Mercury and methylmercury concentrations, sources and distribution in submarine canyon sediments (Capbreton, SW France): Implications for the net methylmercury production

Determinations of methylmercury and mercury methylation/demethylation rate constants in environmental samples using isotope dilution/tracing methods by automatic ethylation-purge and trap-GC-ICP-MS

BACKGROUND

Mercury (Hg), especially methylmercury (MeHg) as a most toxic format of Hg in the environment, has been paid widely concern due to its high bioaccumulative capability and great risk to humans. Great efforts have been made to develop ethylation-purge and trap-gas chromatography-inductively coupled plasma mass spectrometry system for MeHg analysis and Hg biogeochemical cycling investigation. However, the generally manual operation limits the analytical efficiency, and the lack of applications in the real environmental samples restricts the future study. There is a great need for a rapid and accurate method to determine MeHg and Hg methylation/demethylation processes in environmental samples.

RESULTS

Herein, an automatic ethylation-purge and trap-GC-ICP-MS system based on isotope dilution method for MeHg analysis was developed. The results showed that the limit of detection of the developed method was 0.01 ng L-1, the MeHg can be analyzed within 6 min with a relative standard deviation of 4.3 %. The accuracy of this proposed method was verified by the satisfying recoveries of certified reference materials (99.0 ± 0.35 % in ECM-CC580, sediment; 98.0 ± 0.67 % in DORM-4, Fish protein). In addition, comparable concentrations of MeHg in natural water were measured using both of the developed and classical distillation methods. Subsequently, the developed method was adapted for measuring concentrations of MeHg in the water, sediment, and fish muscle collected from the coastal and freshwater systems. Finally, the photic demethylation and biotic methylation/demethylation rate constants in natural surface water and sediment were determined using isotope dilution/tracing methods by automatic ethylation-purge and trap-GC-ICP-MS.

SIGNIFICANCE AND NOVELTY

The developed automatic ethylation-purge and trap-GC-ICP-MS system is promising for accurate and convenient MeHg analysis and Hg biogeochemical cycling investigation in real environmental samples with isotope dilution and tracing methods.

Distributions of total mercury and methylmercury and regulating factors in lake water and surface sediment in the cold-arid Wuliangsuhai Lake region

This study aimed to understand the occurrence of mercury in the water environment of typical cold and arid lakes and the regulating environmental factors. Water and surface sediment samples were collected from July to August, 2022 in the Wuliangsuhai Lake region for the analysis of total mercury (THg) and total methylmercury (TMeHg). Lake water THg and TMeHg ranged between 19.20 ~ 668.10 and 0.10 ~ 11.40 ng/L, respectively, exceeding China's environmental quality standards and contents of other lakes and reservoirs in China and other areas. Surface sediments showed lower mean THg and TMeHg of 261.85 and 0.18 μg/kg, respectively, with the former significantly exceeding the background value of Inner Mongolia and unpolluted natural lakes but lower than those of lakes affected by human factors, such as aquaculture. Sediments showed relatively low methylation and TMeHg (0.01-0.21%) concentrations. Correlation analysis identified salinity, total dissolved solids, conductivity, and redox potential as important factors affecting mercury speciation in water, whereas those in surface sediments were organic matter, pH, and total iron content. This study conducted preliminary research on the different species of Hg in Wuliangsuhai Lake water environment, which can provide scientific evidence for the specific treatment of Hg pollution in agriculture, or industry and other related fields. Our results suggest that upstream and downstream regulatory agencies should strengthen the regulation of agricultural and industrial production, moderately reduce human activities, and reduce the use of mercury-rich substances such as pesticides.

Effect of environmental variables on mercury accumulation in sediments of an anthropogenically impacted tropical estuary (Buenaventura Bay, Colombian Pacific)

Estuaries are the main entry areas of mercury to the marine environment and are important to understand the effect of this contaminant on marine organisms, since it accumulates in the sediments becoming available to enter the food trophic chain. This study aims to determine the environmental variables that mainly influence the spatiotemporal dynamics of total mercury accumulation in sediments of tropical estuaries. Sediment samples were collected from interior and exterior areas of the estuary during the dry and rainy seasons, representing the spatiotemporal gradients of the estuary. The grain size, organic matter content (OM), and total mercury concentration (THg) of the sediment samples were determined. In addition, salinity, temperature, dissolved oxygen, and pH of the water column associated with each sediment sample were assessed. The variations in environmental conditions, OM and THg in sediment were in accordance with a gradient which goes from conditions influenced by fresh water in the inner estuary to conditions influenced by sea water in the outer part of the estuary. The OM and THg in sediments presented similar variation patterns; they were higher in the rainy season than in the dry season and in the interior area of the estuary than in the exterior area. Despite the complex dynamic observed in the distribution and accumulation processes of mercury in sediments, these processes could be modeled from OM and salinity parameters. Due to the correlations found, in the process of accumulation of mercury in sediments the OM could represents the pathway of transport and accumulation of THg, and salinity could represent the influence of the hydroclimatic variations and environmental gradients of the estuary.

