Mercury biogeochemical cycling in the ocean and policy implications

Mercury contamination and potential health risk to French seabirds: A multi-species and multi-site study

Mercury (Hg) is a naturally occurring highly toxic element which circulation in ecosystems has been intensified by human activities. Hg is widely distributed, and marine environments act as its main final sink. Seabirds are relevant bioindicators of marine pollution and chicks are particularly suitable for biomonitoring pollutants as they reflect contamination at short spatiotemporal scales. This study aims to quantify blood Hg contamination and identify its drivers (trophic ecology inferred from stable isotopes of carbon (δ13C) and nitrogen (δ15N), geographical location, chick age and species) in chicks of eight seabird species from 32 French sites representing four marine subregions: the English Channel and the North Sea, the Celtic Sea, the Bay of Biscay and the Western Mediterranean. Hg concentrations in blood ranged from 0.04 μg g-1 dry weight (dw) in herring gulls to 6.15 μg g-1 dw in great black-backed gulls. Trophic position (δ15N values) was the main driver of interspecific differences, with species at higher trophic positions showing higher Hg concentrations. Feeding habitat (δ13C values) also contributed to variation in Hg contamination, with higher concentrations in generalist species relying on pelagic habitats. Conversely, colony location was a weak contributor, suggesting a relatively uniform Hg contamination along the French coastline. Most seabirds exhibited low Hg concentrations, with 74% of individuals categorized as no risk, and < 0.5% at moderate risk, according to toxicity thresholds. However, recent work has shown physiological and fitness impairments in seabirds bearing Hg burdens considered to be safe, calling for precautional use of toxicity thresholds, and for studies that evaluate the impact of Hg on chick development.

Factors controlling methylmercury concentration in soils of Northern Poland

Soil acts as storage for many toxic substances, including mercury and its compounds. However, in addition to its storage function, soil can also be a source of many substances to the aquatic environment. Methylmercury (MeHg) is one of the most toxic form of mercury (Hg) present in the environment. Some studies consider Poland to be one of the major emitters of Hg into both the atmosphere and the Baltic Sea. The purpose of the study was to identify factors affecting the formation and retention of MeHg in the soil as well as it remobilization to the river. Fifteen soil core samples with a length of 200 cm were collected during the fall/winter of 2021-2022. The factors responsible for the inflow and formation of MeHg were precipitation, distance from the riverbank, soil moisture and age of organic matter. MeHg can be transported to topsoil with precipitation. An increase in MeHg concentration was also observed in moist soils located in the vicinity of riverbank. MeHg concentration was lower in soils with degraded organic matter than with fresh organic matter.

System efficiency of methylmercury production, an important indicator for evaluating the long-term risk of mercury: From a case study in the Jiaozhou Bay to global coastal seas

With the implementation of the Minamata Convention, total Hg (THg) in coastal seas are expected to be reduced. However, methylmercury (MeHg) levels in aquatic environments depend not only on THg, but also the system efficiency of MeHg production (represented by MeHg/THg ratio in seawater) whose variations with time remain unclear. By choosing the Jiaozhou Bay (JZB) as a typical coastal system, combined with the published data from the global coastal systems, system efficiency of MeHg production in coastal seas were investigated. The mass budget of MeHg showed that the in situ production and degradation are the major source and sink of MeHg in the JZB. The relationships of MeHg in the seawater and fish with MeHg/THg in seawater indicate that system efficiency of MeHg production may control the risk of MeHg in coastal systems. The sulfate and nitrate in seawater, organic matter in sediment, biotic methylation, and transport of inorganic Hg from the seawater to sediment were identified to be critical parameters and processes for the MeHg/THg in global coastal seas. The findings of this study highlight the importance of monitoring the system efficiency of MeHg production and its controlling processes and parameters for evaluating the long-term risk of Hg.

Thermal desorption technique to speciate mercury in carbonate, silicate, and organic-rich soils

Thermal desorption is a well-assessed technique to speciate mercury (Hg) in soils and sediments. However, the effects related to the different matrices are still not properly assessed. In this study, thermal desorption was applied to Hg-free calcite mixed with Hg standard and soils rich in carbonate and silicate minerals, as well as organic matter. Hg0, HgCl2, HgO, α-HgS, β-HgS and organo-mercuric compounds were recognized, pointing out that the soil matrix operates notable differences in terms of breakdown temperatures of the Hg-compounds and suggesting that the mineralogical composition of soil has to be investigated before applying the thermal desorption technique. Furthermore, the presence of Hg0 was carefully evaluated since, as already observed, it forms Hg2+, which increases mercury mobility in the pedological cover with important consequences for those soils contaminated and located close to decommissioned or active mining areas and/or industrial sites (e.g. chloro-alkali industries). Experimental runs were thus carried out by using carbonate-, silicate- and organic-rich soils doped with liquid Hg. It was observed that Hg0 tends to be oxidized to form Hg+ and then Hg2+ as a function of soil matrix and reaction time. Surprisingly, the oxidation rate is rather fast, since after 42 days the initial content of Hg0 is halved, thus following an exponential decay. This implies that in Hg0-polluted areas, the fate of the resulting Hg2+ can be that to: i) be adsorbed by organic matter and/or Fe-Mn-Al oxides and/or ii) feed shallow aquifers. This study is a further step ahead to understand the behavior of Hg in contaminated soils from industrial and mining areas where liquid Hg is occurring in different soil matrices and may provide useful indications for remediation operations.

Isotope-Based Characterization of Soil Elemental Mercury Emissions from Historical Mercury Mining Areas: Driving Pathways and Relative Contributions

Photo-, microbial, and abiotic dark reduction of soil mercury (Hg) may all lead to elemental mercury (Hg(0)) emissions. Utilizing lab incubations, isotope signatures of Hg(0) emitted from mining soils were characterized to quantify the interplay and contributions of various Hg reduction pathways, which have been scarcely studied. At 15 °C, microbial reduced Hg(0) showed a negative mass-dependent fractionation (MDF) (δ202Hg = -0.30 ± 0.08‰, 1SD) and near-zero mass-independent fractionation (MIF) (Δ199Hg = 0.01 ± 0.04‰, 1SD), closely resembling dark reduced Hg(0) (δ202Hg = -0.18 ± 0.05‰, Δ199Hg = -0.01 ± 0.03‰, 1SD). In comparison, photoreduced Hg(0) exhibited significant MDF and MIF (δ202Hg = -0.55 ± 0.05‰, Δ199Hg = -0.20 ± 0.07‰, 1SD). In the dark, Hg isotopic signatures remained constant over the temperature range of 15-35 °C. Nonetheless, light exposure and temperature changes together altered Hg(0) MIF signatures significantly. Isotope mixing models along with Hg(0) emission flux data highlighted photo- and microbial reduction contributing 79-88 and 12-21%, respectively, of the total Hg(0) emissions from mining soils, with negligible abiotic dark reduction. Microorganisms are the key driver of soil Hg(0) emissions by first dissolving HgS and then promoting ionic Hg formation, followed by facilitating the photo- and microbial reduction of organically bound Hg. These insights deepen our understanding of the biogeochemical processes that influence Hg(0) releases from surface soils.

Mercury and stable nitrogen isotope ratios in the hair of bearded seals (Erignathus barbatus nauticus) from the Sea of Okhotsk

The mercury pollution status in the northwestern Sea of Okhotsk remains largely unexplored. In this study, hair samples were collected from 40 bearded seals harvested between August and October 2021 in the region. Total mercury (THg) concentrations in the samples exhibited a wide range from 137 to 1885 ng/g (median: 407 ng/g). While no significant differences in THg concentrations were found between male and female seals, distinctions were observed between young and potentially mature seals. Stable nitrogen isotope analysis indicated that juveniles and mature adults did not differ, although sample sizes were limiting. The higher THg concentrations in juveniles were attributed to variations in the seals' diets and/or variations in foraging locations during the juvenile stage which likely contribute to THg differences due to greater seasonal migration to offshore habitats. Notably, THg levels in bearded seals from the northwestern Sea of Okhotsk were lower in comparison to other pinniped species in the North Pacific. These findings, representing the first dataset for this pinniped species in the Russian segment of its habitat, contribute insights into mercury exposure in the Sea of Okhotsk mammalian population.

