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  • 1.
    Adediran, Gbotemi A.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Liem-Nguyen, Van
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Song, Yu
    Schaefer, Jeffra K.
    Slcyllberg, Ulf
    Björn, Erik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Microbial Biosynthesis of Thiol Compounds: Implications for Speciation, Cellular Uptake, and Methylation of Hg(II)2019In: Environmental Science and Technology, ISSN 0013-936X, E-ISSN 1520-5851, Vol. 53, no 14, p. 8187-8196Article in journal (Refereed)
    Abstract [en]

    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)<INF><INF><INF>2</INF></INF> </INF>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)<INF>2</INF> complexes. The Hg(II) methylation rate across different Hg(LMM-RS)<INF>2</INF> 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.

  • 2. Liem-Nguyen, Van
    et al.
    Nguyen-Ngoc, Hoang-Tung
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Adediran, Gbotemi A.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Björn, Erik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Determination of picomolar levels of methylmercury complexes with low molecular mass thiols by liquid chromatography tandem mass spectrometry and online preconcentration2020In: Analytical and Bioanalytical Chemistry, ISSN 1618-2642, E-ISSN 1618-2650Article in journal (Refereed)
    Abstract [en]

    Methylmercury (MeHg) is one of the most potent neurotoxins. It is produced in nature through the methylation of inorganic divalent mercury (HgII) 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 HgII methylating bacterium, Geobacter sulfurreducens PCA, exposed to 100 nM of HgII.

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  • 3.
    Zhu, Wei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Song, Yu
    Adediran, Gbotemi A.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Jiang, Tao
    Reis, Ana T.
    Pereira, Eduarda
    Skyllberg, Ulf
    Björn, Erik
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mercury transformations in resuspended contaminated sediment controlled by redox conditions, chemical speciation and sources of organic matter2018In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 1872-9533, Vol. 220, p. 158-179Article in journal (Refereed)
    Abstract [en]

    Mercury (Hg) contaminated sediments can be significant sources of Hg in aquatic ecosystems and, through re-emission processes, to the atmosphere. Transformation and release of Hg may be enhanced by various sediment perturbation processes, and controlling biogeochemical factors largely remain unclear. We investigated how rates of Hg transformations in pulp-fiber enriched sediment contaminated by Hg from chlor-alkali industry were controlled by (i) transient redox-changes in sulfur and iron chemistry, (ii) the chemical speciation and solubility of Hg, and (iii) the sources and characteristics of organic matter (OM). Sediment-bottom water microcosm systems were exposed to four combinations of air and nitrogen gas for a total time of 24 h. The treatments were: 24 h N-2, 0.5 h air + 23.5 h N-2, 4 h air + 20 h N-2 and 24 h of air exposure. As a result of these treatments, microcosms spanned a wide range of redox potential, as reflected by the dissolved sulfide concentration range of <= 0.3-97 mu M. Four different chemical species of inorganic divalent Hg (Hg-II) and methyl mercury (MeHg), enriched in different Hg isotope tracers, were added to the microcosms: 201 Hg(NO3)(2)(aq), Hg-202(II) adsorbed to OM (Hg-202(II)-OM(ads)), Hg-198(II) as microcrystalline metacinnabar (beta-(HgS)-Hg-198(s)) and (MeHgCl)-Hg-204(aq). Microcosm systems were composed of bottom water mixed with sediment taken at 0-2, 0-5 and 0-10 cm depth intervals. The composition of OM varied with sediment depth such that compared to deeper sediment, the 0-2 cm depth-interval had a 2-fold higher contribution of labile OM originating from algal and terrestrial inputs, serving as metabolic electron-donors for microorganisms. The potential methylation rate constant (k(meth)) of Hg tracers and net formation of ambient MeHg (MeHg/THg molar ratio) increased up to 50% and 400%, respectively at intermediate oxidative conditions, likely because of an observed 2-fold increase in sulfate concentration stimulating the activity of sulfate reducing bacteria with a capability of methylating Hg-II. Due to differences in Hg-II water-sediment partitioning, k(meth) varied by a factor of 11-70 for the different isotope-enriched Hg tracers. The chemical form of Hg-II was a major controlling factor for k(meth) and its response to the resuspension-oxidation of the system. The beta-(HgS)-Hg-198(s) tracer had the lowest k(meth) and it was mainly constrained by redox-driven Hg-II solubility. The Hg-202(II)-OM(ads) tracer showed an intermediate value on k(meth). It was controlled by both Hg-II solubility and availability of electron donors and acceptors, regulating bacterial activity. The Hg-201(NO3)(2)(aq) tracer had the highest value on k(meth) which was limited mainly by bacterial activity. The k(meth) was up to a factor of 3 higher in the 0-2 cm sediment depth-interval than in 0-5 and 0-10 cm intervals due to a larger contribution of labile OM in the 0-2 cm sediment. Reduction of Hg-II to Hg-0 followed by volatilization exclusively occurred at high sulfidic conditions in the top 0-2 cm sediment. Aromatic moieties of terrestrial OM, present mainly in the top sediment, is suggested to control the reduction of Hg-II. The Hg-0 volatilization rate constant for the Hg-202(II)-OM (ads) tracer exceeded that for beta-(HgS)-Hg-198(s) by one order of magnitude. Our results suggest that contaminated sediments posing a high risk for reactivation of legacy Hg following transient redox resuspension events are characterized by depletion of sulfate in the sediment porewater prior to resuspension, predominance of Hg-II species with solubility exceeding that of crystalline beta-HgS(s), and conditions promoting in situ formation and/ or import of labile OM from algal and terrestrial sources.

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