Assessing and predicting the changes for inorganic mercury and methylmercury concentrations in surface waters of a tidal estuary (Adour Estuary, SW France)

Total and dissolved concentrations of inorganic mercury (IHg) and methylmercury (MeHg) in water (Adour Estuary) were determined during three sampling campaigns and related to biogeochemical variables (nutrients, organic matter). Factors (sampling time, sample type) were included in analysis of covariance with effect separation. The urban estuary suffered historically from anthropogenic sources, however, decreased emissions have reduced Hg concentrations. Total IHg (0.51-3.42 ng L^-1) and MeHg (25-81 pg L^-1) concentrations are additively described by suspended particulate matter and particulate organic carbon. Higher total concentrations, carried by organic-rich particles, were found near specific discharge points (0.79-8.02 ng L^-1 and 34-235 pg L^-1 for IHg and MeHg, respectively). The associated high dissolved MeHg concentrations could not be explained only by biogeochemical variables. Better efficiency of the models is found for total than for dissolved concentrations. Models should be checked with other contaminants or with estuaries, suffering from downstream contamination.

Appraising ecotoxicological risk of mercury species and their mixtures in sediments to aquatic biota using diffusive gradients in thin films (DGT)

Mercury (Hg) is a global, persistent and inevitable pollutant, the toxicity of which is mostly reflected in its species including inorganic Hg (InHg) and methyl mercury (MeHg). Using diffusive gradients in thin films (DGT) is deemed as a reliable technique to determine the bioavailability of pollutants. This study is the first attempt to assess the integrated toxicity of mercury species mixtures in sediments to the aquatic biota based on the DGT technique. In the course, the Daya Bay under serious anthropogenic influences was selected as the study case. The results showed that the DGT concentrations of InHg and MeHg were detected as 0.30-1.93 μg/L and 0.28-1.94 μg/L respectively in the surface sediments collected from the Daya Bay. In terms of the toxicity of single mercury species, the risk quotient (RQ) values of InHg and MeHg significantly exceeded 1, indicating that the adverse effects of InHg and MeHg should not be ignored. In terms of the integrated toxicity of mercury species mixtures, the probabilistic biological risk assessment results demonstrate that Daya Bay features low (3.32%) probability of toxic effects in its surface sediments to the aquatic biota.

Demethylation─The Other Side of the Mercury Methylation Coin: A Critical Review

The public and environmental health consequences of mercury (Hg) methylation have drawn much attention and considerable research to Hg methylation processes and their dynamics in diverse environments and under a multitude of conditions. However, the net methylmercury (MeHg) concentration that accumulates in the environment is equally determined by the rate of MeHg degradation, a complex process mediated by a variety of biotic and abiotic mechanisms, about which our knowledge is limited. Here we review the current knowledge on MeHg degradation and its potential pathways and mechanisms. We describe detoxification by resistant microorganisms that employ the Hg resistance (mer) system to reductively break the carbon-mercury (C-Hg) bond producing methane (CH₄) and inorganic mercuric Hg(II), which is then reduced by the mercuric reductase to elemental Hg(0). Very recent research has begun to elucidate a mechanism for the long-recognized mer-independent oxidative demethylation, likely involving some strains of anaerobic bacteria as well as aerobic methane-oxidizing bacteria, i.e., methanotrophs. In addition, photochemical and chemical demethylation processes are described, including the roles of dissolved organic matter (DOM) and free radicals as well as dark abiotic demethylation in the natural environment about which little is currently known. We focus on mechanisms and processes of demethylation and highlight the uncertainties and known effects of environmental factors leading to MeHg degradation. Finally, we suggest future research directions to further elucidate the chemical and biochemical mechanisms of biotic and abiotic demethylation and their significance in controlling net MeHg production in natural ecosystems.