Near surface oxidation of elemental mercury leads to mercury exposure in the Arctic Ocean biota

Atmospheric mercury (Hg(0), Hg(II)) and riverine exported Hg (Hg(II)) are proposed as important Hg sources to the Arctic Ocean. As plankton cannot passively uptake Hg(0), gaseous Hg(0) has to be oxidized to be bioavailable. Here, we measured Hg isotope ratios in zooplankton, Arctic cod, total gaseous Hg, sediment, seawater, and snowpack from the Bering Strait, the Chukchi Sea, and the Beaufort Sea. The Δ200Hg, used to differentiate between Hg(0) and Hg(II), shows, on average, 70% of Hg(0) in all biota and differs with seawater Δ200Hg (Hg(II)). Since Δ200Hg anomalies occur via tropospheric Hg(0) oxidation, we propose that near-surface Hg(0) oxidation via terrestrial vegetation, coastally evaded halogens, and sea salt aerosols, which preserve Δ200Hg of Hg(0) upon oxidation, supply bioavailable Hg(II) pools in seawater. Our study highlights sources and pathways in which Hg(0) poses potential ecological risks to the Arctic Ocean biota.

Mediterranean Marine Mammals: Possible Future Trends and Threats Due to Mercury Contamination and Interaction with Other Environmental Stressors

Despite decreasing anthropogenic mercury (Hg) emissions in Europe and the banning and restriction of many persistent organic pollutants (POPs) under the Stockholm Convention, Mediterranean marine mammals still have one of the highest body burdens of persistent pollutants in the world. Moreover, the Mediterranean basin is one of the most sensitive to climate change, with likely changes in the biogeochemical cycle and bioavailability of Hg, primary productivity, and the length and composition of pelagic food webs. The availability of food resources for marine mammals is also affected by widespread overfishing and the increasing number of alien species colonizing the basin. After reporting the most recent findings on the biogeochemical cycle of Hg in the Mediterranean Sea and the physico-chemical and bio-ecological factors determining its exceptional bioaccumulation in odontocetes, this review discusses possible future changes in the bioavailability of the metal. Recent ocean-atmosphere-land models predict that in mid-latitude seas, water warming (which in the Mediterranean is 20% faster than the global average) is likely to decrease the solubility of Hg and favor the escape of the metal to the atmosphere. However, the basin has been affected for thousands of years by natural and anthropogenic inputs of metals and climate change with sea level rise (3.6 ± 0.3 mm year-1 in the last two decades), and the frequency of extreme weather events will likely remobilize a large amount of legacy Hg from soils, riverine, and coastal sediments. Moreover, possible changes in pelagic food webs and food availability could determine dietary shifts and lower growth rates in Mediterranean cetaceans, increasing their Hg body burden. Although, in adulthood, many marine mammals have evolved the ability to detoxify monomethylmercury (MMHg) and store the metal in the liver and other organs as insoluble HgSe crystals, in Mediterranean populations more exposed to the metal, this process can deplete the biological pool of Se, increasing their susceptibility to infectious diseases and autoimmune disorders. Mediterranean mammals are also among the most exposed in the world to legacy POPs, micro- and nanoplastics, and contaminants of emerging interest. Concomitant exposure to these synthetic chemicals may pose a much more serious threat than the Se depletion. Unfortunately, as shown by the literature data summarized in this review, the most exposed populations are those living in the NW basin, the main feeding and reproductive area for most Mediterranean cetaceans, declared a sanctuary for their protection since 2002. Thus, while emphasizing the adoption of all available approaches to mitigate anthropogenic pressure with fishing and maritime traffic, it is recommended to direct future research efforts towards the assessment of possible biological effects, at the individual and population levels, of chronic and simultaneous exposure to Hg, legacy POPs, contaminants of emerging interest, and microplastics.

Microorganisms Involved in Methylmercury Demethylation and Mercury Reduction are Widely Distributed and Active in the Bathypelagic Deep Ocean Waters

The ocean's mercury (Hg) content has tripled due to anthropogenic activities, and although the dark ocean (>200 m) has become an important Hg reservoir, concentrations of the toxic and bioaccumulative methylmercury (MeHg) are low and therefore very difficult to measure. As a consequence, the current understanding of the Hg cycle in the deep ocean is severely data-limited, and the factors controlling MeHg, as well as its transformation rates, remain largely unknown. By analyzing 52 globally distributed bathypelagic deep-ocean metagenomes and 26 new metatranscriptomes from the Malaspina Expedition, our study reveals the widespread distribution and expression of bacterial-coding genes merA and merB in the global bathypelagic ocean (∼4000 m depth). These genes, associated with HgII reduction and MeHg demethylation, respectively, are particularly prevalent within the particle-attached fraction. Moreover, our results indicate that water mass age and the organic matter composition shaped the structure of the communities harboring merA and merB genes living in different particle size fractions, their abundance, and their expression levels. Members of the orders Corynebacteriales, Rhodobacterales, Alteromonadales, Oceanospirillales, Moraxellales, and Flavobacteriales were the main taxonomic players containing merA and merB genes in the deep ocean. These findings, together with our previous results of pure culture isolates of the deep bathypelagic ocean possessing the metabolic capacity to degrade MeHg, indicated that both methylmercury demethylation and HgII reduction likely occur in the global dark ocean, the largest biome in the biosphere.

Modelling the anthropogenic Hg pollution fingerprint on the marine fishery production worldwide: A preliminary exposure assessment for people living in countries having different income levels

Mercury is a toxic pollutant that poses risks for the human population, mainly by eating contaminated fish. Mercury is released into the atmosphere from a variety of anthropogenic activities, with levels of emissions and under policy controls that largely vary across the world, leading thus to different relative contributions to the environmental matrices. Establishing the exact sources of this contaminant in the environment is crucial to optimising the policies aimed at mitigating the exposure risks for specific populations or ecosystems. In this study, we modelled, for the first time, the fingerprint of mercury anthropogenic emissions, jointly released by source-sectors (11) and source-regions (13), on the deposition over (19) FAO fishery zones, and on the FAO official fishery productions worldwide over the 2012-2021 decade. Using mercury anthropogenic emissions for 2012 from EDGAR, East Asia and "Artisanal and Small scale Gold Mining" result the source-region and the source-sector, respectively, that contribute the most to the mercury deposition over all the FAO fishery zones. The only exception applies for the FAO fishery zone 37, the Mediterranean Sea, where the "Industrial Combustion" from the closest Europe is the pair region-sector whose joint contribution is the greatest. When normalised to the overall fishery production worldwide, representing the global fish consumption, the anthropogenic mercury fingerprint showed a similar general pattern, however with notable differences, amplifying the relative contributions of all source-sectors from East Asia and attenuating the relative contributions of the regions in the Southern Hemisphere. This fingerprint further changes when the fish consumption in countries, classified by the World Bank as having different incomes, is considered. These results demonstrate that the same anthropogenic mercury deposited on any fishery zone actually affects in a different way the different population segments worldwide. This study aims to urge the science community as well as the policy makers to use a measure that better represents the mercury hazard for human health. Further, we hope that this study, using nomenclatures that are largely used on final shelf-product, could increase the people's awareness regarding the products they consume.

Distribution of mercury and methylmercury in aquacultured fish in special waters formed by coal mining subsidence

In China, fence net aquaculture practices have been established in some subsidence waters that have been formed in coal mining subsidence areas. Within this dynamic ecological context, diverse fish species grow continuously until being harvested at the culmination of their production cycle. The purpose of this study was to investigate diverse factors influencing the bioavailability and distribution of mercury (Hg) and methylmercury (MeHg), which have high physiological toxicity in fish, in the Guqiao coal mining subsidence area in Huainan, China. Mercury and MeHg were analyzed in 38 fish samples of eight species using direct mercury analysis (DMA-80) and gas chromatography-cold vapor atomic fluorescence spectrometry (GC-CVAFAS). The analysis results show that the ranges of Hg and MeHg content and methylation rate in the fish were 7.84-85.18 ng/g, 0.52-3.52 ng/g, and 0.81-42.68 %, respectively. Meanwhile, conclusions are also summarized as following: (1) Monophagous herbivorous fish that were fed continuously in fence net aquaculture areas had higher MeHg levels and mercury methylation rates than carnivorous fish. Hg and MeHg contents were affected by different feeding habits of fish. (2) Bottom-dwelling fish show higher MeHg levels, and habitat selection in terms of water depth also partially affected the MeHg content of fish. (3) The effect of fence net aquaculture on methylation of fish in subsidence water is mainly from feed and mercury-containing bottom sediments. However, a time-lag is observed in the physiological response of benthic fishes to the release of Hg from sediments. Our findings provides baseline reference data for the ecological impact of fence net aquaculture in waters affected by soil subsidence induced by coal mining in China. Prevalent environmental contaminants within coal mining locales, notably Hg, may infiltrate rain-induced subsidence waters through various pathways.

