Soil Geobacteraceae are the key predictors of neurotoxic methylmercury bioaccumulation in rice.

Contamination of rice by the potent neurotoxin methylmercury (MeHg) originates from microbe-mediated Hg methylation in soils. However, the high diversity of Hg methylating microorganisms in soils hinders the prediction of MeHg formation and challenges the mitigation of MeHg bioaccumulation via regulating soil microbiomes. Here we explored the roles of various cropland microbial communities in MeHg formation in the potentials leading to MeHg accumulation in rice and reveal that Geobacteraceae are the key predictors of MeHg bioaccumulation in paddy soil systems. We characterized Hg methylating microorganisms from 67 cropland ecosystems across 3,600 latitudinal kilometres. The simulations of a rice-paddy biogeochemical model show that MeHg accumulation in rice is 1.3-1.7-fold more sensitive to changes in the relative abundance of Geobacteraceae compared to Hg input, which is recognized as the primary parameter in controlling MeHg exposure. These findings open up a window to predict MeHg formation and accumulation in human food webs, enabling more efficient mitigation of risks to human health through regulations of key soil microbiomes.

A hidden demethylation pathway removes mercury from rice plants and mitigates mercury flux to food chains.

Dietary exposure to methylmercury (MeHg) causes irreversible damage to human cognition and is mitigated by photolysis and microbial demethylation of MeHg. Rice (Oryza sativa L.) has been identified as a major dietary source of MeHg. However, it remains unknown what drives the process within plants for MeHg to make its way from soils to rice and the subsequent human dietary exposure to Hg. Here we report a hidden pathway of MeHg demethylation independent of light and microorganisms in rice plants. This natural pathway is driven by reactive oxygen species generated in vivo, rapidly transforming MeHg to inorganic Hg and then eliminating Hg from plants as gaseous Hg°. MeHg concentrations in rice grains would increase by 2.4- to 4.7-fold without this pathway, which equates to intelligence quotient losses of 0.01-0.51 points per newborn in major rice-consuming countries, corresponding to annual economic losses of US$30.7-84.2 billion globally. This discovered pathway effectively removes Hg from human food webs, playing an important role in exposure mitigation and global Hg cycling.

New insights into HgSe antagonism: Minor impact on inorganic Hg mobility while potential impacts on microorganisms.

Selenium (Se) is a crucial antagonistic factor of mercury (Hg) methylation in soil, with the transformation of inorganic Hg (IHg) to inert mercury selenide (HgSe) being the key mechanism. However, little evidence has been provided of the reduced Hg mobility at environmentally relevant doses of Hg and Se, and the potential impacts of Se on the activities of microbial methylators have been largely ignored. This knowledge gap hinders effective mitigation for methylmercury (MeHg) risks, considering that Hg supply and microbial methylators serve as materials and workers for MeHg production in soils. By monitoring the mobility of IHg and microbial activities after Se spike, we reported that 1) active methylation might be the premise of HgSe antagonism, as higher decreases in MeHg net production were found in soils with higher constants of Hg methylation rate; 2) IHg mobility did not significantly change upon Se addition in soils with high DOC concentrations, challenging the long-held view of Hg immobilization by Se; and 3) the activities of iron-reducing bacteria (FeRB), an important group of microbial methylators, might be potentially regulated by Se addition at a dose of 4 mg/kg. These findings provide empirical evidence that IHg mobility may not be the limiting factor under Se amendment and suggest the potential impacts of Se on microbial activities.

Event-Based Atmospheric Precipitation Uncovers Significant Even and Odd Hg Isotope Anomalies Associated with the Circumpolar Vortex.

