms and the mussels, but it did not affect the assimilation from the ingested diatoms. The influence of tissue body burden of Se in the green mussels following pre-exposure to selenite and selenomethionine for different periods (1–5 weeks) on the accumulation of Hg(II) and MeHg was further investigated. Our results showed that tissue Se concentrations did not significantly affect the dietary assimilation of mercury, but the influences on the aqueous uptake were variable. Our study thus, strongly highlights the specificity of the Se–Hg interaction in marine mussels for different Se and Hg species. Both dissolved and dietary uptake appeared to be equally important in the accumulation of Hg(II) and MeHg in the green mussels.

Uptake of Hg(II) and methylmercury by the green mussel Perna viridis under different organic carbon conditions

The bioavailability and toxicity of mercury to aquatic organisms are greatly dependent on its chemical species, such as inorganic and methylated mercury. We examined the influences of various physico-chemical parameters (metal concentration, salinity, dissolved organic carbon (DOC) and colloidal organic carbon (COC) on the bioavailability of inorganic mercury (Hg(II)) and mono- methylmercury (MeHg) to the green mussel Perna viridis. The uptake rate constant of MeHg was about 8.5x higher than that of Hg(II). The uptake increased by 75% for Hg(II) and 117% for MeHg with decreasing salinity from 30 psu to 15 psu. Biogenic DOC derived from diatom decomposition decreased the uptake of Hg(II), consistent with the results of DOC originating from natural seawater. In contrast, the uptake of Hg(II) was enhanced considerably in the presence of humic acid. The bioavailability of MeHg was only weakly influenced by differences in DOC quality or quantity, but it was significantly decreased by estuarine DOC. The influence of COC on Hg bioavailability was also dependent on the colloidal structural properties and the colloid-metal complexation. Binding with colloidal nanoparticles increased the uptake of Hg(II) by 3.3 to 7.3 times compared to low molecular weight-complexed Hg, but decreased the uptake of MeHg by 42 to 73%. For both Hg(II) and MeHg, the uptake of the 9 d aged radiolabeled metals was lower than the uptake of the 2 d aged radiola- beled forms. Our study suggests that various mechanisms, such as facilitated transport and direct col- loidal ingestion, are involved in the uptake of Hg(II) and MeHg under different dissolved and col- loidal organic carbon conditions. COC and DOC need to be separately considered, given their different influences on mercury uptake.

Uptake and elimination routes of inorganic mercury and methylmercury in Daphnia magna.

Mercury (Hg) is an important environmental pollutant due to its highly toxic nature and widespread occurrence in aquatic systems. The biokinetics of Hg in zooplankton have been largely ignored in previous studies. This study examines the assimilation, dissolved uptake, and efflux of inorganic mercury [Hg(II)] and methylmercury (MeHg) in a freshwater cladoceran, Daphnia magna, and models the exposure pathways of Hg(II) and MeHg in the daphnids. The assimilation efficiencies (AEs) of both Hg species decreased significantly with increasing algal carbon concentrations. The dissolved uptake of Hg(II) and MeHg was proportional to the ambient concentration (ranging from environmentally realistic to high concentration over a 3-4 orders of magnitude variation), whereas MeHg had a slightly higher uptake rate constant (0.46 L g(-1) h(-1)) than Hg(II) (0.35 L g(-1) h(-1)). Surprisingly, the efflux rate constants of Hg(ll) and MeHg were rather comparable (0.041 -0.063 day(-1)). The release of both Hg(II) and MeHg via different routes (excretion, egestion, molting, and neonate production) was further examined at different food concentrations. It was found that regeneration into the dissolved phase was important for D. magna to eliminate both Hg species, but maternal transfer of Hg(II) (11-15%) and MeHg (32-41%) to neonates represented another important pathway for the elimination of Hg(II) and MeHg from the mothers. Modeling results suggest that food is an important source for MeHg exposure (47-98%), but water exposure represents 31-96% of Hg(II) accumulation in D. magna, depending on the variation of Hg bioconcentration factor in ingested food. Furthermore, MeHg predominates the bioaccumulation of Hg in D. magna even though MeHg constitutes only a small percentage of the total Hg in the water. The results strongly indicate that maternal transfer of Hg(II) and MeHg in freshwater zooplankton should be considered in manytoxicity testings and risk assessment in aquatic food chains.

Use of an Integrated Mercury Food Web Model for Ecological Risk Assessment

We developed an integrated food web model for mercury, using the CATS (Contaminants in Aquatic and Terrestrial ecoSytems) model developed by Traas and co-workers (Traas, T.P.; Stäb, J.A.; Kramer, P.R.G.; Cofino, W.P.; Aldenberg, T. Modeling and risk assessment of tributyltin accumulation in the food web of a shallow freshwater lake. Environ. Sci. Technol. 1996, 30 (4), 1227–1237). It was translated into the MatLab mathematical programming language and been adapted for the modeling of mercury. Mercury CATS now models four species of mercury (unreactive mercury, zero valent mercury, divalent mercury, and methylmercury) and their environmental and interactions using the mercury speciation kinetics model article by Bale (Bale, A.E. Modeling aquatic mercury fate in Clear Lake, Calif. J. Env. Eng. 2000, 126 (2), 153–163.) as a guideline. The mercury cycling kinetics include methylation of divalent mercury, photo-oxidation of zero valent mercury, reduction of divalent mercury, adsorption and desorption of divalent and methylmercury, volatilization of zero valent mercury, and uptake into the food web. A lake food web consisting of phytoplankton, rooted vegetation, zooplankton, crayfish, freshwater clams, amphipods, chironomids, eels, killifish, bass, and ospreys traces growth of the various compartments, uptake of both divalent and methylmercury, and the movement of both divalent and methylmercury through the food web via feeding relationships. This model design is sufficiently flexible that the incorporation of new scientific information can be easily done as our understanding of both the organisms and mercury's environmental chemistry increases.