Particles-involved photochemical processes: A review for the case of mercury reduction in relation to aquatic mercury cycling

Mercury (Hg) is one of the toxic metals of global and environmental concern, with aquatic Hg cycling being central in determining the production of highly toxic methylmercury and the air-water Hg exchange influencing the long-range intercontinental atmospheric Hg transport. Both inorganic and organic forms of Hg can be bound by suspended particles, including inorganic minerals (in particular metal oxides/sulfides) and particulate organic matter. Photochemical transformation is a critical process in surface water, and the role of suspended particles in Hg redox photoreactions has increasingly emerged, albeit in limited studies in comparison to extensive studies on aqueous (homogeneous) photoreactions of Hg. The lack of understanding of what roles suspended particles play might result in inaccurate estimation of how Hg species transform and/or cycle in the environment. In view of this gap, this paper critically reviews and synthesizes information on the studies conducted on different natural surface waters with respect to the potential roles of suspended particles on Hg photo-redox reactions. It robustly discusses the various possible pathways and/or mechanisms of particle-mediated Hg (II) reduction, in enhancing or lowering the production of dissolved gaseous mercury. These processes include photo hole-electron pair formation and reactive oxygen species generation from particle excitation and their involvement in Hg photoreduction, in addition to the light attenuation effect of particles. This paper highlights the necessity of future studies exploiting these particles-mediated Hg photoreactions pathways and the implications of including these heterogeneous photoreactions (together with particulate elemental Hg species) on the air-water Hg exchange estimation.

Mercury and Selenium Accumulation in the Tissues of Stranded Bottlenose Dolphins (Tursiops truncatus) in Northeast Florida, 2013-2021

Bottlenose dolphins (Tursiops truncatus) are long-lived marine mammals, upper-level predators, and they inhabit near-shore environments, which increases their exposure to pollution. Mercury is a ubiquitous and persistent metal pollutant that can bioaccumulate and biomagnify up the food chain. Dolphins are known to accumulate mercury, and limited research has shown that mercury exposure can weaken the immune system of dolphins. The objectives of this study were to assess the mercury concentrations in the tissues (muscle, small intestine, liver) of stranded bottlenose dolphins and to compare the tissue mercury levels in dolphins that were stranded during the 2013-2015 morbillivirus Unusual Mortality Event (UME; immunosuppressed individuals) with the levels of those that were stranded at a normal rate (2016-2021). Selenium has been shown to reduce mercury toxicity in many animals; therefore, tissue selenium concentration and the molar ratio of selenium to mercury were also assessed. The tissue mercury (muscle, liver) and selenium (liver) concentrations increased with the age of the dolphins, with the liver accumulating the highest concentrations. No sex differences were observed in the mercury and selenium concentrations. While differences in tissue mercury concentrations were not observed due to the UME, the selenium accumulation profiles were significantly different between the two time periods. These results suggest that selenium may not have been as protective against mercury toxicity in the bottlenose dolphins that were stranded during the UME, possibly due to infection with morbillivirus.

Light-induced degradation of dimethylmercury in different natural waters

Photo-induced degradation of dimethylmercury (DMHg) is considered to be an important source for the generation of methylmercury (MMHg). However, studies on DMHg photodegradation are scarce, and it is even debatable about whether DMHg can be degraded in natural waters. Herein, we found that both DMHg and MMHg could be photodegraded in three natural waters collected from the Yellow River Delta, while in pure water only DMHg photodegradation occurred under visible light irradiation. The effects of different environmental factors on DMHg photodegradation were investigated, and the underlying mechanisms were elucidated by density functional theory calculations and a series of control experiments. Our findings revealed that the DMHg degradation rate was higher in the tidal creek water compared to Yellow River, Yan Lake, and purified water. NO3-, NO2-, and DOM could promote the photodegradation with DOM and NO3- showing particularly strong positive effects. Different light sources were employed, and UV light was found to be more effective in DMHg photodegradation. Moreover, MMHg was detected during the photodegradation of DMHg, confirming that the photochemical demethylation of DMHg is a source of MMHg in sunlit water. This work may provide a novel mechanistic insight into the DMHg photodegradation in natural waters and enrich the study of the global biogeochemical cycle of Hg.

Distribution of mercury in modern bottom sediments of the Beaufort Sea in relation to the processes of early diagenesis: Microbiological aspect

This study investigated the contents of total mercury (THg), trace metals, and CH4 and determined the signature microbes involved in various biogeochemical processes in the sediment of the Canadian Beaufort Sea. The THg ranged between 32 and 63 μg/kg and the trace metals such as Fe, Al, Mn, and Zn were significant in distributions. The pH, SO42-, Fe2+, and redox proxy metals were crucial factors in the spatial and vertical heterogeneity of geochemical distributions. CH4 was detected only at the mud volcano site. Microbial analyses identified Clostridium, Desulfosporosinus, Desulfofustis, and Desulftiglans as the predominant Hg methylators and sulfate reducers; Nitrosopumilus and Hyphomicrobium as the major nitrifiers and denitrifiers; Methanosarcina and Methanosaeta as keystone methanogens; and Methyloceanibacter and Methyloprofundus as signature methanotrophs. Altogether, this study expands the current understanding of the microbiological and geochemical features and could be helpful in predicting ecosystem functions in the Canadian Beaufort Sea.

Genomic and transcriptomic characterization of methylmercury detoxification in a deep ocean Alteromonas mediterranea ISS312

Methylmercury (MeHg) is one of the most worrisome pollutants in marine systems. MeHg detoxification is mediated by merB and merA genes, responsible for the demethylation of MeHg and the reduction of inorganic mercury, respectively. Little is known about the biological capacity to detoxify this compound in marine environments, and even less the bacterial transcriptional changes during MeHg detoxification. This study provides the genomic and transcriptomic characterization of the deep ocean bacteria Alteromonas mediterranea ISS312 with capacity for MeHg degradation. Its genome sequence revealed four mer operons containing three merA gene and two merB gene copies, that could be horizontally transferred among distant related genomes by mobile genetic elements. The transcriptomic profiling in the presence of 5 μM MeHg showed that merA and merB genes are within the most expressed genes, although not all mer genes were equally transcribed. Besides, we aimed to identify functional orthologous genes that displayed expression profiles highly similar or identical to those genes within the mer operons, which could indicate they are under the same regulatory controls. We found contrasting expression profiles for each mer operon that were positively correlated with a wide array of functions mostly related to amino acid metabolism, but also to flagellar assembly or two component systems. Also, this study highlights that all merAB genes of the four operons were globally distributed across oceans layers with higher transcriptional activity in the mesopelagic deeper waters. Our study provides new insights about the transcriptional patterns related to the capacity of marine bacteria to detoxify MeHg, with important implications for the understanding of this process in marine ecosystems.

What does scientometry tell us about mercury toxicology and its biological impairments?

Mercury is a toxic pollutant that poses risks to both human and environmental health, making it a pressing public health concern. This study aimed to summarize the knowledge on mercury toxicology and the biological impairments caused by exposure to mercury in experimental studies and/or diagnosis in humans. The research was conducted on the main collection of Web of Science, employing as a methodological tool a bibliometric analysis. The selected articles were analyzed, and extracted data such as publication year, journal, author, title, number of citations, corresponding author's country, keywords, and the knowledge mapping was performed about the type of study, chemical form of mercury, exposure period, origin of exposure, tissue/fluid of exposure measurement, mercury concentration, evaluation period (age), mercury effect, model experiments, dose, exposure pathway, and time of exposure. The selected articles were published between 1965 and 2021, with Clarkson TW being the most cited author who has also published the most articles. A total of 38% of the publications were from the USA. These studies assessed the prenatal and postnatal effects of mercury, emphasizing the impact of methylmercury on neurodevelopment, including motor and cognitive evaluations, the association between mercury and autism, and an evaluation of its protective effects against mercury toxicity. In observational studies, the blood, umbilical cord, and hair were the most frequently used for measuring mercury levels. Our data analysis reveals that mercury neurotoxicology has been extensively explored, but the association among the outcomes evaluated in experimental studies has yet to be strengthened. Providing metric evidence on what is unexplored allows for new studies that may help governmental and non-governmental organizations develop guidelines and policies.

