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The effect of aqueous speciation and cellular ligand binding on the biotransformation and bioavailability of methylmercury in mercury-resistant bacteria

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Abstract

Mercury resistant bacteria play a critical role in mercury biogeochemical cycling in that they convert methylmercury (MeHg) and inorganic mercury to elemental mercury, Hg(0). To date there are very few studies on the effects of speciation and bioavailability of MeHg in these organisms, and even fewer studies on the role that binding to cellular ligands plays on MeHg uptake. The objective of this study was to investigate the effects of thiol complexation on the uptake of MeHg by measuring the intracellular demethylation-reduction (transformation) of MeHg to Hg(0) in Hg-resistant bacteria. Short-term intracellular transformation of MeHg was quantified by monitoring the loss of volatile Hg(0) generated during incubations of bacteria containing the complete mer operon (including genes from putative mercury transporters) exposed to MeHg in minimal media compared to negative controls with non-mer or heat-killed cells. The results indicate that the complexes MeHgOH, MeHg-cysteine, and MeHg-glutathione are all bioavailable in these bacteria, and without the mer operon there is very little biological degradation of MeHg. In both Pseudomonas stutzeri and Escherichia coli, there was a pool of MeHg that was not transformed to elemental Hg(0), which was likely rendered unavailable to Mer enzymes by non-specific binding to cellular ligands. Since the rates of MeHg accumulation and transformation varied more between the two species of bacteria examined than among MeHg complexes, microbial bioavailability, and therefore microbial demethylation, of MeHg in aquatic systems likely depends more on the species of microorganism than on the types and relative concentrations of thiols or other MeHg ligands present.

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Acknowledgments

We thank G. Endo for providing bacterial strains. This project was supported by grants from NSF-EAR (Geobiology & Low Temperature Geochemistry, EAR-0952291) and the U.S. Department of Energy, Office of Science (BER, DE-SC0007051), and a Hatch/McIntyre-Stennis grant through the New Jersey Agricultural Experiment Station.

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Authors and Affiliations

  1. Marine Sciences Department, University of Connecticut, Groton, CT, USA

    Udonna Ndu & Robert P. Mason

  2. Department of Biochemistry and Microbiology, Rutgers University, New Brunswick, NJ, USA

    Tamar Barkay

  3. Department of Environmental Health, Harvard School of Public Health, Boston, MA, USA

    Amina Traore Schartup

  4. Department of Environmental Sciences, Rutgers University, New Brunswick, NJ, USA

    Udonna Ndu & John R. Reinfelder

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  1. Udonna Ndu
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  2. Tamar Barkay
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Correspondence to Udonna Ndu.

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Ndu, U., Barkay, T., Schartup, A.T. et al. The effect of aqueous speciation and cellular ligand binding on the biotransformation and bioavailability of methylmercury in mercury-resistant bacteria. Biodegradation 27, 29–36 (2016). https://doi.org/10.1007/s10532-015-9752-3

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