Binding strength of mercury (II) to different dissolved organic matter: The roles of DOM properties and sources

Dissolved organic matter (DOM) influences the environmental fate and toxic effects of trace metals such as mercury (Hg). However, because of limits in DOM analytical techniques and lack of sample diversity in past studies, it remains unclear whether the binding strength of DOM complexed with Hg(II) is related to the DOM properties. In this study, different DOM isolates (n = 26) from various sources were used to determine the conditional stability constant (logK) of DOM-Hg complexes using the equilibrium dialysis ligand exchange (EDLE) method. UV-Vis and fluorescence spectrometry were used to evaluate the correlation between logK values and DOM properties, such as chromophoric moieties, aromaticity, and molecular weight. Results demonstrated that the DOM from different sources presented an extensive range of binding strengths to Hg(II), because of their heterogeneous properties. Moreover, DOM chromophores, including aromaticity and molecular weight, are critical indicators of the DOM-Hg affinity in ambient-relevant circumstances. Significantly, higher terrestrial DOM led to greater DOM-Hg affinity. Additionally, this study supports that UV-Vis and fluorescence spectroscopy can be used to estimate DOM composition and its binding strength with Hg(II). Furthermore, the observed relationship between logK and DOM properties provided a possible pathway of explanation for the spatial co-variations between Hg(II) concentrations and DOM characters observed in previous field investigations.

Distribution of mercury isotope signatures in Yundang Lagoon, Xiamen, China, after long-term interventions

Isotope signatures of mercury (Hg) were determined for Hg fractions in seawater, sediments, porewaters, core sediments and fish from the Yundang Lagoon, Xiamen, China. Sequential extraction was used to extract Hg fractions in sediments and the purge-trap method was used to preconcentrate Hg in seawater. A large variation in mass dependent fractionation (δ²⁰²Hg: -2.50‰ to -0.36‰) was observed in the lagoon. Seawater and fish samples showed positive mass-independent fractionation (Δ¹⁹⁹Hg: -0.06‰-0.45‰), while most of sediment and porewater samples displayed insignificant mass-independent fractionation (Δ¹⁹⁹Hg: -0.10‰-0.07‰). Ancillary parameters (total organic carbon, sulfide, pH, Eh, water content and grain size) were also measured in the sediments to investigate correlations with Hg isotopes. Three sources (domestic sewage, sediments and atmospheric deposition) were identified as the main sources of Hg in the lagoon seawater. Photochemical reaction was the main process causing isotope fractionation in seawater. Through Hg partitioning and deposition, light isotopes were enriched from dissolved Hg to particulate Hg, then to sediments, and then to porewaters. Finally, Hg isotope signatures were used to identify the Hg sources and fractionation processes in core sediments from different depths. Our results demonstrate that Hg isotopes are powerful tools for tracing Hg sources and arriving at a better understanding of Hg biogeochemical cycling in the lagoon after long-term interventions.

Distribution of mercury species in different tissues and trophic levels of commonly consumed fish species from the south Bay of Biscay (France)

Mercury (Hg) is a contaminant of global concern in marine ecosystems, notably due to its ability to accumulate and concentrate in food webs. Concentrations of total mercury (THg), methylmercury (MeHg) and inorganic mercury (IHg) were assessed and compared in different tissues (liver, muscle, and gonads) of three common fish species (hake Merluccius merluccius, red mullet Mullus surmuletus, and sole Solea solea) from the continental shelf from the southern part of the Bay of Biscay. Several studies investigated Hg concentration in fish muscle, but few assessed concentrations in other organs, despite the importance of such data to understand contaminant organotropism and metabolization. Results showed that trophic position and feeding habitat are required to understand the variability of Hg concentration in muscle between fish species. In addition, high MeHg/THg ratio in muscle could be explained by the predatory behavior of the studied fish species and the biomagnification of this Hg species within the food web, MeHg. Despite differences between species, Hg concentration was always higher in muscle (from 118 ± 64 to 338 ± 101 ng g^-1 w.w.) and liver (from 122 ± 108 to 271 ± 95 ng g^-1 w.w.). These results can be related to physiological processes especially the MeHg detoxification strategies.

Mercury distribution and enrichment in coastal sediments from different geographical areas in the North Atlantic Ocean

This study evaluated mercury (Hg) in three coastal areas in the North Atlantic Ocean (Azores archipelago, Northwest Portugal and Southeast Mexico) and calculated their enrichment factor and geoaccumulation index. Hg presented greater affinity with fine sediment fraction. In all sampling areas the sites with proximity to possible Hg hotspots presented increased [Hg] in fine sediments. However, this fraction only represents less than 1% of the whole sediment, resulting in a significantly lower [Hg] in the total sediment fraction. After comparing [Hg] in the fine fraction and the values established by the Sediment Quality Guidelines, two sites in the Azores exceeded these values. Nevertheless, when considering the total sediment fraction, adverse biological effects are not expected to occur. Finally, both enrichment factor and geoaccumulation index indicated that the Hg present in 86% comes from natural sources and only sampling sites close to potential hotspots showed some degree of enrichment and contamination.