A Comparative Study of Mercury Bioaccumulation in Bivalve Molluscs from a Shallow Estuarine Embayment

In estuarine food webs, bivalve molluscs transfer nutrients and pollutants to higher trophic levels. Mercury (Hg) pollution is ubiquitous, but it is especially elevated in estuaries historically impacted by industrial activities, such as those in the U.S. Northeast. Monomethylmercury (MeHg), the organic form of Hg, is highly bioaccumulative and transferable in the food web resulting in the highest concentrations in the largest and oldest marine predators. Patterns of Hg concentrations in marine bivalve molluscs, however, are poorly understood. In this study, inorganic Hg (iHg), MeHg, and the total Hg (THg) in soft tissues of the northern quahogs (Mercenaria mercenaria), eastern oysters (Crassostrea virginica), and ribbed mussels (Geukensia demissa) from eastern Long Island sound, a temperate estuary of the western North Atlantic Ocean was investigated. In all three species, concentrations of THg remained similar between the four sampling months (May, June, July, and September), and were mostly independent of animal size. In quahogs, MeHg and iHg displayed significant (p < 0.05) positive (iHg in May and June) and negative (MeHg in July and September) changes with shell height. Variability in concentrations of THg, MeHg, and iHg, both inter- and intra-specifically was high and greater in quahogs and oysters (THg: 37, 39%, MeHg: 28, 39%, respectively) than in mussels (THg: 13%, MeHg: 20%). The percentage of THg that was MeHg (%MeHg) was also highly variable in the three species (range: 10-80%), highlighting the importance of measuring MeHg and not only THg in molluscs.

Foraging behavior and age affect maternal transfer of mercury to northern elephant seal pups

Deep ocean foraging northern elephant seals (Mirounga angustirostris) consume fish and squid in remote depths of the North Pacific Ocean. Contaminants bioaccumulated from prey are subsequently transferred by adult females to pups during gestation and lactation, linking pups to mercury contamination in mesopelagic food webs (200-1000 m depths). Maternal transfer of mercury to developing seal pups was related to maternal mercury contamination and was strongly correlated with maternal foraging behavior (biotelemetry and isotopes). Mercury concentrations in lanugo (hair grown in utero) were among the highest observed worldwide for young pinnipeds (geometric mean 23.01 μg/g dw, range 8.03-63.09 μg/g dw; n = 373); thus, some pups may be at an elevated risk of sub-lethal adverse health effects. Fetal mercury exposure was affected by maternal foraging geographic location and depth; mercury concentrations were highest in pups of the deepest diving, pelagic females. Moreover, pup lanugo mercury concentrations were strongly repeatable among successive pups of individual females, demonstrating relative consistency in pup mercury exposure based on maternal foraging strategies. Northern elephant seals are biosentinels of a remote deep-sea ecosystem. Our results suggest that mercury within North Pacific mesopelagic food webs may also pose an elevated risk to other mesopelagic-foraging predators and their offspring.

Continuous Flow-Double Purge and Trap Method for Preconcentrating Mercury in Large Volumes of Seawater for Stable Isotope Analysis

Mercury (Hg) isotopes provide a useful tool to understand Hg sources and processes in the environment. The Hg isotopic composition of seawater remains poorly constrained due to the lack of an efficient method to process large volumes of low-Hg-concentration seawater samples. Here, we develop a continuous flow-double purge and trap device for the in situ preconcentration of Hg in seawater. This method yielded a good Hg recovery of 91.7 ± 3.3% (n = 4, 1SD) for spiked seawater samples and gave reasonably similar Hg isotope ratios of NIST 8610, indicating a limited matrix effect and limited Hg isotope fractionation during processing of seawater. NIST 8610 δ202Hg (-0.55 ± 0.09‰, n = 4, 1SD) and Δ199Hg (0.07 ± 0.02‰, n = 4, 1SD) were similar to previously published data. The method was successfully applied to seawater collected from the Xiamen Bay and the South China Sea. The seawater samples showed a Hg recovery of 91.6 ± 5.4% (n = 12, 1SD). Seawater Δ199Hg (-0.04 ± 0.05‰, n = 7, 1SD) in the Xiamen Bay was different from seawater Δ199Hg (0.05 ± 0.07‰, n = 5, 1SD) in the South China Sea, which implies distinct Hg sources to coastal and open ocean areas and highlights the robustness of our method in understanding the Hg isotopic composition of seawater.

Mercury concentrations in Antarctic zooplankton with a focus on the krill species, Euphausia superba

The Antarctic is the most isolated region in the world; nevertheless, it has not avoided the negative impact of human activity, including the inflow of toxic mercury (Hg). Hg deposited in the Antarctic marine environment can be bioavailable and accumulate in the food web, reaching elevated concentrations in high-trophic-level biota, especially if methylated. Zooplankton, together with phytoplankton, are critical for the transport of pollutants, including Hg to higher trophic levels. For the Southern Ocean ecosystem, one of the key zooplankton components is the Antarctic krill Euphausia superba, the smaller euphausiid Thysanoessa macrura, and the amphipod Themisto gaudichaudii - a crucial food source for most predatory fish, birds, and mammals. The main goal of this study was to determine the Hg burden, as well as the distribution of different Hg forms, in these dominant Antarctic planktonic crustaceans. The results showed that the highest concentrations of Hg were found in T. gaudichaudii, a typically predatory taxon. Most of the Hg in the tested crustaceans was labile and potentially bioavailable for planktivorous organisms, with the most dangerous methylmercury (MeHg) accounting for an average of 16 % of the total mercury. Elevated Hg concentrations were observed close to the land, which is influenced by the proximity to penguin and pinniped colonies. In areas near the shore, volcanic activity might be a possible cause of the increase in mercury sulfide (HgS) content. The total Hg concentration increased with the trophic position and ontogenetic stage of predation, specific to adult organisms. In contrast, the proportion of MeHg decreased with age, indicating more efficient demethylation or elimination. The Hg magnification kinetics in the study area were relatively high, which may be related to climate-change induced alterations of the Antarctic ecosystem: additional food sources and reshaped trophic structure.

An improved method for rapid and safe preparation and measurement of dimethylmercury using gas chromatography-atomic fluorescence spectrometry

Transformations between dimethylmercury (DMHg) and other mercury (Hg) species have been one of the critical knowledge gaps in the Hg global biogeochemical cycle due to the lack of detailed studies. The preparation and measurement of DMHg are challenging due to the high toxicity and volatility of DMHg. In this work, we invented a new DMHg generator for successfully preparing high-purity DMHg in a highly controllable and safe way. The DMHg could be spontaneously volatilized and diffused from the original preparation solution to the solution to be studied. The parameters for generating DMHg were optimized to be the pH value of 4.0 with a MeCo/Hg2+ molar ratio of 10 at 20 °C. The following measurement method of DMHg in the presence of various species of Hg was also investigated and optimized. Hg0 and DMHg could be separated effectively with the carrier gas flow rate of 15 mL min-1 and the gas chromatography column temperature of 30 °C. The interferences of Hg0, monomethylmercury and other species were excluded by systematic control experiments. A sensitive and reliable approach for quantifying trace DMHg in water was developed. Under the optimal conditions, the limits of detection for Hg0, MMHg and DMHg were 0.03, 0.002 and 0.024 ng L-1, respectively, with the relative standard deviation below 8.2%. The developed method was validated by the determination Hg species of different natural water samples. This work is expected to provide a new and safe strategy for DMHg preparation and a verified method for DMHg measurement.

Assessing comparability and uncertainty of analytical methods for methylated mercury species in seawater

BACKGROUND

The relative distribution and importance of monomethylmercury (MMHg) and dimethylmercury (DMHg) in seawater is still under debate. A lack of comparability between measurements at sub-picomolar levels hampered the further understanding of the biogeochemical Hg cycle. To overcome this, we assessed the relative standard measurement uncertainties (Uex,r) for direct measurements of MMHg and DMHg by species-specific isotope dilution ICP-MS and cryo-focusing GC-ICP-MS at femtomolar concentrations. Furthermore, Uex,r was determined for the indirect determination of DMHg (DMHgcalc = MeHg - MMHg) and MeHg (MeHgcalc = MMHg + DMHg) to compare the two methodologies.

RESULTS

Expanded Uex,r (confidence interval of 95%) for cryo-focusing GC-ICP-MS was 14.4 (<50 fM) and 14.2% (>50 fM) and for SS-ID GC-ICP-MS 5.6 (<50 fM) and 3.7% (>50 fM). For concentrations above 50 fM, Uex,r for DMHgcalc was always lower than for direct measurements (14.2%). For MeHgcalc, on the other hand, Uex,r was always higher for concentrations above 115 fM (range: 3.7-13.9%) than for direct measurements (3.7%). We evaluated the comparability of directly measured and calculated DMHg and MeHg concentrations based on Hg speciation measurements for two vertical profiles in the Mediterranean Sea. We show that directly measured and indirectly determined DMHg and MeHg concentrations yield comparable results.