The determination of the mass-independent fractionation of even Hg isotopes (even-MIF, Δ200Hg) in atmospheric samples adds another intriguing feature to the Hg isotope system. Despite our lack of sufficient experimental verification and the momentary absence of a valid mechanism to explain its occurrence, even-MIF could be instrumental in understanding the cycle and deposition of atmospheric Hg. In contrast to slightly positive Δ200Hg values (<0.30‰) frequently observed in most atmospheric samples, large Δ200Hg values (up to 1.24‰) determined in precipitation from Peterborough (Ontario, Canada) stand out and could provide valuable information for the origin of the even-MIF mystery. We now report a systematic analysis of high-resolution rainfall and snowfall samples collected in winter during cold weather at Peterborough, Canada. Dissolved and particulate Hg both displayed large variations of odd-MIF (from -0.93‰ to 2.02‰ for Δ199Hg), which may result from long-range transportation, as the negative odd-MIF in particulate Hg is likely a result of long-distance transport of arctic atmospheric Hg(II). Dissolved Hg revealed significant even-MIF values (from 0.25‰ to 1.19‰ for Δ200Hg) and a negative relationship between Δ200Hg and Δ204Hg, which provide further evidence for the previously proposed conceptual model of Δ200Hg. Disconnected odd-MIF and even-MIF trends were detected in sequentially collected precipitation samples, which further suggests different sources or mechanisms for Δ199Hg and Δ200Hg. Particularly, the high Δ200Hg values highlight the transport of stratospheric Hg through a polar vortex to the sampling region, stimulating further systematic investigation. The new Δ200Hg data for particulate Hg add to existing information on atmospheric Hg(II) worldwide, suggesting a global distribution of Hg characterized by even-MIF in the atmosphere, and further constrain the model of atmospheric Hg deposition.

Mercury Isotope Variations in Lake Sediment Cores in Response to Direct Mercury Emissions from Non-Ferrous Metal Smelters and Legacy Mercury Remobilization.

Nature archives record atmospheric mercury (Hg) depositions from directly emitted Hg and re-emitted legacy Hg. Tracing the legacy versus newly deposited Hg is still, however, challenging. Here, we measured Hg isotope compositions in three dated sediment cores at different distances from the Flin Flon smelter, the largest Canadian Hg sources to the atmosphere during the 1930s-2000s. During the smelter's operative period, Hg isotope compositions showed limited variations in the near-field lake (<10 km) sediments but were rather variable in middle- (20-75 km) and far-field lake (∼800 km) sediments. Only the post-2000 sediments in middle/far-field lakes showed significantly negative Hg isotope shifts, while sediments from the 1970s-1990s had Hg isotope values resembling those of near-field lake post-1930 sediments. We suggest that the smelter's peak Hg emissions during the 1970s-1990s, which coincided with the deployment of a super stack in the mid-1970s, largely increased the long-range dispersion of smelter plumes. For the top post-2000 sediments, the fugitive dust from ore tailings and terrestrial legacy Hg re-emissions dominated Hg deposition in near-field lakes and middle/far-field lakes, respectively. Our study demonstrates that legacy Hg remobilization now exports substantial amounts of Hg to ecosystems, highlighting the need for aggressive remediation measures of Hg-contaminated sites.

Sources of riverine mercury across the Mackenzie River Basin; inferences from a combined HgC isotopes and optical properties approach.

The Arctic environment harbors a complex mosaic of mercury (Hg) and carbon (C) reservoirs, some of which are rapidly destabilizing in response to climate warming. The sources of riverine Hg across the Mackenzie River basin (MRB) are uncertain, which leads to a poor understanding of potential future release. Measurements of dissolved and particulate mercury (DHg, PHg) and carbon (DOC, POC) concentration were performed, along with analyses of Hg stable isotope ratios (incl. ∆199Hg, δ202Hg), radiocarbon content (∆14C) and optical properties of DOC of river water. Isotopic ratios of Hg revealed a closer association to terrestrial Hg reservoirs for the particulate fraction, while the dissolved fraction was more closely associated with atmospheric deposition sources of shorter turnover time. There was a positive correlation between the ∆14C-OC and riverine Hg concentration for both particulate and dissolved fractions, indicating that waters transporting older-OC (14C-depleted) also contained higher levels of Hg. In the dissolved fraction, older DOC was also associated with higher molecular weight, aromaticity and humic content, which are likely associated with higher Hg-binding potential. Riverine PHg concentration increased with turbidity and SO4 concentration. There were large contrasts in Hg concentration and OC age and quality among the mountain and lowland sectors of the MRB, which likely reflect the spatial distribution of various terrestrial Hg and OC reservoirs, including weathering of sulfate minerals, erosion and extraction of coal deposits, thawing permafrost, forest fires, peatlands, and forests. Results revealed major differences in the sources of particulate and dissolved riverine Hg, but nonetheless a common positive association with older riverine OC. These findings reveal that a complex mixture of Hg sources, supplied across the MRB, will contribute to future trends in Hg export to the Arctic Ocean under rapid environmental changes.