Relationships between mercury body concentrations, standard metabolic rate, and body mass in eastern mosquitofish (Gambusia holbrooki) from three experimental populations

Eastern mosquitofish (Gambusia holbrooki) were sampled from three experimental populations (two Hg-exposed populations and one reference population) to determine whether transgenerational exposure (lifelong exposure of multiple generations) to Hg adversely affects standard metabolic rate (SMR). Mosquitofish subjected to lifelong Hg exposure accumulated significant concentrations of Hg in their tissues compared to fish from the reference population (mean: 3.89-4.13 vs 0.08 microHg/g wet mass, respectively). Less than 10% of the variability in Hg tissue concentrations could be explained by fish body mass, likely because of the short life span and/or dietary habits of this species. Despite the high body burdens of Hg in exposed fish, we found no significant difference in SMR among individuals from Hg-exposed or reference populations. Our findings contrast recent laboratory work describing elevated SMR in mosquitofish exposed to 100 microg/L dissolved inorganic Hg for 48 h. To account for contrasting results between studies, we hypothesize that acute exposure to dissolved inorganic Hg damages gill epithelium, resulting in increased metabolic rate, but that lifelong Hg exposure via trophic uptake of methyl mercury does not affect fish respiratory structures. Alternative hypotheses include the possibility that G. holbrooki is a species that can tolerate high body burdens of Hg or that more than four years of genetic isolation during Hg exposure (8-12 generations) resulted in selection for Hg-tolerant or -resistant individuals.

The role of ground vegetation in the uptake of mercury and methylmercury in a forest ecosystem

Litterfall from trees has been identified as an important pathway for deposition of mercury (Hg) and methylmercury (MeHg) in forested catchments, but very little is known about the role of ground vegetation in deposition and cycling of Hg compounds. This study was conducted to identify the origin of Hg compounds in the ground vegetation, and to estimate the role of its litterfall with respect to pools and fluxes of Hg in a coniferous forest in the German Fichtelgebirge mountains. Above and below ground biomass of the dominant ground vegetation (Vaccinium myrtillus, Deschampsia flexuosa and Calamagrostis villosa) were sampled at several plots successively during the growing season. The fluxes to the soil via litterfall of the ground vegetation were calculated using contents of Hg and MeHg in the annual fractions of aboveground biomass. With fluxes of 0.4 – 7.8 mg Hgtotal ha−1 a−1 and 0.01 – 0.04 mg MeHg ha−1 a−1 (depending on the plant species) this pathway contributes only a few percent to the total deposition of both compounds in the catchment. To identify the uptake pathways of Hg compounds, the same plant species were grown in a pot experiment with addition of isotope labelled Hg compounds (202Hg2+, Me198Hg) to a clean sand substrate. Only small proportions of 202Hg and Me198Hg in the substrate were taken up by the plants, but in all cases the proportion translocated into aboveground biomass after uptake was greater in case of Me198Hg. Thus, internal recycling in the plant-soil system is a source especially for MeHg in the ground vegetation. However, as compared to the input of Hg compounds by tree litterfall and storage in the forest floor, Hgtotal and MeHg in ground vegetation are of minor importance. High volatilization of added Hg isotopes raises the question of a re-emission of Hg compounds by the transpiration flux of the ground vegetation.

Methyl mercury uptake and associations with the induction of chromosomal aberrations in Chinese hamster ovary (CHO) cells

In order to evaluate possible health effects of environmental exposure of humans towards methyl mercury species, relevant exposure experiments using methyl mercury chloride in aqueous solution and Chinese hamster ovary (CHO) cells were performed. The solution was monitored for the presence of monomethyl, dimethyl and elemental mercury by several analytical techniques including chromatographic as well as atomic absorption and mass spectrometric methods. Methyl mercury induces structural chromosomal aberrations (CA) and sister chromatid exchanges (SCE) in CHO cells. At a concentration of methyl mercury in the culture medium of 1.0×10 −6 M where the frequencies of CA and SCE are significantly elevated, the intracellular concentration was 1.99×10 −16 mol/cell. Possible biochemical processes leading to the cytogenetic effects are discussed together with toxicological consequences, when humans (e.g. workers at waste deposits) are exposed to environmental concentrations of methyl mercury.

Methylmercury accumulation and fluxes across the intestine of channel catfish, Ictalurus punctatus

The excised intestines of channel catfish, Ictalurus punctatus, were perfused at 20 or 4 °C for 1 h 45 min, with methylmercury (CH 3HgCl) alone, or in the presence of excess l-cysteine ( l-Cys), d-cysteine ( d-Cys), l-methionine ( l-Met); or with ouabain or probenecid to identify the potential CH 3Hg(II) uptake pathways in fish intestines. A temperature effect was noted, with CH 3Hg(II) concentrations in tissues perfused at 20 °C being higher than at 4 °C, substantiating the idea that mechanisms requiring metabolic energy are involved in CH 3Hg(II) uptake in fish intestines. The results indicate that, when CH 3Hg(II) is complexed as the CH 3Hg- l-Cys complex, it is taken up via an l-neutral amino acid carrier and rapidly transported to the serosal side of the intestine. Methylmercury uptake could be inhibited by probenecid and ouabain, although probenecid had less impact on CH 3Hg(II) uptake than ouabain. Our results for CH 3Hg(II) uptake in the presence of d-Cys, l-Met in excess of l-Cys, or with a metal mixture further established that CH 3Hg(II) uptake across fish intestines occurs via a variety of pathways, including an energy-dependent l-neutral amino acid carrier, and that the route and amount of accumulation were a function of CH 3Hg(II) speciation in the digestive tract of the fish.