SIGNIFICANCE

Our results validate the application of the indirect method for the determination of DMHg if a direct measurement method with a low Uex,r such as isotope dilution is used for MMHg and MeHg measurements. The validation of the indirect measurement approach opens new possibilities to generate more precise and accurate DMHg data in the global ocean.

Gaseous mercury exchange between air and highly dynamic tidal flats: A laboratory incubation experiment

Gaseous mercury (mainly elemental mercury, Hg(0)) exchange between air and Earth's surfaces is one of the most critical fluxes governing global Hg cycle. As an important and unique part of intertidal ecosystem, tidal flat is characterized by periodic inundation and exposure due to tidal cycle, generating varying hydrological, photochemical and biogeochemical processes. However, quantitative and mechanistic understanding of Hg(0) dynamics between air and exceptionally dynamic tide flats has remained limited to date. In this study, we select five representative tidal flat sediments from typical coastal habits of Chinese coastlines to perform laboratory incubation experiments for deciphering the effect of the interaction of tidal cycle and solar radiation on Hg(0) dynamics over tidal flats with different sediment compositions. We show that sediment Hg concentration, tidal cycle and solar radiation collectively modulate the air-surface Hg(0) exchange over tidal flats and highlight that the photochemistry dominates the Hg(0) production and emission over tidal flats. We find that the daytime inundation presents highest Hg(0) emission fluxes for Hg-poor sediment, but the daytime exposure is the hot moment of Hg(0) emission from Hg-rich sediments and substantially contributes to daily Hg(0) emission fluxes. In the treatment to mimic semidiurnal tide, the daily Hg(0) fluxes are positively correlated to sediment Hg concentrations. Combining our mechanistic insights on air-surface Hg(0) exchange over tidal flats and related data and knowledge reported by other studies, we discuss the implications of our study for field measurement and model development of Hg(0) dynamics over highly dynamic tidal flats. We conclude that the air-surface Hg(0) dynamics over tidal flats are extremely complex and highly variable, and a greater understanding the interactions between natural processes, human impacts and climate forcings will better constrain current and future Hg biogeochemical cycle in global tidal flats.

Mercury (Hg) in the "Skin" of the Ocean: Dissolved Gaseous Hg, Total Hg, and Hg Redox Chemistry in Sea Surface Microlayer and Implication for Air-Sea Hg Exchange

The sea surface microlayer (SML) is the uppermost ∼1000 μm of the surface of the ocean. With distinct physicochemical properties and position relative to the adjacent subsurface waters (SSWs), the ubiquitous distribution and high dynamics of the SML greatly regulate the global air-sea gas exchange and biogeochemistry. Mercury (Hg) redox chemistry in surface seawaters and air-sea exchange of gaseous Hg (mainly Hg(0)) fundamentally control the global oceanic Hg cycle. However, the occurrence and transformation of Hg in the SML have been poorly quantified. Here we optimize the traditional SML sampling system to make it more suitable for dissolved gaseous Hg (DGM, mainly Hg(0)) sampling. We then assess the temporal and spatial variability of DGM, total Hg, dissolved organic carbon (DOC), and Hg redox chemistry in the SML and SSWs of diverse marine environments. Our data suggest a general DGM, total Hg, and DOC enrichment in the SML relative to the SSWs but with complex variability in time and space. The incubation experiments further reveal the complex characteristics of Hg redox chemistry between the SML and SSWs. We discuss important implications of the SML Hg cycle on air-sea Hg exchange and suggest wider investigations of the SML Hg cycle in the global hydrosphere.

Associations between total mercury, trace minerals, and blood health markers in Northwest Atlantic white sharks (Carcharodon carcharias)

The ecology and life-histories of white sharks make this species susceptible to mercury bioaccumulation; however, the health consequences of mercury exposure are understudied. We measured muscle and plasma total mercury (THg), health markers, and trace minerals in Northwest Atlantic white sharks. THg in muscle tissue averaged 10.0 mg/kg dry weight, while THg in blood plasma averaged 533 μg/L. THg levels in plasma and muscle were positively correlated with shark precaudal length (153-419 cm), and THg was bioaccumulated proportionally in muscle and plasma. Nine sharks had selenium:mercury molar ratios in blood plasma >1.0, indicating that for certain individuals the potential protective effects of the trace mineral were diminished, whereas excess selenium may have protected other individuals. No relationships between plasma THg and any trace minerals or health markers were identified. Thus, we found no evidence of negative effects of Hg bioaccumulation, even in sharks with very high THg.

Using Crocodylians for monitoring mercury in the tropics

Mercury contamination is a widespread phenomenon that impacts ecosystems worldwide. Artisanal Small Scale Gold Mining (ASGM) activities are responsible for more than a third of atmospheric Hg emission. Due to Hg toxicity and its broad and elevated prevalence in the environment resulting from ASGM activities in the tropics, its biomonitoring is essential to better understand the availability of its methylmercury (MeHg) form in the environment. The Minamata Convention was ratified with the objective to "protect human health and the environment from anthropogenic emissions and releases of mercury compounds". Biomagnification of MeHg occurs through the trophic food web, where it biomagnifies and bioaccumulates in top predators. To monitor environmental MeHg contamination, studies have evaluated the use of living organisms; however, reptiles are among the least documented vertebrates regarding MeHg exposure. In this review we evaluate the use of crocodylians for Hg biomonitoring in tropical ecosystems. We found that out of the 28 crocodiles species, only 10 have been evaluated regarding Hg contamination. The remaining challenges when using this taxon for Hg biomonitoring are inconsistencies in the applied methodology (e.g., wet versus dry weight, tissues used, quantification method). However, due to their life history traits, crocodylians are particularly relevant for monitoring MeHg contamination in regions where ASGM activities occur. In conclusion and given their ecological and socio-economic importance, crocodylians are at great risk of MeHg contamination and are excellent bioindicators for tropical ecosystems.

Coastal streams and sewage outfalls: Hot spots of mercury discharge, pollution and cycling in nearshore environments

The coastal streams (CSs) and sewage outfalls (SOs) are widely distributed and direct anthropogenic stress on global coastal ecosystems. However, the CS/SO-associated mercury (Hg) discharge, pollution and cycle in nearshore environment are less quantified. Here, we report that total Hg (THg) and methylmercury (MMHg) concentrations in waters of CSs (n = 8) and SOs (n = 15) of the northern China were ∼102 to 103 times of coastal surface waters and 10 to 102 times of major rivers in China and other regions. The CS/SO discharges resulted in the increase of total organic carbon (TOC) contents, THg and MMHg concentrations and TOC-normalized THg and MMHg concentrations in sediments of CS/SO-impacted coasts. The laboratory experiments further illustrated that the CS/SO-impacted sediments characterized with high potentials of dissolved THg and MMHg productions and releases. Our findings indicate that the layout optimization of SOs is able to reduce the Hg risk in coastal environment.

Riverine Discharge Fuels the Production of Methylmercury in a Large Temperate Estuary

Estuaries are an important food source for the world's growing population, yet human health is at risk from elevated exposure to methylmercury (MeHg) via the consumption of estuarine fish. Moreover, the sources and cycling of MeHg in temperate estuarine ecosystems are poorly understood. Here, we investigated the seasonal and tidal patterns of mercury (Hg) forms in Long Island Sound (LIS), in a location where North Atlantic Ocean waters mix with the Connecticut River. We found that seasonal variations in Hg and MeHg in LIS followed the extent of riverine Hg delivery, while tides further exacerbated the remobilization of earlier deposited riverine Hg. The net production of MeHg near the river plume was significant compared to that in other locations and enhanced during high tide, possibly resulting from the enhanced microbial activity and organic carbon remineralization in the river plume. Statistical models, driven by our novel data, further support the hypothesis that the river-delivered organic matter and inorganic Hg drive net MeHg production in the estuarine water column. Our study sheds light on the significance of water column biogeochemical processes in temperate tidal estuaries in regulating MeHg levels and inspires new questions in our quest to understand MeHg sources and dynamics in coastal oceans.

Interaction of Naturally Occurring Phytoplankton with the Biogeochemical Cycling of Mercury in Aquatic Environments and Its Effects on Global Hg Pollution and Public Health

The biogeochemical cycling of mercury in aquatic environments is a complex process driven by various factors, such as ambient temperature, seasonal variations, methylating bacteria activity, dissolved oxygen levels, and Hg interaction with dissolved organic matter (DOM). As a consequence, part of the Hg contamination from anthropogenic activity that was buried in sediments is reinserted into water columns mainly in highly toxic organic Hg forms (methylmercury, dimethylmercury, etc.). This is especially prominent in the coastal shallow waters of industrial regions worldwide. The main entrance point of these highly toxic Hg forms in the aquatic food web is the naturally occurring phytoplankton. Hg availability, intake, effect on population size, cell toxicity, eventual biotransformation, and intracellular stability in phytoplankton are of the greatest importance for human health, having in mind that such Hg incorporated inside the phytoplankton cells due to biomagnification effects eventually ends up in aquatic wildlife, fish, seafood, and in the human diet. This review summarizes recent findings on the topic of organic Hg form interaction with natural phytoplankton and offers new insight into the matter with possible directions of future research for the prevention of Hg biomagnification in the scope of climate change and global pollution increase scenarios.