Experimental evidence for recovery of mercury-contaminated fish populations.

Anthropogenic releases of mercury (Hg)1-3 are a human health issue4 because the potent toxicant methylmercury (MeHg), formed primarily by microbial methylation of inorganic Hg in aquatic ecosystems, bioaccumulates to high concentrations in fish consumed by humans5,6. Predicting the efficacy of Hg pollution controls on fish MeHg concentrations is complex because many factors influence the production and bioaccumulation of MeHg7-9. Here we conducted a 15-year whole-ecosystem, single-factor experiment to determine the magnitude and timing of reductions in fish MeHg concentrations following reductions in Hg additions to a boreal lake and its watershed. During the seven-year addition phase, we applied enriched Hg isotopes to increase local Hg wet deposition rates fivefold. The Hg isotopes became increasingly incorporated into the food web as MeHg, predominantly from additions to the lake because most of those in the watershed remained there. Thereafter, isotopic additions were stopped, resulting in an approximately 100% reduction in Hg loading to the lake. The concentration of labelled MeHg quickly decreased by up to 91% in lower trophic level organisms, initiating rapid decreases of 38-76% of MeHg concentration in large-bodied fish populations in eight years. Although Hg loading from watersheds may not decline in step with lowering deposition rates, this experiment clearly demonstrates that any reduction in Hg loadings to lakes, whether from direct deposition or runoff, will have immediate benefits to fish consumers.

Mercury methylation rates in Deception Island (Maritime Antarctica) waters and pyroclastic gravel impacted by volcanic mercury.

Deception Island is an active volcano in the Antarctica being volcanism a source of mercury. To improve the understanding of the Hg cycle in this remote ecosystem, pyroclastic gravel and water samples were collected and total (THg) and monomethylmercury (MMHg) concentrations were measured as well as the potential for Hg methylation. Gravel samples collected close to active fumaroles showed the highest THg levels (72 ng/g) while in water samples the highest concentrations of THg (1.2 ng/L) and MMHg (0.45 ng/L) where found. Methylation activity was barely observable in gravel samples. Biotic methylation rates in water were up to 13 times higher compared to those recorded previously in other polar waters. Abiotic methylation processes also play an important role, with up to 0.54 ± 0.43% of added Hg converted instantaneously to MMHg. These results suggest that Deception Island presents favourable conditions for MMHg explaining the elevated concentrations of both THg and MMHg in this ecosystem.

Mercury and methylmercury levels in soils associated with coal-fired power plants in central-northern Chile.

Mercury pollution is a worldwide problem, and is associated with a number of natural and anthropogenic processes. The present work, conducted in Chile, a country that has traditionally depended heavily on fossil fuels for power generation, examines total mercury (THg) and monomethylmercury (MMHg) concentrations in soils across different sites exposed to coal fired power plant emissions. Samples from four selected (Renca, Laguna Verde, Las Ventanas, Huasco) and 1 control (Quintay) sites were analyzed using cold vapour and fluorescence spectroscopy (CV-AFS) for THg determination and chromatographic separation with atomic fluorescence detection (DI-GC-AFS) was followed for speciation analysis. From the sites analyzed, Renca and Las Ventanas showed high concentrations of total mercury, exhibiting ranges between 135 - 568 and 94-464 ng g-1 respectively, while Laguna Verde and Huasco exhibited lower values ranged 5-27 and 9-44 ng g-1 respectively. Conversely, analysis of MMHg concentrations showed that only Renca site possessed high values, ranging between 0.1 and 3.0 ng g-1, resulting in this site being considered contaminated. Conversely, other sites showed minimal values comparable to the control site (0.024 ±â€¯0.003 ng g-1) in terms of MMHg concentrations. An analysis of the differences between MMHg and THg concentrations in contaminated sites, suggests an overall absence of methylation in soils of Las Ventanas, probably related to the very high levels of soil heavy metals, especially copper. Moreover, the influence of the composition and physicochemical properties of the different soils on the mobility of the species was assessed. Results obtained (as Log Kd) were 3.5 and 4.1 for Renca and Las Ventanas respectively, suggesting low mobility of mercury species in the environment for both sites. Finally, the data obtained allowed us to establish a first approximation of the differences in concentration and mobility of total and MMHg associated with coal fired power plants emission in central-northern Chile, an area previously understudied in a country heavily dependent on fossil-fuels.

