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Research article - Peer-reviewed, 2023

The Combined Effect of Hg(II) Speciation, Thiol Metabolism, and Cell Physiology on Methylmercury Formation by Geobacter sulfurreducens

Gutensohn, Mareike; Schaefer, Jeffra K.; Yunda, Elena; Skyllberg, Ulf; Bjorn, Erik

Abstract

The chemical and biological factors controlling microbial formation of methylmercury (MeHg) are widely studied separately, but the combined effects of these factors are largely unknown. We examined how the chemical speciation of divalent, inorganic mercury (Hg(II)), as controlled by low-molecular-mass thiols, and cell physiology govern MeHg formation by Geobacter sulfurreducens. We compared MeHg formation with and without addition of exogenous cysteine (Cys) to experimental assays with varying nutrient and bacterial metabolite concentrations. Cysteine additions initially (0-2 h) enhanced MeHg formation by two mechanisms: (i) altering the Hg(II) partitioning from the cellular to the dissolved phase and/or (ii) shifting the chemical speciation of dissolved Hg(II) in favor of the Hg(Cys)2 complex. Nutrient additions increased MeHg formation by enhancing cell metabolism. These two effects were, however, not additive since cysteine was largely metabolized to penicillamine (PEN) over time at a rate that increased with nutrient addition. These processes shifted the speciation of dissolved Hg(II) from complexes with relatively high availability, Hg(Cys)2, to complexes with lower availability, Hg(PEN)2, for methylation. This thiol conversion by the cells thereby contributed to stalled MeHg formation after 2-6 h Hg(II) exposure. Overall, our results showed a complex influence of thiol metabolism on microbial MeHg formation and suggest that the conversion of cysteine to penicillamine may partly suppress MeHg formation in cysteine-rich environments like natural biofilms.

Keywords

mercury methylation; low molecular mass thiols; mercury speciation; anaerobe microorganisms

Published in

Environmental Science and Technology
2023, Volume: 57, number: 18, pages: 7185-7195
Publisher: AMER CHEMICAL SOC

    UKÄ Subject classification

    Biochemistry and Molecular Biology
    Microbiology

    Publication identifier

    DOI: https://doi.org/10.1021/acs.est.3c00226

    Permanent link to this page (URI)

    https://res.slu.se/id/publ/122251