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) 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 NOM by Hg L-edge EXAFS spectroscopy. The Hg(Cys) 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 H NMR spectroscopy andC isotope labeled Cys. The log K ± SD for the formation of the Hg(NOM-RS) molecular structure, Hg + 2NOM-RS = Hg(NOM-RS), and for the Hg(Cys)(NOM-RS) mixed complex, Hg + 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 H NMR spectroscopy and the Hg(NOM-RS) structure was verified by Hg L-edge EXAFS spectroscopy. An important finding is that the thermodynamic stabilities of the complexes Hg(NOM-RS), Hg(Cys)(NOM-RS) and Hg(Cys) 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) 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) is transformed to methylmercury (MeHg) by anaerobic microbes. In order to understand this transformation and the mobility and transport of Hg and MeHg, factors and conditions in control of the solubility and chemical speciation of Hg and MeHg need to be clarified. Here we explore the ability of thermodynamic models to simulate measured solubility of Hg 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-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 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 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.

Oxygen‐depleted areas are spreading in coastal and offshore waters worldwide, but the implication for production and bioaccumulation of neurotoxic methylmercury (MeHg) is uncertain. We combined observations from six cruises in the Baltic Sea with speciation modeling and incubation experiments to gain insights into mercury (Hg) dynamics in oxygen depleted systems. We then developed a conceptual model describing the main drivers of Hg speciation, fluxes, and transformations in water columns with steep redox gradients. MeHg concentrations were 2–6 and 30–55 times higher in hypoxic and anoxic than in normoxic water, respectively, while only 1–3 and 1–2 times higher for total Hg (THg). We systematically detected divalent inorganic Hg (HgII) methylation in anoxic water but rarely in other waters. In anoxic water, high concentrations of dissolved sulfide cause formation of dissolved species of HgII: HgS2H−(aq) and Hg (SH)20(aq). This prolongs the lifetime and increases the reservoir of HgII readily available for methylation, driving the high MeHg concentrations in anoxic zones. In the hypoxic zone and at the hypoxic‐anoxic interface, Hg concentrations, partitioning, and speciation are all highly dynamic due to processes linked to the iron and sulfur cycles. This causes a large variability in bioavailability of Hg, and thereby MeHg concentrations, in these zones. We find that zooplankton in the summertime are exposed to 2–6 times higher MeHg concentrations in hypoxic than in normoxic water. The current spread of hypoxic zones in coastal systems worldwide could thus cause an increase in the MeHg exposure of food webs.

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.

Environmental contextThe chemical speciation of mercury (Hg) largely controls its biogeochemical cycling and exposure to biota. Here, we investigate the thermodynamic stabilities of complexes formed between inorganic divalent Hg (HgII) and 15 biogeochemically relevant low-molecular-mass (LMM) thiol ligands. This information is critical for accurate modelling of the chemical speciation of HgII and to clarify the role of HgII–LMM thiol complexes in the cycling of Hg in the environment. AbstractInorganic divalent mercury (HgII) has a very high affinity for reduced sulfur functional groups. Reports from laboratory experiments suggest that HgII complexes with specific low-molecular-mass (LMM) thiol (RSH) ligands control rates of HgII transformation reactions. Because of methodological limitations for precise determination of the highly stable HgII complexes with LMM thiol ligands, constants reported in the literature remain inconsistent. This uncertainty impedes accurate modelling of the chemical speciation of HgII and the possibility to elucidate the role of HgII complexes with LMM thiols for Hg transformation reactions. Here, we report values of thermodynamic stability constants for 15 monodentate, two-coordinated HgII complexes, Hg(SR)2, formed with biogeochemically relevant LMM thiol ligands. The constants were determined by a two-step ligand-exchange procedure where the specific Hg(SR)2 complexes were quantified by liquid chromatography–inductively coupled plasma mass spectrometry. Thermodynamic stability constants (log β2) determined for the Hg(SR)2 complexes ranged from 34.6, N-cysteinylglycine, to 42.1, 3-mercaptopropionic acid, for the general reaction Hg2++2RS– ⇌ Hg(SR)2. Density functional theory (DFT) calculations showed that electron-donating carboxyl and carbonyl groups have a stabilising effect on the HgII–LMM thiol complexes, whereas electron-withdrawing protonated primary amino groups have a destabilising effect. Experimental results and DFT calculations demonstrated that the presence of such functional groups in the vicinity of the RSH group caused significant differences in the stability of Hg(SR)2 complexes. These differences are expected to be important for the chemical speciation of HgII and its transformation reactions in environments where a multitude of LMM thiol compounds are present.

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.

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The potent neurotoxin methylmercury (MeHg) is a major concern due to its negative effects on wildlife and human health. Boreal wetlands play a crucial role in Hg cycling on a global scale, and therefore, it is crucial to understand the biogeochemical processes involved in MeHg formation in this landscape element. By combining high-throughput hgcA amplicon sequencing with molecular barcoding, we reveal diverse clades of potential HgII methylators in a wide range of wetland soils. Among Bacteria, Desulfuromonadota (14% of total reads), Desulfurobacterota_A, and Desulfurobacterota (up to 6% of total reads), previously classified as Deltaproteobacteria, were important members of the hgcA+ microbial community in the studied wetlands. We also identified Actinobacteriota (9.4% of total reads), Bacteroidota (2% of total reads), and Firmicutes (1.2% of total reads) as members of the hgcA+ microbial community. Within Archaea, Methanosarcinales represented up to 2.5% of the total reads. However, up to half of the hgcA+ community could not be resolved beyond domain Bacteria. Our survey also shows that local physicochemical conditions, such as pH, nutrient concentrations, water content, and prevailing redox states, are important for shaping the hgcA+ microbial community structure across the four studied wetlands. Furthermore, we observed a significant correlation between HgII methylation rate constants and the structure of the hgcA+ microbial community. Our findings expand the current knowledge on the hgcA+ microbial community composition in wetlands and the physicochemical factors underpinning spatial heterogeneity in such communities.