Abstract
To advance the scientific understanding of bacteria-driven mercury (Hg) transformation processes in natural environments, thermodynamics and kinetics of divalent mercury Hg(II) chemical speciation need to be understood. Based on Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopic information, combined with competitive ligand exchange (CLE) experiments, we determined Hg(II) structures and thermodynamic constants for Hg(II) complexes formed with thiol functional groups in bacterial cell membranes of two extensively studied Hg(II) methylating bacteria: Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. The Hg EXAFS data suggest that 5% of the total number of membranethiol functionalities (Mem-RStot = 380 ± 50 μmol g–1 C) are situated closely enough to be involved in a 2-coordinated Hg(Mem-RS)2 structure in Geobacter. The remaining 95% of Mem-RSH is involved in mixed-ligation Hg(II)-complexes, combining either with low molecular mass (LMM) thiols like Cys, Hg(Cys)(Mem-RS), or with neighboring O/N membrane functionalities, Hg(Mem-RSRO). We report log K values for the formation of the structures Hg(Mem-RS)2, Hg(Cys)(Mem-RS), and Hg(Mem-RSRO) to be 39.1 ± 0.2, 38.1 ± 0.1, and 25.6 ± 0.1, respectively, for Geobacter and 39.2 ± 0.2, 38.2 ± 0.1, and 25.7 ± 0.1, respectively, for ND132. Combined with results obtained from previous studies using the same methodology to determine chemical speciation of Hg(II) in the presence of natural organic matter (NOM; Suwannee River DOM) and 15 LMM thiols, an internally consistent thermodynamic data set is created, which we recommend to be used in studies of Hg transformation processes in bacterium–NOM–LMM thiol systems.
Highlights
Methylmercury (MeHg), a highly toxic pollutant that poses a serious risk to humans and ecosystems, is primarily produced by microbial methylation of inorganic divalent mercury, Hg(II).[1−3] In addition to the activity of the methylating organisms, MeHg formation is controlled by the bioavailability and biouptake of Hg(II) species,[4] where reactions with reduced sulfur are of utmost importance.[1,5]
Total organic carbon (TOC) content accounted for 51 and 41% of the membrane mass of Geobacter and ND132, respectively, which is in agreement with a previous study.[42]
Sulfur K-edge X-ray absorption nearedge structure (XANES) spectra (Figure S1 and Table S2) of whole cell and membrane samples demonstrated a high dominance of Org-SRED, representing the sum of reduced organic S functionalities organic disulfide (RSSR), monosulfide (RSR), and thiol (RSH) that cannot be separated by this methodology
Summary
Methylmercury (MeHg), a highly toxic pollutant that poses a serious risk to humans and ecosystems, is primarily produced by microbial methylation of inorganic divalent mercury, Hg(II).[1−3] In addition to the activity of the methylating organisms, MeHg formation is controlled by the bioavailability and biouptake of Hg(II) species,[4] where reactions with reduced sulfur (including inorganic sulfide and organic thiol functional groups) are of utmost importance.[1,5]. The reaction of Hg(II) with Mem-RSH is expected to be kinetically controlled, 24 h should with marginal be sufficient to allow equilibrium to be reached according to previous studies of Hg(II) with thiols of NOM, cells and membranes.[19,21,28,37] aliquots of Cys stock solution were added to obtain a final concentration of 2.0 μM. This solution was sampled for total Hg(II) (Hgtot) and Hg(Cys)[2] analysis at the time of 1, 24, 48 and 72 h.
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