Assessment of the impact of small-scale mining on soil contamination by mercury and hydrocarbons in the kadey catchment (East Cameroon)

The objective of the present study was to assess soil contamination by mercury and hydrocarbon products used in gold mining in the Kadey catchment area. The results obtained show that gold mining by the small mine is the cause of chemical contamination of the soil caused by hydrocarbon products with concentrations 800 times higher than the threshold value on certain points of the former mining sites. The use of mercury has led to an increase in its concentration to values that are still below the critical thresholds. It also appears from this work that organic matter plays a very important role in the retention of mercury on the soil surface. In the case of hydrocarbons are concerned, although high concentrations above are found exclusively at the surface, the rainfall contributes to their infiltration into the soil and to a horizontal redistribution of the contamination.

Interaction of mercury species with proteins: towards possible mechanism of mercurial toxicology

The nature of the binding of mercurials (organic and inorganic) and their subsequent transformations in biological systems is a matter of great debate as several different hypotheses have been proposed and none of them has been conclusively proven to explain the characteristics of Hg binding with the proteins. Thus, the chemical nature of Hg-protein binding through the possible transportation mechanism in living tissues is critically reviewed herein. Emphasis is given to the process of transportation, and binding of Hg species with selenol-containing biomolecules that are appealing for toxicological studies as well as the advancement of environmental and biological research.

Mercury isotope compositions in seawater and marine fish revealed the sources and processes of mercury in the food web within differing marine compartments

Anthropogenic activities and climate change have significantly increased mercury (Hg) levels in seawater. However, the processes and sources of Hg in differing marine compartments (e.g. estuary, marine continental shelf (MCS) or pelagic area) have not been well studied, which makes it difficult to understand Hg cycling in marine ecosystems. To address this issue, the total Hg (THg) concentration, methylmercury (MeHg) concentration and stable Hg isotopes were determined in seawater and fish samples collected from differing marine compartments of the South China Sea (SCS). The results showed that the estuarine seawater exhibited substantially higher THg and MeHg concentrations than those in the MCS and pelagic seawater. Significantly negative δ²⁰²Hg (-1.63‰ ± 0.42‰) in estuarine seawater compared with that in pelagic seawater (-0.58‰ ± 0.08‰) may suggest watershed input and domestic sewage discharge of Hg in the estuarine compartment. The Δ¹⁹⁹Hg value in estuarine fish (0.39‰ ± 0.35‰) was obviously lower than that in MCS (1.10‰ ± 0.54‰) and pelagic fish (1.15‰ ± 0.46‰), which showed that relatively little MeHg photodegradation occurred in the estuarine compartment. The Hg isotope binary mixing model based on Δ²⁰⁰Hg revealed that approximately 74% MeHg in pelagic fish is derived from atmospheric Hg(II) deposition, and over 60% MeHg in MCS fish is derived from sediments. MeHg sources for estuarine fish may be highly complex (e.g. sediment or riverine/atmospheric input) and further investigations are warranted to clarify the contribution of each source. Our study showed that Hg stable isotopes in seawater and marine fish can be used to identify the processes and sources of Hg in different marine compartments. This finding is of great relevance to the development of marine Hg food web models and the management of Hg in fish.

Mercury stable isotopes in the ocean: Analytical methods, cycling, and application as tracers

Mercury (Hg) has seven stable isotopes that can be utilized to trace the sources of Hg and evaluate the importance of transport and transformation processes in the cycling of Hg in the environment. The ocean is an integral part of the Earth and plays an important role in the global mercury cycle. However, there is a lack of a systematic review of Hg stable isotopes in marine environments. This review is divided into four sections: a) advances in Hg stable isotope analysis, b) the isotope ratios of Hg in various marine environmental matrices (seawater, sediment, and organisms), c) processes governing stable Hg isotope ratios in the ocean, and d) application of Hg stable isotopes to understand biotic uptake and migration. Mercury isotopes have provided much useful information on marine Hg cycling that cannot be given by Hg concentrations alone. This includes (i) sources of Hg in coastal or estuarine environments, (ii) transformation pathways and mechanisms of different forms of Hg in marine environments, (iii) trophic levels and feeding guilds of marine fish, and (iv) migration/habitat changes of marine fish. With the improvement of methods for seawater Hg isotope analysis (especially species-specific methods) and the measurement of Hg isotope fractionation during natural biogeochemical processes in the ocean, Hg stable isotopes will advance our understanding of the marine Hg cycle in the future, e.g., mercury exchange at the sea-atmosphere interface and seawater-sediment interface, contributions of different water masses to Hg in the ocean, fractionation mechanisms of Hg and MeHg transformation in seawater.

The Combined Effect of Hg(II) Speciation, Thiol Metabolism, and Cell Physiology on Methylmercury Formation by Geobacter sulfurreducens

The chemical and biological factors controlling microbial formation of methylmercury (MeHg) are widely studied separately, but the combined effects of these factors are largely unknown. We examined how the chemical speciation of divalent, inorganic mercury (Hg(II)), as controlled by low-molecular-mass thiols, and cell physiology govern MeHg formation by Geobacter sulfurreducens. We compared MeHg formation with and without addition of exogenous cysteine (Cys) to experimental assays with varying nutrient and bacterial metabolite concentrations. Cysteine additions initially (0-2 h) enhanced MeHg formation by two mechanisms: (i) altering the Hg(II) partitioning from the cellular to the dissolved phase and/or (ii) shifting the chemical speciation of dissolved Hg(II) in favor of the Hg(Cys)₂ complex. Nutrient additions increased MeHg formation by enhancing cell metabolism. These two effects were, however, not additive since cysteine was largely metabolized to penicillamine (PEN) over time at a rate that increased with nutrient addition. These processes shifted the speciation of dissolved Hg(II) from complexes with relatively high availability, Hg(Cys)₂, to complexes with lower availability, Hg(PEN)₂, for methylation. This thiol conversion by the cells thereby contributed to stalled MeHg formation after 2-6 h Hg(II) exposure. Overall, our results showed a complex influence of thiol metabolism on microbial MeHg formation and suggest that the conversion of cysteine to penicillamine may partly suppress MeHg formation in cysteine-rich environments like natural biofilms.

A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes

Recent studies demonstrate a short 3-6-month atmospheric lifetime for mercury (Hg). This implies Hg emissions are predominantly deposited within the same hemisphere in which they are emitted, thus placing increasing importance on considering Hg sources, sinks and impacts from a hemispheric perspective. In the absence of comprehensive Hg data from the Southern Hemisphere (SH), estimates and inventories for the SH have been drawn from data collected in the NH, with the assumption that the NH data are broadly applicable. In this paper, we centre the uniqueness of the SH in the context of natural biogeochemical Hg cycling, with focus on the midlatitudes and tropics. Due to its uniqueness, Antarctica warrants an exclusive review of its contribution to the biogeochemical cycling of Hg and is therefore excluded from this review. We identify and describe five key natural differences between the hemispheres that affect the biogeochemical cycling of Hg: biome heterogeneity, vegetation type, ocean area, methylation hotspot zones and occurence of volcanic activities. We review the current state of knowledge of SH Hg cycling within the context of each difference, as well as the key gaps that impede our understanding of natural Hg cycling in the SH. The differences demonstrate the limitations in using NH data to infer Hg processes and emissions in the SH.

