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The most critical step for methylmercury (MeHg) bioaccumulation in aquatic food webs is phytoplankton uptake of dissolved MeHg. Dissolved organic matter (DOM) has been known to influence MeHg uptake, but the mechanisms have remained unclear. Here we show that the concentration of DOM-associated thiol functional groups (DOM-RSH) varies substantially across contrasting aquatic systems and dictates MeHg speciation and bioavailability to phytoplankton. Across our 20 study sites, DOM-RSH concentrations decrease 40-fold from terrestrial to marine environments whereas dissolved organic carbon (DOC), the typical proxy for MeHg binding sites in DOM, only has a 5-fold decrease. MeHg accumulation into phytoplankton is shown to be directly linked to the concentration of specific MeHg binding sites (DOM-RSH), rather than DOC. Therefore, MeHg bioavailability increases systematically across the terrestrial-marine aquatic continuum as the DOM-RSH concentration decreases. Our results strongly suggest that measuring DOM-RSH concentrations will improve empirical models in phytoplankton uptake studies and will form a refined basis for modeling MeHg incorporation in aquatic food webs under various environmental conditions., (© 2023. Springer Nature Limited.)

Sediments represent the main reservoir of mercury (Hg) in aquatic environments and may act as a source of Hg to aquatic food webs. Yet, accumulation routes of Hg from the sediment to benthic organisms are poorly constrained. We studied the bioaccumulation of inorganic and methylmercury (Hg II and MeHg, respectively) from different geochemical pools of Hg into four groups of benthic invertebrates (amphipods, polychaetes, chironomids, and bivalves). The study was conducted using mesocosm experiments entailing the use of multiple isotopically enriched Hg tracers and simulation of estuarine systems with brackish water and sediment. We applied different loading regimes of nutrients and terrestrial organic matter and showed that the vertical localization and the chemical speciation of Hg II and MeHg in the sediment, in combination with the diet composition of the invertebrates, consistently controlled the bioaccumulation of Hg II and MeHg into the benthic organisms. Our results suggest a direct link between the concentration of MeHg in the pelagic planktonic food web and the concentration of MeHg in benthic amphipods and, to some extent, in bivalves. In contrast, the quantity of MeHg in benthic chironomids and polychaetes seems to be driven by MeHg accumulation via the benthic food web. Accounting for these geochemical and dietary drivers of Hg bioaccumulation in benthic invertebrates will be important to understand and predict Hg transfer between the benthic and the pelagic food web, under current and future environmental scenarios.