Microbial degradation of hydrophobic emerging contaminants from marine sediment slurries (Capbreton Canyon) to pure bacterial strain

Despite emerging contaminants (ECs) are more and more monitored in environmental matrices, there is still a lack of data in marine ecosystems, especially on their fate and degradation potentials. In this work, for the first time, the degradation potential of synthetic musks (galaxolide and tonalide), UV filters (padimate O and octocrylene) and a pharmaceutical compound (carbamazepine) was studied in marine sediment samples, under laboratory conditions using sediment slurry incubations under biotic and abiotic conditions. Minimum half life times under biotic conditions were found at 21 days, 129 days and 199 days for padimate O, galaxolide and carbamazepine, respectively. Enrichments conducted under anoxic and oxic conditions demonstrated that degradations after one month of incubation either under both biotic and abiotic conditions were limited under anoxic conditions compared to oxic conditions for all the contaminants. Novel aerobic bacteria, able to degrade synthetic musks and UV filters have been isolated. These novel strains were mainly related to the Genus Bacillus. Based on these results, the isolated strains able to degrade such ECs, can have a strong implication in the natural resilience in marine environment, and could be used in remediation processes.

Marine mercury-methylating microbial communities from coastal to Capbreton Canyon sediments (North Atlantic Ocean)

Microbial mercury (Hg) methylation transforms inorganic mercury to neurotoxic methylmercury (MeHg) mainly in aquatic anoxic environments. Sampling challenges in marine ecosystems, particularly in submarine canyons, leads to a lack of knowledge about the Hg methylating microbia in marine sediments. A previous study showed an enrichment of mercury species in sediments from the Capbreton Canyon where both geochemical parameters and microbial activities constrained the net MeHg production. In order to characterize Hg-methylating microbial communities from coastal to deeper sediments, we analysed the diversity of microorganisms' (16S rDNA-based sequencing) and Hg methylators (hgcA based cloning and sequencing). Both, 16S rDNA and hgcA gene analysis demonstrated that the putative Hg-methylating prokaryotes were likely within the Deltaproteobacteria, dominated by sulfur-compounds based reducing bacteria (mainly sulfate reducers). Additionally, others clades were also identified as carrying HgcA gene, such as, Chloroflexi, Spirochaetes, Elusimicrobia, PVC superphylum (Plantomycetes, Verrucomicrobia and Chlamydiae) and Euryarchaea. Nevertheless, 61% of the hgcA sequences were not assigned to specific clade, indicating that further studies are needed to understand the implication of new microorganisms carrying hgcA in the Hg methylation in marine environments. These first results suggest that sulfur cycle drives the Hg-methylation in marine ecosystem.

Priority and emerging micropollutants distribution from coastal to continental slope sediments: A case study of Capbreton Submarine Canyon (North Atlantic Ocean)

Incising continental margins, submarine canyons are key issue for understanding shelf/deep sea exchange of particulate pollutant, impact on marine ecosystem and global geochemical cycling. The occurrence and distribution of 100 priority and emerging micropollutants were investigated in sediments within the first 25 km of the Capbreton submarine area. The most predominant compounds were polycyclic aromatic hydrocarbons (PAHs), trace metals and metalloid (TMs) (e.g. mercury, lead and arsenic), synthetical musks (e.g. musk ketone, galaxolide), UV filters (e.g. octocrylene and 2-ethylhexyl 4-methoxycinnamate, EHMC) as well as some pharmaceuticals (e.g. azithromycin, acetaminophen). Highest concentrations were measured in submarine canyon sediments, distant from the coast and were correlated with both organic carbon and fine fraction contents, where PAHs, EHMC and musk ketone concentrations up to 7116, 32 and 7 ng g^-1 dry weight, respectively. Those results likely demonstrate, that atmospheric inputs of pyrogenic PAHs, and both trapping and transporting of polluted particles along the continuum shore/deep sea by the Capbreton Canyon, might lead to an accumulation of anthropogenic micropollutants. The ecological risk assessment indicates that priority pollutants raise a potentially high risk for benthic organisms (e.g. PAHs, TMs). This might raised a specific concern about how the human can impact this ecosystem.