Mercury Isotopes in Deep-Sea Epibenthic Biota Suggest Limited Hg Transfer from Photosynthetic to Chemosynthetic Food Webs

Deep oceans receive mercury (Hg) from upper oceans, sediment diagenesis, and submarine volcanism; meanwhile, sinking particles shuttle Hg to marine sediments. Recent studies showed that Hg in the trench fauna mostly originated from monomethylmercury (MMHg) of the upper marine photosynthetic food webs. Yet, Hg sources in the deep-sea chemosynthetic food webs are still uncertain. Here, we report Hg concentrations and stable isotopic compositions of indigenous biota living at hydrothermal fields of the Indian Ocean Ridge and a cold seep of the South China Sea along with hydrothermal sulfide deposits. We find that Hg is highly enriched in hydrothermal sulfides, which correlated with varying Hg concentrations in inhabited biota. Both the hydrothermal and cold seep biota have small fractions (<10%) of Hg as MMHg and slightly positive Δ¹⁹⁹Hg values. These Δ¹⁹⁹Hg values are slightly higher than those in near-field sulfides but are 1 order of magnitude lower than the trench counterparts. We suggest that deep-sea chemosynthetic food webs mainly assimilate Hg from ambient seawater/sediments and hydrothermal fluids formed by percolated seawater through magmatic/mantle rocks. The MMHg transfer from photosynthetic to chemosynthetic food webs is likely limited. The contrasting Hg sources between chemosynthetic and trench food webs highlight Hg isotopes as promising tools to trace the deep-sea Hg biogeochemical cycle.

National-Scale Assessment of Total Gaseous Mercury Isotopes Across the United States

With the 2011 promulgation of the Mercury and Air Toxics Standards by the U.S. Environmental Protection Agency, and the successful negotiation by the United Nations Environment Program of the Minamata Convention, emissions of mercury (Hg) have declined in the United States. While the declines in atmospheric Hg concentrations in North America are encouraging, linking the declines to changing domestic and global source portfolios remains challenging. To address these research gaps, the U.S. Geological Survey initiated the first national-scale effort to establish a baseline of total gaseous mercury stable isotope values at 31 sites distributed across the United States. Results indicated that unique Hg sources, such as Hg evasion from an elemental Hg contaminated site or free tropospheric intrusions in high altitude sites, were distinguishable from background atmospheric values. Minor gradients were observed across the nation, with regions of heavy industrial activity demonstrating lower δ 202 Hg , but no consistent changes in other isotopes such as Δ 199 Hg and Δ 200 Hg were observed. Furthermore, δ 202 Hg was impacted by foliar uptake and senescence but trends varied between forested regions in the northeastern and midwestern United States. These data demonstrate regional emission sources and other environmental variables can impact total gaseous Hg (TGM) isotope values, highlighting the need to characterize atmospheric Hg isotopes over larger geographical areas to evaluate changes related to national and international Hg regulations.

A century of mercury: Ecosystem-wide changes drive increasing contamination of a tropical seabird species in the South Atlantic Ocean

Mercury (Hg) is a highly toxic metal that adversely impacts human and wildlife health. The amount of Hg released globally in the environment has increased steadily since the Industrial Revolution, resulting in growing contamination in biota. Seabirds have been extensively studied to monitor Hg contamination in the world's oceans. Multidecadal increases in seabird Hg contamination have been documented in polar, temperate and subtropical regions, whereas in tropical regions they are largely unknown. Since seabirds accumulate Hg mainly from their diet, their trophic ecology is fundamental in understanding their Hg exposure over time. Here, we used the sooty tern (Onychoprion fuscatus), the most abundant tropical seabird, as bioindicator of temporal variations in Hg transfer to marine predators in tropical ecosystems, in response to trophic changes and other potential drivers. Body feathers were sampled from 220 sooty terns, from museum specimens (n = 134) and free-living birds (n = 86) from Ascension Island, in the South Atlantic Ocean, over 145 years (1876-2021). Chemical analyses included (i) total- and methyl-Hg, and (ii) carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes, as proxies of foraging habitat and trophic position, respectively, to investigate the relationship between trophic ecology and Hg contamination over time. Despite current regulations on its global emissions, mean Hg concentrations were 58.9% higher in the 2020s (2.0 μg g^-1, n = 34) than in the 1920s (1.2 μg g^-1, n = 107). Feather Hg concentrations were negatively and positively associated with δ¹³C and δ¹⁵N values, respectively. The sharp decline of 2.9 ‰ in δ¹³C values over time indicates ecosystem-wide changes (shifting primary productivity) in the tropical South Atlantic Ocean and can help explain the observed increase in terns' feather Hg concentrations. Overall, this study provides invaluable information on how ecosystem-wide changes can increase Hg contamination of tropical marine predators and reinforces the need for long-term regulations of harmful contaminants at the global scale.

Potential risks, source apportionment, and health risk assessment of dissolved heavy metals in Zhoushan fishing ground, China

Dissolved heavy metal pollution in the ocean is one of the most severe environmental concerns; however, the potential sources of heavy metals and the resulting health risks are not fully understood. To explore the distribution characteristics, source apportionment, and health risks of dissolved heavy metals (As, Cd, Cu, Hg, Pb, and Zn) in the Zhoushan fishing ground, this study analyzed heavy metals in surface seawater during the wet and dry seasons. The concentrations of heavy metals varied greatly between seasons, and the mean concentration in the wet season was generally higher than that in the dry season. A positive matrix factorization model coupled with correlation analysis was applied to identify promising sources of heavy metals. Four potential sources (agricultural, industrial, traffic, atmospheric deposition, and natural sources) were identified as the determinants of the accumulation of heavy metals. The health risk assessment results revealed that non-carcinogenic risk (NCR) for adults and children were acceptable (HI < 1), and carcinogenic risk (CR) were at a low level (1 × 10^-6 < TCR ≤ 1 × 10^-4). The source-oriented risk assessment indicated that industrial and traffic sources were the main sources of pollution, contributing 40.7 % of NCR and 27.4 % of CR, respectively. This study proposes forming reasonable, effective policies to control industrial pollution and improve the ecological environment of Zhoushan fishing grounds.

Microorganisms isolated from seabirds feathers for mercury bioremediation

Environmental pollution caused by mercury has received increasing attention in recent years. Several studies have warned of the high rates of biomagnification in superior levels of marine food networks affecting seabirds. Although seabird feathers are reported as bioindicators of mercury, the possibility of using the microbiota associated with them for the bioremediation of this metal has not been considered. Despite the potential of the seabird feather microbiota, the cultivable microorganisms from this sample matrix have not been identified. In this study, we isolated and identified the organisms in the feathers from three types of seabirds, two species of penguins (Pygoscelis antartica and Pygoscelis papua) and the brown skua bird (Catharacta lonnbergi) through poisoned media a final concentration of 10 mg / L Hg2+ in the culture medium for the microbial consortia. Yeast isolates belonged to the genus Debaryomyces, Meyerozyma, Papiliotrema, and Rhodotorula, and fungi genera Leiotrametes, Penicillium, Pseudogymnoascus, and Cladosporium were identified. Adult bird feathers with high mercury concentrations can serve as a matrix to isolate microorganisms capable of removing mercury. Keywords: Antarctica, bioremediation, feathers, mercury, microorganisms

Sample Preparation and Analytical Techniques in the Determination of Trace Elements in Food: A Review

Every human being needs around 20 essential elements to maintain proper physiological processes. However, trace elements are classified as beneficial, essential, or toxic for living organisms. Some trace elements are considered essential elements for the human body in adequate quantities (dietary reference intakes, DRIs), while others have undetermined biological functions and are considered undesirable substances or contaminants. Pollution with trace elements is becoming a great concern since they can affect biological functions or accumulate in organs, causing adverse effects and illnesses such as cancer. These pollutants are being discarded in our soils, waters, and the food supply chain due to several anthropogenic factors. This review mainly aims to provide a clear overview of the commonly used methods and techniques in the trace element analysis of food from sample preparations, namely, ashing techniques, separation/extraction methods, and analytical techniques. Ashing is the first step in trace element analysis. Dry ashing or wet digestion using strong acids at high pressure in closed vessels are used to eliminate the organic matter. Separation and pre-concentration of elements is usually needed before proceeding with the analytical techniques to eliminate the interferences and ameliorate the detection limits.

Total mercury, methylmercury, and selenium concentrations in blue marlin Makaira nigricans from a long-term dataset in the western north Atlantic

Mercury in seafood is a neurotoxicant that threatens human health. Dynamic rates of mercury emission, re-emission, and atmospheric deposition warrant studies into mercury concentrations in fish because many are consumed by humans and can serve as sentinels of mercury levels in the environment. We modeled trends in total mercury content in an apex marine fish predator, Atlantic blue marlin Makaira nigricans, whose muscle tissues were opportunistically sampled from North Carolina (USA) sportfishing tournaments over a discontinuous time period: between 1975 and 77 and 1998-2021 (n = 148). The model-estimated influence of marlin weight on total mercury concentration was constant across years (shared slope) allowing for comparisons of weight-corrected mercury concentrations among years. Weight-corrected total mercury concentrations revealed an inter-decadal decline of approximately 45 % between the 1970s and late 1990s and then variable but relatively stable concentrations through 2021. The mean (SD) wet weight concentration of total mercury was 9.47 (4.11) from 1975 to 77 and 4.17 (2.61) from 2020 to 2021. Methylmercury and selenium were measured on a subset of fish to address questions related to human health and consumption. Methylmercury levels (mean = 0.72 μg/g) were much lower than total mercury (mean = 4.69 μg/g) indicating that total mercury is not a good proxy for methylmercury in Atlantic blue marlin. Selenium, examined as a Se:Hg molar ratio and as a selenium health benefit value (HBV_Se), showed high protective value against mercury toxicity. Long-term trends in the concentration of mercury in blue marlin should continue to be monitored to determine whether policies to mitigate anthropogenic contributions to global mercury are achieving their intended goals and to provide information to inform safe human consumption.