Risk assessment of chemicals is still primarily focusing on single compound evaluation, even if environmental contamination consists of a mixture of pollutants. The concentration addition (CA) and independent action (IA) models have been developed to predict mixture toxicity. Both models assume no interaction between the components, resulting in an additive mixture effect. In the present study, the embryo toxicity test (OECD TG no. 236) with zebrafish embryos (Danio rerio) was performed to investigate whether the toxicity caused by binary, ternary, and quaternary mixtures of organic (Benzo[a]pyrene, perfluorooctanesulfonate, and 3,3´,4,4´,5-pentachlorobiphenyl 126) and inorganic (arsenate) pollutants can be predicted by CA and IA. The acute toxicity and sub-lethal alterations such as lack of blood circulation were investigated. The models estimated the mixture toxicity well and most of the mixtures were additive. However, the binary mixture of PFOS and PCB126 caused a synergistic effect, with almost a ten-fold difference between the observed and predicted LC 50 -value. For most of the mixtures, the CA model was better in predicting the mixture toxicity than the IA model, which was not expected due to the chemicals' different modes of action. In addition, some of the mixtures caused sub-lethal effects not observed in the single compound toxicity tests. The mixture of PFOS and BaP caused a division of the yolk and imbalance was caused by the combination of PFOS and As and the ternary mixture of PFOS, As, and BaP. Interestingly, PFOS was part of all three mixtures causing the mixture specific sub-lethal effects. In conclusion, the present study shows that CA and IA are mostly resulting in good estimations of the risks that mixtures with few components are posing. However, for a more reliable assessment and a better understanding of mixture toxicity, further investigations are required to study the underlying mechanisms., (Copyright © 2022. Published by Elsevier B.V.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Gadani shipbreaking area, Pakistan, is the world's third largest shipbreaking unit. However, to date, only a few studies on the environmental impacts of the industry, including mercury (Hg) pollution, have been conducted. To address this, concentrations of total Hg (HgT) and methylmercury (MeHg) were measured in surface sediments collected from the Gadani shipbreaking area as well as a local reference area. The highest concentrations of HgT and MeHg (median ± interquartile range) were detected in samples from the beach at the yard zone (HgT: 270 ± 230 μg kg -1 , MeHg: 0.65 ± 0.69 μg kg -1 ), followed by sediment samples from the inter/sub-tidal zone where ships are dismantled (HgT: 20 ± 5.8 μg kg -1 , MeHg: 0.043 ± 0.016 μg kg -1 ). These concentrations were on average 4-50 and 3-30 times greater than the concentrations of HgT and MeHg, respectively, observed in the reference area. CAPSULE: Elevated concentrations of total and methylated mercury observed in the Gadani Shipbreaking area sediments., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Methylmercury (MeHg) is a neurotoxin formed from inorganic divalent mercury (Hg II ) via microbial methylation, and boreal wetlands have been identified as major sources of MeHg. There is however a lack of studies investigating the relationship between the chemical speciation of Hg II and MeHg formation in such environments, in particular regarding to role of thiol compounds. We determined Hg II methylation potentials, k meth , in boreal wetland soils using two Hg II isotope tracers: 198 Hg(OH) 2 (aq) and Hg II bonded to thiol groups in natural organic matter, 200 Hg II -NOM(ads), representing Hg II sources with high and low availability for methylation. The 198 Hg(OH) 2 (aq) tracer was consistently methylated to a 5-fold higher extent than 200 Hg II -NOM(ads), independent of environmental conditions. This suggests that the concentration of Hg II in porewater was a decisive factor for Hg II methylation. A comprehensive thermodynamic speciation model (including Hg II complexes with inorganic sulfide (H 2 S), polysulfides (H 2 S n ), thiols associated with natural organic matter (NOM-RSH) and specific low molecular mass thiols (LMM-RSH) provided new insights on the speciation of Hg II in boreal wetland porewaters, but did not demonstrate any clear relationship between k meth and the calculated chemical speciation. In contrast, significant positive relationships were observed between k meth and the sum of LMM thiol compounds of biological origin. We suggest two possible mechanisms underlying these correlations: 1) LMM thiols kinetically control the size and composition of the Hg II pool available for microbial uptake, and/or 2) LMM thiols are produced by microbes such that the correlation reflects a relation between microbial activity and MeHg formation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.)

Potential degradation pathways of dimethylmercury (DMHg) remain as one of the critical knowledge gaps in the marine biogeochemical cycle of mercury (Hg). Although Hg is known to be highly reactive with reduced sulfur, demethylation of DMHg in the presence of sulfide has until now remained experimentally untested. Here, we provide the first experimental support for demethylation of DMHg to monomethylmercury (MMHg) in the presence of both dissolved sulfide and mackinawite (FeS(s) m ). The degradation of DMHg was shown to be pH dependent, with higher demethylation rates at pH 9 than pH 5. At room temperature and environmentally relevant DMHg to sulfide molar ratios, we observed demethylation rates up to 0.05 d -1 . When comparing the number of active sites available, FeS(s) m was found to have a higher capacity to demethylate DMHg, in comparison with dissolved sulfide. Our study suggests that dissolved sulfide and FeS(s) m mediated demethylation of DMHg may act as a sink for DMHg, and a potential source of MMHg, in aquatic systems.

Thiol compounds (R-SH) have many important biological functions and are principal controls of the speciation of several toxic metals in the environment. However, determining the concentration of thiols associated with environmental matrices is challenging due to the compounds' low abundance and interferences from non-thiol compounds for many available methods. Here a novel method has been developed and validated to quantify the total concentration of thiol functional groups in aqueous samples using derivatization with monobromo(trimethylammonio)bimane (qBBr) and quantification with tandem mass spectrometry. The thiol concentration was determined by titration of the sample with qBBr, which reacts selectively with thiols, and quantification of the residual qBBr. We systematically evaluated potential interferences from various organic compounds, inorganic ions (including sea water matrices), sulfide and mercury (Hg) species, and demonstrate that the method is highly sensitive, selective and robust. The limit of detection (LOD) for total thiols is in the nanomolar concentration range (~6 nM). The method performance was also demonstrated by determination of the total thiol concentration in different natural samples including boreal stream water (1.16 μM), wetland porewater (0.96 μM) and the Suwanee River natural organic matter (NOM) reference material SR101 N (7.9 μmol g -1 ). The developed method represents a combination of low LOD and high selectivity and robustness that is unsurpassed for total thiol concentration measurements., (Copyright © 2020 Elsevier B.V. All rights reserved.)