Sources of mercury in deep-sea sediments of the Mediterranean Sea as revealed by mercury stable isotopes.

Mercury (Hg) and its stable isotope composition were used to determine the sources of Hg in deep-sea sediments of the Mediterranean Sea. Surface and down-core sediment δ202Hg values varied widely between -2.30 and +0.78‰, showed consistently positive values for mass independent fractionation of odd Hg isotopes (with average values of Δ199Hg = +0.10 ± 0.04‰ and Δ201Hg = +0.04 ± 0.02‰) and near-zero Δ200Hg values, indicating either multiple Hg sources or a combination of different Hg isotope fractionating processes before and after sediment deposition. Both mass-dependent and mass-independent fractionation processes influence the isotopic composition of Hg in the Mediterranean Sea. Positive Δ199Hg values are likely the result of enhanced Hg2+ photoreduction in the Mediterranean water column before incorporation of Hg into sediments, while mass-dependent fractionation decreases δ202Hg values due to kinetic isotope fractionation during deposition and mobilization. An isotope mixing model based on mass-dependent and mass-independent fractionation (δ202Hg and Δ199Hg) suggests at least three primary Hg sources of atmospheric deposition in the surface sediments: urban, industrial and global precipitation-derived. Industry is the main source of Hg in Algerian and Western Basin surface sediments and at two sites in the Adriatic Sea, while the urban contribution is most prominent at the Strait of Otranto (MS3) and in Adriatic surface sediments. The contribution from precipitation ranged from 10% in Algerian to 37% in W Basin sediments. Overall, results suggest that atmospheric Hg deposition to Mediterranean surface sediments is dominated by gaseous elemental mercury (58 ± 11%) rather than wet deposition.

Mercury speciation and mercury stable isotope composition in sediments from the Canadian Arctic Archipelago.

Total mercury (THg) and monomethylmercury (MMHg) concentrations as well as mercury (Hg) isotope ratios were determined in sediment cores sampled from six locations from the Canadian Arctic Archipelago (CAA). At most sites, THg concentrations showed a decreasing trend with depth, ranging from 5 to 61 ng/g, implicating possible increased Hg deposition and/or riverine inputs in top sediment layers. MMHg values showed large oscillations within the top 10 cm of the cores. This variability decreased at the bottom of the cores with MMHg concentrations ranging from less than12 to up to 1073 pg/g. Average concentrations of THg and MMHg in the top 10 cm were linearly correlated, whereas no correlation was observed with organic matter (loss on ignition). Mercury isotope ratios showed negative values for both δ202Hg (-1.59 to -0.55‰) and Δ199Hg (-0.62 to -0.01‰). δ202Hg values became more negative with depth, while the opposite was observed for Δ199Hg. The former is consistent with predicted historical atmospheric Hg trends as a result of increased coal burning worldwide. Hg isotope ratio measurements in CAA sediments offer additional opportunities to trace Hg processes and sources in the Arctic.

Increased Methylmercury Accumulation in Rice after Straw Amendment.

Consumption of rice has been shown to be an important route of dietary exposure to methylmercury (MeHg, a neurotoxin) for Asians having a low fish but high rice diet. Therefore, factors that increase MeHg production and bioaccumulation in soil-rice systems, could enhance the risk of MeHg exposure. On the basis of a national-scale survey in China (64 sites in 12 provinces) and rice cultivation experiments, we report that straw amendment, a globally prevalent farming practice, could increase MeHg concentrations in paddy soils (11-1043%) and rice grains (95%). By carrying out a series of batch incubation, seedling uptake and sand culture experiments, we demonstrate that these increases could be attributed to (1) enhanced abundances/activities of microbial methylators and the transformation of refractory HgS to organic matter-complexed Hg, facilitating microbial Hg methylation in soils; (2) enhanced MeHg mobility, and increased root lengths (35-41%) and tip numbers (60-105%), increasing MeHg uptake by rice roots; and (3) enhanced MeHg translocation to rice grains from other tissues. Results of this study emphasize fresh organic matter-enhanced MeHg production and bioaccumulation, and highlight the increased risk of MeHg after straw amendment and thus the need for new policies concerning straw management.