Gaseous elemental mercury and its evasion fluxes in the marine boundary layer of the marginal seas of the northwestern Pacific: Results from two cruises in September-December 2019

There are many questions regarding the behavior of mercury in the sea-atmosphere system of the northwestern Pacific. Continuous underway measurements of atmospheric gaseous elemental mercury (GEM) and measurements of sea-air GEM evasion fluxes were carried out in the marginal seas of northwestern Pacific from the South China Sea to the Sea of Okhotsk in fall-winter 2019. The median GEM concentration (1.1 ng/m³) was lower than both the background value and the averages previously observed in these areas. A latitudinal gradient of atmospheric GEM and GEM evasion fluxes with maximum values at southern latitudes was found. The following areas have been identified as potential source areas: the Kurill area of the Pacific Ocean Northeast China, Korean Peninsula, and the territory from the southwest coast of the Yellow Sea to the south of Indochina. Seasonal variations were observed in the Sea of Japan and East China Sea with higher GEM concentrations in winter than in fall. Our data and analysis of published data showed significant relationships between GEM evasion fluxes, latitude and sea surface temperature (SST). It seems that on a global scale, along with the GEM gradient between water and atmosphere, SST is the most significant parameter for sea-air GEM evasion fluxes.

Anaerobic mercury methylators inhabit sinking particles of oxic water columns

Increased concentration of mercury, particularly methylmercury, in the environment is a worldwide concern because of its toxicity in severely exposed humans. Although the formation of methylmercury in oxic water columns has been previously suggested, there is no evidence of the presence of microorganisms able to perform this process, using the hgcAB gene pair (hgc⁺ microorganisms), in such environments. Here we show the prevalence of hgc⁺ microorganisms in sinking particles of the oxic water column of Lake Geneva (Switzerland and France) and its anoxic bottom sediments. Compared to anoxic sediments, sinking particles found in oxic waters exhibited relatively high proportion of hgc⁺genes taxonomically assigned to Firmicutes. In contrast hgc⁺members from Nitrospirae, Chloroflexota and PVC superphylum were prevalent in anoxic sediment while hgc⁺ Desulfobacterota were found in both environments. Altogether, the description of the diversity of putative mercury methylators in the oxic water column expand our understanding on MeHg formation in aquatic environments and at a global scale.

Impacts of the invasive Spartina anglica on C-S-Hg cycles and Hg(II) methylating microbial communities revealed by hgcA gene analysis in intertidal sediment of the Han River estuary, Yellow Sea

We investigated the impact of invasive vegetation on mercury cycles, and identified microorganisms directly related to Hg(II) methylation using hgcA gene in vegetated mud flats (VMF) inhabited by native Suaeda japonica (SJ) and invasive Spartina anglica (SA), and unvegetated mud flats (UMF) in Ganghwa intertidal sediments. Sulfate reduction rate (SRR) and rate constants of Hg(II) methylation (K_m) and methyl-Hg demethylation (K_d) were consistently greater in VMF than in UMF, specifically 1.5, 2 and 11.7 times higher, respectively, for SA. Both K_m and K_d were significantly correlated with SRR and the abundance of sulfate-reducing bacteria. These results indicate that the rhizosphere of invasive SA provides a hotspot for Hg dynamics coupled with sulfate reduction. HgcA gene analysis revealed that Hg(II)-methylators were dominated by Deltaproteobacteria, Chloroflexi and Euryarchaeota, comprising 37.9%, 35.8%, and 6.5% of total hgcA gene sequences, respectively, which implies that coastal sediments harbor diverse Hg(II)-methylating microorganisms that previously underrepresented.

Distribution and phylogeny of mercury methylation, demethylation, and reduction genes in the Seto Inland Sea of Japan

Mercury (Hg) adversely affects human and environmental health. To evaluate the mercury (Hg) speciation (methylation, demethylation, and reduction) of microorganisms in coastal seawater, we analyzed the microbial functional gene sets involved in Hg methylation (hgcA and hgcB), demethylation (merB), and reduction (merA) using a metagenomic approach in the eastern and western parts (the Kii and Bungo channels, respectively) of the Seto Inland Sea (SIS) of Japan. We determined the concentration of dissolved total mercury (dTHg) and methylated mercury (dMeHg) in seawater. The metagenomic analysis detected hgcAB, merA, and merB in both channels, whereas the phylogenies of these genes differed between them. A correlation between Hg concentration (both dTHg and dMeHg) and the relative abundance of each gene was not observed. Our data suggests that microbial Hg methylation and demethylation could occur in the SIS and there could be a distinct microbial Hg speciation process between the Kii and Bungo channels.

Arctic methylmercury cycling

Anthropogenic mercury (Hg) undergoes long-range transport to the Arctic where some of it is transformed into methylmercury (MeHg), potentially leading to high exposure in some Arctic inhabitants and wildlife. The environmental exposure of Hg is determined not just by the amount of Hg entering the Arctic, but also by biogeochemical and ecological processes occurring in the Arctic. These processes affect MeHg uptake in biota by regulating the bioavailability, methylation and demethylation, bioaccumulation and biomagnification of MeHg in Arctic ecosystems. Here, we present a new budget for pools and fluxes of MeHg in the Arctic and review the scientific advances made in the last decade on processes leading to environmental exposure to Hg. Methylation and demethylation are key processes controlling the pool of MeHg available for bioaccumulation. Methylation of Hg occurs in diverse Arctic environments including permafrost, sediments and the ocean water column, and is primarily a process carried out by microorganisms. While microorganisms carrying the hgcAB gene pair (responsible for Hg methylation) have been identified in Arctic soils and thawing permafrost, the formation pathway of MeHg in oxic marine waters remains less clear. Hotspots for methylation of Hg in terrestrial environments include thermokarst wetlands, ponds and lakes. The shallow sub-surface enrichment of MeHg in the Arctic Ocean, in comparison to other marine systems, is a possible explanation for high MeHg concentrations in some Arctic biota. Bioconcentration of aqueous MeHg in bacteria and algae is a critical step in the transfer of Hg to top predators, which may be dampened or enhanced by the presence of organic matter. Variable trophic position has an important influence on MeHg concentrations among populations of top predator species such as ringed seal and polar bears distributed across the circumpolar Arctic. These scientific advances highlight key processes that affect the fate of anthropogenic Hg deposited to Arctic environments.

Environmental assessment model for scrubbers versus alternative mitigation systems for feeder vessels in liner shipping

Implementation of the Global Sulphur Cap (GSC), in January 2020, boosted scrubber installation in vessels to fulfill the new air emission limitations. This increase in scrubbers' use has intensified concern about its environmental performance. Even though achievement of GSC requirements through this mitigation system has been widely proven, the impact of wash water discharge on the marine environment remains under discussion. In this paper, an assessment environmental model is introduced to quantify in monetary terms the performance of feeder vessels that operate with several mitigation systems. This model attempts to improve traditional air emission evaluations by including the impact of scrubbers' discharges on the marine environmental. In this way, the analysis not only allows different mitigations systems to be ranked by considering their capacity to reduce air emissions, but also provides further information about the marine eutrophication and ecotoxicity impact from scrubbers' discharge. Through the model's application to a regular shipping line between the Canary Islands and the Iberian Peninsula, it was found that, the scrubber, regardless of its operation mode (open- or closed loop), is the most efficient mitigation option after the Liquefied Natural Gas (LNG) fuel shift. The impact of scrubbers' discharge was not as significant as expected on the feeder vessel's total pollution since this provides similar relative weight to the methane emissions from a dual-engine option by operating with LNG. The results also show the need to more closely research the marine eutrophication impact of closed-loop scrubbers. Finally, this paper warns about a significant dispersion on the monetary values of marine ecotoxicity and eutrophication, due to a high dependence of the results on the frameworks' localization. Consequently, further research is needed on the homogenization of pollution monetization in the marine environment.