In the recent decade, the hydroelectric reservoir is identified as a methylmercury (MeHg) hotspot and gained much attention. The artificial water level management in the Three Gorges Reservoir (TGR) in China formed a water-level-fluctuation zone (WLFZ) undergoing flooding drying rotations annually. However, the mercury (Hg) methylation and major geochemical driving factors at different elevations in the WLFZ remain unclear. Here we use total Hg (Hg T ) normalized MeHg (MeHg/Hg T ratio) to evaluate Hg methylation degree in a one-year field study at 155, 165 m elevations in the WLFZ and with >175 m elevation as the reference. Results demonstrate that MeHg/Hg T ratio at the WLFZ could reach 4.1% in soils, and both 155 and 165 m elevations have a higher Hg methylation degree than the >175 m elevation. However, the differences in MeHg/Hg T ratios both in soils and waters between 155 and 165 m elevations are not significant. This indicates the influence of different submerging periods on the MeHg/Hg T at the WLFZ elevations is not observed. The significant correlation between the MeHg/Hg T ratio and soil organic carbon (SOC) content implies a MeHg retention in re-exposed soils after flooding. Decoupling of MeHg/Hg T ratios between submerged soil and overlying water are found at both elevations and therefore make MeHg/Hg T in waters alone cannot be used to evaluate Hg methylation degree in this study. The calculation of Hg T and MeHg partitioning coefficient (K d ) found an immobilization of MeHg by submerged soils at the WLFZ during the flooding period. Major geochemical factors, determined through principal component analysis (PCA), in affecting Hg methylation are the redox cycling of sulfur and the distribution of organic matters in the WLFZ., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Methylmercury (MeHg) is one of the most potent neurotoxins. It is produced in nature through the methylation of inorganic divalent mercury (Hg II ) by phylogenetically diverse anaerobic microbes. The mechanistic understanding of the processes that govern the extent of bacterial export of MeHg, its bioaccumulation, and bio-toxicity depends on accurate quantification of its species, especially its complexation with low molecular mass thiols; organometallic complexes that are difficult to detect and measure in natural conditions. Here, we report the development of a novel analytical method based on liquid chromatography tandem mass spectrometry (LC-MS/MS) to determine 13 MeHg complexes with important thiol compounds which have been observed in the environment and in biological systems. By using online preconcentration via solid phase extraction (SPE), the method offers picomolar (12-530 pM) detection limits, the lowest reported so far for the determination of MeHg compounds. Among three different SPE materials, a weak cation exchange phase showed the best efficiency at a low pH of 2.5. We further report the presence of MeHg-cysteine, MeHg-cysteamine, MeHg-penicillamine, MeHg-cysteinylglycine, and MeHg-glutamylcysteine as the predominant MeHg-thiol complexes in the extracellular milieu of an important Hg II methylating bacterium, Geobacter sulfurreducens PCA, exposed to 100 nM of Hg II .