Discerning the Sources of Silver Nanoparticle in a Terrestrial Food Chain by Stable Isotope Tracer Technique.

The increasing use of silver-containing nanoparticles (NPs) in commercial products has led to NP accumulation in the environment and potentially in food webs. Identifying the uptake pathways of different chemical species of NPs, such as Ag2S-NP and metallic AgNPs, into plants is important to understanding their entry into food chains. In this study, soybean Glycine max L. was hydroponically exposed to Ag2S-NPs via their roots (10-50 mg L-1) and stable-isotope-enriched 109AgNPs via their leaves [7.9 µg (g fresh weight)-1]. Less than 29% of Ag in treated leaves (in direct contact with 109AgNP) was accumulated from root uptake of Ag2S-NPs, whereas almost all of the Ag in soybean roots and untreated leaves sourced from Ag2S-NPs. Therefore, Ag2S-NPs are phytoavailable and translocate upward. During trophic transfer the Ag isotope signature was preserved, indicating that accumulated Ag in snails most likely originated from Ag2S-NPs. On average, 78% of the Ag in the untreated leaves was assimilated by snails, reinforcing the considerable trophic availability of Ag2S-NPs via root uptake. By highlighting the importance of root uptake of Ag2S-NPs in plant uptake and trophic transfer to herbivores, our study advances current understanding of the biogeochemical fate of Ag-containing NPs in the terrestrial environment.

Mercury mobility and effects in the salt-marsh plant Halimione portulacoides: Uptake, transport, and toxicity and tolerance mechanisms.

The plant Halimione portulacoides, an abundant species widely distributed in temperate salt-marshes, has been previously assessed as bioindicator and biomonitor of mercury contamination in these ecosystems. The present study aims to assess uptake and distribution of total mercury (THg) and methylmercury (MMHg) within H. portulacoides, potential mercury release by volatilization through leaves, and toxicity and tolerance mechanisms by investigating plant photochemical responses. Stem cuttings of H. portulacoides were collected from a salt-marsh within the Tagus estuary natural protected area, and grown under hydroponic conditions. After root development, plants were exposed to 199HgCl2 and CH3201HgCl, and sampled at specific times (0, 1, 2, 4, 24, 72, 120, 168 (7 days) and 432 h (18 days)). After exposure, roots, stems and leaves were analysed for total 199Hg (T199Hg) and MM201Hg content. Photobiology parameters, namely efficiency and photoprotection capacity, were measured in leaves. Both THg and MMHg were incorporated into the plant root system, stems and leaves, with roots showing much higher levels of both isotope enriched spikes than the other plant tissues. Presence of both mercury isotopes in the stems and leaves and high significant correlations found between roots and stems, and stems and leaves, for both THg and MMHg concentrations, indicate Hg translocation between the roots and above-ground organs. Long-term uptake in stems and leaves, leading to higher Hg content, was more influenced by temperature and radiation than short-term uptake. However, the relatively low levels of both THg and MMHg in the aerial parts of the plant, which were influenced by temperature and radiation, support the possibility of mercury release by stems and leaves, probably via stomata aperture, as a way to eliminate toxic mercury. Regarding photochemical responses, few differences between control and exposed plants were observed, indicating high tolerance of this salt marsh plant to THg and MMHg.

Evidence of Mercury Methylation and Demethylation by the Estuarine Microbial Communities Obtained in Stable Hg Isotope Studies.

Microbial activity is a critical factor controlling methylmercury formation in aquatic environments. Microbial communities were isolated from sediments of two highly mercury-polluted areas of the Tagus Estuary (Barreiro and Cala do Norte) and differentiated according to their dependence on oxygen into three groups: aerobic, anaerobic, and sulphate-reducing microbial communities. Their potential to methylate mercury and demethylate methylmercury was evaluated through incubation with isotope-enriched Hg species (199HgCl and CH3201HgCl). The results showed that the isolated microbial communities are actively involved in methylation and demethylation processes. The production of CH3199Hg was positively correlated with sulphate-reducing microbial communities, methylating up to 0.07% of the added 199Hg within 48 h of incubation. A high rate of CH3201Hg degradation was observed and >20% of CH3201Hg was transformed. Mercury removal of inorganic forms was also observed. The results prove the simultaneous occurrence of microbial methylation and demethylation processes and indicate that microorganisms are mainly responsible for methylmercury formation and accumulation in the polluted Tagus Estuary.

Accumulation of Silver in Yellow Perch ( Perca flavescens) and Northern Pike ( Esox lucius) From a Lake Dosed with Nanosilver.

A total of 15 kg of silver nanoparticles (AgNPs) was added continuously over two ice-free field seasons to a boreal lake (i.e., Lake 222) at the IISD Experimental Lakes Area in Canada. We monitored the accumulation of silver (Ag) in the tissues of yellow perch ( Perca flavescens) and northern pike ( Esox lucius) exposed to the AgNPs under environmentally relevant conditions. The greatest accumulation was observed in the liver tissues of pike, and a single pike sampled in the second year of additions had the highest concentration observed in liver of 5.1 micrograms per gram of wet weight. However, the Ag concentrations in gill and muscle tissue of both pike and perch did not exceed 0.35 micrograms per gram of wet weight. Following additions of AgNP, the Ag residues in fish tissues declined, with a half-life of Ag in pike liver of 119 days. Monitoring using passive sampling devices and single-particle inductively coupled plasma mass spectrometry during the AgNP addition phase confirmed that Ag nanoparticles were present in the water column and that estimated mean concentrations of Ag increased over time to a maximum of 11.5 µg/L. These data indicate that both a forage fish and a piscivorous fish accumulated Ag in a natural lake ecosystem dosed with AgNPs, leading to Ag concentrations in some tissues of the piscivorous species that were 3 orders of magnitude greater than the concentrations in the water.

Spatial and temporal trends in the fate of silver nanoparticles in a whole-lake addition study.

Studies of the fate and toxicity of nanoparticles, including nanosilver (AgNPs), have been primarily conducted using bench scale studies over relatively short periods of time. To better understand the fate of AgNPs in natural aquatic ecosystems over longer time scales and ecological settings, we released suspensions of AgNPs (30-50 nm, capped with polyvinylpyrrolidone) into a boreal lake at the Experimental Lakes Area in Canada. Approximately 9 kg of silver was added from a shoreline point source from June to October 2014, which resulted in total Ag (TAg) concentrations of about 10 µg L-1 or less. In addition, dissolved Ag concentrations (DAg) were typically very low. Using single particle inductively coupled plasma mass spectrometry (sp-ICP-MS) analysis of grab samples, we found that the nanoparticles typically ranged in the 40-60 nm size class and were widely distributed throughout the lake, while larger aggregates (i.e. >100 nm) were infrequently detected. The highest occurrence of aggregates was found near the addition site; however, size distributions did not vary significantly among spatial locations or time suggesting rapid dispersal upon entry into the lake. Lake stratification at the thermocline was not a barrier to mobility of the AgNPs, as the particles were also detected in the hypolimnion. Environmental factors influenced Ag size distributions over sampling locations and time. Total dissolved phosphorus, bacterioplankton chlorophyll-a, and sampling time strongly correlated with aggregation and dissolution dynamics. AgNPs thus appear to be relatively mobile and persistent over the growing season in lake ecosystems.

Distribution of mercury species and mercury isotope ratios in soils and river suspended matter of a mercury mining area.