We present a sensitive, selective and robust method for the determination of 14 thiol compounds in aqueous samples. Thiols were derivatized with ω-bromoacetonylquinolinium bromide (BQB) and its deuterium labeled equivalent D7-ω-bromoacetonylquinolinium bromide (D7). Derivatized thiols were preconcentrated by online solid-phase extraction (SPE) followed by liquid chromatography separation and electrospray ionization tandem mass spectrometry determination (SPE/LC-ESI-MS/MS). The robustness of the method was validated for wide ranges in pH, salinity, and concentrations of sulfide and dissolved organic carbon (DOC) to cover contrasting natural water types. The limits of detection (LODs) for the thiols were 3.1-66 pM. Between 6 and 14 of the thiols were detected in different natural sample types at variable concentrations: boreal wetland porewater (0.7-51 nM), estuarine sediment porewater (50 pM-11 nM), coastal sea water (60 pM-16 nM), and sulfate reducing bacterium cultures (80 pM-4 nM). MS/MS fragmentation of the compounds produces two pairs of common product ions, m/z 130.2/137.1 and 218.1/225.1, which enables scanning for unknown thiols in precursor ion scan mode. Using this approach, we identified cysteine, mercaptoacetic acid, N-acetyl-L-cysteine and sulfurothioic S-acid in boreal wetland porewater. The performance of the developed method sets a new state of the art for the determination of thiol compounds in environmental and biological samples., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Cellular uptake of inorganic divalent mercury (Hg(II)) is a key step in microbial formation of neurotoxic methylmercury (MeHg), but the mechanisms remain largely unidentified. We show that the iron reducing bacterium Geobacter sulfurreducens produces and exports appreciable amounts of low molecular mass thiol (LMM-RSH) compounds reaching concentrations of about 100 nM in the assay medium. These compounds largely control the chemical speciation and bioavailability of Hg(II) by the formation of Hg(LMM-RS) 2 complexes (primarily with cysteine) in assays without added thiols. By characterizing these effects, we show that the thermodynamic stability of Hg(II)-complexes is a principal controlling factor for Hg(II) methylation by this bacterium such that less stable complexes with mixed ligation involving LMM-RSH, OH - , and Cl - are methylated at higher rates than the more stable Hg(LMM-RS) 2 complexes. The Hg(II) methylation rate across different Hg(LMM-RS) 2 compounds is also influenced by the chemical structure of the complexes. In contrast to the current perception of microbial uptake of Hg, our results adhere to generalized theories for metal biouptake based on metal complexation with cell surface ligands and refine the mechanistic understanding of Hg(II) availability for microbial methylation.

Elevated methylmercury (MeHg) production in mining-contaminated paddy soils, despite the high fraction of refractory HgS(s), has been frequently reported, while the underlying mechanisms are not fully understood. Here, we hypothesized that sulfate input, via fertilization, rainfall, and irrigation, is critical in mobilizing refractory HgS(s) and thus enhancing Hg methylation in mining-contaminated paddy soils. To test this hypothesis, the effects of sulfate amendment on Hg methylation and MeHg bioaccumulation in mining-contaminated soil-rice systems were examined. The results indicated 28-61% higher net MeHg production in soils under sulfate amendment (50-1000 mg kg -1 ), which in turn increased grain MeHg levels by 22-55%. The enhancement of Hg methylation by Hg mobilization in sulfate-amended soils was supported by two observations: (1) the increased Hg(aq) release from HgS(s), the dominant Hg species in the paddy soils, in the presence of sulfide produced following sulfate reduction and (2) the decreases of refractory HgS(s) in soils under sulfate amendment. By contrast, changes in the abundances/activities of potential microbial Hg methylators in different Hg-contaminated soils were not significant following sulfate amendment. Our results highlight the importance to consider enhanced Hg mobility and thus methylation in soils under sulfate amendment.

A molecular level understanding of the thermodynamics and kinetics of the chemical bonding between mercury, Hg(II), and natural organic matter (NOM) associated thiol functional groups (NOM-RSH) is required if bioavailability and transformation processes of Hg in the environment are to be fully understood. This study provides the thermodynamic stability of the Hg(NOM-RS) 2 structure using a robust method in which cysteine (Cys) served as a competing ligand to NOM (Suwannee River 2R101N sample) associated RSH groups. The concentration of the latter was quantified to be 7.5 ± 0.4 μmol g -1 NOM by Hg L III -edge EXAFS spectroscopy. The Hg(Cys) 2 molecule concentration in chemical equilibrium with the Hg(II)-NOM complexes was directly determined by HPLC-ICPMS and losses of free Cys due to secondary reactions with NOM was accounted for in experiments using 1 H NMR spectroscopy and 13 C isotope labeled Cys. The log K ± SD for the formation of the Hg(NOM-RS) 2 molecular structure, Hg 2+ + 2NOM-RS - = Hg(NOM-RS) 2 , and for the Hg(Cys)(NOM-RS) mixed complex, Hg 2+ + Cys - + NOM-RS - = Hg(Cys)(NOM-RS), were determined to be 40.0 ± 0.2 and 38.5 ± 0.2, respectively, at pH 3.0. The magnitude of these constants was further confirmed by 1 H NMR spectroscopy and the Hg(NOM-RS) 2 structure was verified by Hg L III -edge EXAFS spectroscopy. An important finding is that the thermodynamic stabilities of the complexes Hg(NOM-RS) 2 , Hg(Cys)(NOM-RS) and Hg(Cys) 2 are very similar in magnitude at pH values <7, when all thiol groups are protonated. Together with data on 15 low molecular mass (LMM) thiols, as determined by the same method ( Liem-Ngyuen et al. Thermodynamic stability of mercury(II) complexes formed with environmentally relevant low-molecular-mass thiols studied by competing ligand exchange and density functional theory . Environ. Chem. 2017 , 14 , ( 4 ), 243 - 253 .), the constants for Hg(NOM-RS) 2 and Hg(Cys)(NOM-RS) represent an internally consistent thermodynamic data set that we recommend is used in studies where the chemical speciation of Hg(II) is determined in the presence of NOM and LMM thiols.