Mercury (Hg) released by mining activities can be dispersed in the environment, where it is subject to species transformations. Hg isotope ratios have been used to track sources in Hg contaminated areas, although it is unclear to what extent variations in δ-values are attributed to distinct Hg species. Hg was mined as Hg sulphide (cinnabar) in Idrija, Slovenia for centuries. Sediments are loaded with mining-residues (cinnabar and calcine), whereas contaminated soils mainly contain Hg bound to natural organic matter (NOM-Hg) related to atmospheric Hg deposition. Hg released from soils and sediments is transported as suspended matter (SM) in the Idrijca river to the Gulf of Trieste (GT), Italy. We determine Hg isotope ratios in river SM, sediments and soils from the Idrijca-catchment to decipher the Hg isotope ratio variability related to Hg species distribution in different grain-size fractions. δ202Hg values of SM collected from tributaries corresponded to those found in soils ranging from -2.58 to 0.19‰ and from -2.27 to -0.88‰, respectively. Speciation measurements reveal that fine fractions (0.45-20 µm) are dominated by NOM-Hg, while larger fractions contain more cinnabar. More negative δ202Hg values were related to higher proportions of NOM-Hg, which are predominant in soils and SM. Rain events increase SM-loads in the river, mainly due to resuspension of coarse grain-size fractions of bottom sediments bearing larger proportions of cinnabar, which leads to more positive δ202Hg values. The large magnitude of variation in δ202Hg and the smaller magnitude of variation in Δ199Hg (-0.37 to 0.09‰) are likely related to fractionation during ore roasting. Soil samples with high NOM-Hg content show more negative δ202Hg values and larger variation of Δ199Hg. More negative δ202Hg values in GT sediments were rather linked to distant sedimentation of soil derived NOM-Hg than to sedimentation of autochthonous marine material. Heterogeneity in the Idrija ore and ore processing likely produce large variations in the Hg isotopic composition of cinnabar and released metallic Hg, which complicate the differentiation of Hg sources. Combining Hg isotope measurements with solid phase Hg speciation reveals that Hg isotope ratios rather indicate different Hg species and are not necessarily symptomatic for Hg pollution sources.

Risk-Benefit Assessment of Monomethylmercury and Omega-3 Fatty Acid Intake for Ringed Seal Consumption with Particular Emphasis on Vulnerable Populations in the Western Canadian Arctic.

Many northern Inuit communities rely on traditional food as major source of nourishment. An essential part of the traditional Arctic diet is marine mammals such as ringed seals or beluga. Being top predators, these animals are often highly contaminated with various toxins. In contrast, some tissues of marine mammals are also characterized by high amounts of n3-PUFAs (omega-3 polyunsaturated fatty acids). Here, we try to balance the risks associated with the consumption of different tissue types of ringed seals in terms of the neurotoxin monomethylmercury (MMHg) with the benefits of consumption due to high n3-PUFA concentrations. Fetuses are at the highest risk of neurological impairments because MMHg can easily cross the placental barrier. Therefore, women of childbearing age served as an indicator population for especially susceptible subpopulations. We calculated maximal weekly maternal portions sizes if mutual consumption of muscle and blubber tissue or liver and blubber tissue was assumed. Those weekly portion sizes resulted in an estimated overall IQ point gain of infants of 0, whereas the consumption of liver or muscle tissue without blubber could lead to an IQ loss. In contrast to former studies, our data do not generally prohibit the consumption of liver tissue. Instead, our results suggest that a maximal weekly consumption of 125 g liver tissue together with 1 g of blubber tissue is acceptable and does not lead to neurological damages in the long term. Similarly, the consumption of maximal 172 g muscle tissue can be balanced by the mutual consumption of 1 g blubber tissue.

Effects of exposure pathways on the accumulation and phytotoxicity of silver nanoparticles in soybean and rice.

The widespread use of silver nanoparticles (AgNPs) raises concerns both about their accumulation in crops and human exposure via crop consumption. Plants take up AgNPs through their leaves and roots, but foliar uptake has been largely ignored. To better understand AgNPs-plant interactions, we compared the uptake, phytotoxicity and size distribution of AgNPs in soybean and rice following root versus foliar exposure. At similar AgNP application levels, foliar exposure led to 17-200 times more Ag bioaccumulation than root exposure. Root but not foliar exposure significantly reduced plant biomass, while root exposure increased the malondialdehyde and H2O2 contents of leaves to a larger extent than did foliar exposure. Following either root or foliar exposure, Ag-containing NPs larger (36.0-48.9 nm) than the originally dosed NPs (17-18 nm) were detected within leaves. These particles were detected using a newly developed macerozyme R-10 tissue extraction method followed by single-particle inductively coupled plasma mass spectrometry. In response to foliar exposure, these NPs were stored in the cell wall and plamalemma of leaves. NPs were also detected in planta following Ag ion exposure, indicating their in vivo formation. Leaf-to-leaf and root-to-leaf translocation of NPs in planta was observed but the former did not alter the size distribution of the NPs. Our observations point to the possibility that fruits, seeds and other edible parts may become contaminated by translocation processes in plants exposed to AgNPs. These results are an important contribution to improve the risk assessment of NPs under environmental exposure scenarios.