Boreal wetlands have been identified as environments in which inorganic divalent mercury (Hg II ) is transformed to methylmercury (MeHg) by anaerobic microbes. In order to understand this transformation and the mobility and transport of Hg II and MeHg, factors and conditions in control of the solubility and chemical speciation of Hg II and MeHg need to be clarified. Here we explore the ability of thermodynamic models to simulate measured solubility of Hg II and MeHg in different types of boreal wetland soils. With the input of measured concentrations of MeHg, sulfide, eight low molecular mass thiols and thiol groups associated with natural organic matter (NOM), as determined by sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy and Hg L III -edge extended X-ray absorption fine structure spectroscopy (EXAFS), the model could accurately predict porewater concentrations of MeHg in the wetlands. A similar model for Hg II successfully predicted the average level of its concentration in the porewaters, but the variability among samples, driven mainly by the concentration of aqueous inorganic sulfide, was predicted to be larger than measurements. The smaller than predicted variability in Hg II solubility is discussed in light of possible formation of colloidal HgS(s) passing the 0.22 μm filters used to define the aqueous phase. The chemical speciation of the solid/adsorbed and aqueous phases were dominated by NOM associated thiol complexes for MeHg and by an equal contribution from NOM associated thiols and HgS(s) for Hg II .

Net formation of methylmercury (MeHg) in sediments is known to be affected by the availability of inorganic divalent mercury (Hg(II)) and by the activities of Hg(II) methylating and MeHg demethylating bacteria. Enhanced autochthonous organic matter deposition to the benthic zone, following increased loading of nutrients to the pelagic zone, has been suggested to increase the activity of Hg(II) methylating bacteria and thus the rate of net methylation. However, the impact of increased nutrient loading on the biogeochemistry of mercury (Hg) is challenging to predict as different geochemical pools of Hg may respond differently to enhanced bacterial activities. Here, we investigate the combined effects of nutrient (N and P) supply to the pelagic zone and the chemical speciation of Hg(II) and of MeHg on MeHg formation and degradation in a brackish sediment-water mesocosm model ecosystem. By use of Hg isotope tracers added in situ to the mesocosms or ex situ in incubation experiments, we show that the MeHg formation rate increased with nutrient loading only for Hg(II) tracers with a high availability for methylation. Tracers with low availability did not respond significantly to nutrient loading. Thus, both microbial activity (stimulated indirectly through plankton biomass production by nutrient loading) and Hg(II) chemical speciation were found to control the MeHg formation rate in marine sediments.

Low molecular mass (LMM) thiols is a diverse group of compounds, which play several important roles in aquatic ecosystems, even though they typically occur at low concentrations. Comprehensive studies of LMM thiols in natural waters have so far been hampered by selectivity and limit of detection constraints of previous analytical methods. Here, we describe a selective and robust method for the quantification of 16 LMM thiols in natural waters. Thiols were derivatized with 4-(hydroxymercuri)benzoate (PHMB) and preconcentrated online by solid-phase extraction (SPE) before separation by liquid chromatography and determination by electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Their quantification was performed by selective reaction monitoring (SRM), while the presence of a product ion at m/z 355, specific for thiols and common for the investigated compounds, also allows to screen samples for unknown thiols by a precursor ion scan approach. The robustness of the method was validated for aqueous matrices with different pH, sulfide, and dissolved organic carbon (DOC) concentrations. The limits of detection for the thiols were in the sub-nanomolar range (0.06-0.5 nM) and the methodology allowed determination of both reduced and total thiol concentrations (using tris(2-carboxyethyl)phosphine (TCEP) as reducing agent). Six thiols (mercaptoacetic acid, cysteine, homocysteine, N-acetyl-cysteine, mercaptoethane-sulfonate, and glutathione) were detected with total concentrations of 7-153 nM in boreal lake or wetland pore waters while four thiols (mercaptoacetic acid, cysteine, homocysteine, and N-acetyl-cysteine) were detected in their reduced form at concentrations of 5-80 nM.