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

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.

New Insights into MeHg Accumulation in Rice (Oryza sativa L.): Evidence from Cysteine

“…We hypothesize that Cys acts as a carrier to facilitate MeHg transportation in rice plants, as observed in chlorophytes and microorganisms. , The presence of Cys promotes the formation of the Hg–Cysteine complex, which can be actively taken up by Geobacter sulfurreducens via the plasma membrane transporters . The chemical structures of the MeHg–Cysteine complex depend on the reaction between Cys and MeHg, particularly on the molecular ratio of MeHg to Cys.…”

“…71,72 The presence of Cys promotes the formation of the Hg−Cysteine complex, which can be actively taken up by Geobacter sulfurreducens via the plasma membrane transporters. 73 The chemical structures of the MeHg−Cysteine complex depend on the reaction between Cys and MeHg, particularly on the molecular ratio of MeHg to Cys. The variety of chemical structures of MeHg−Cysteine complex affects their bioavailability and accumulation.…”

CdSe Core and CdSe@ZnSe Core–Shell QDs Decorated with Human Serum Albumin: Deciphering the Antimicrobial Properties and Sensing of Mercury Ion

“…Thus, from FTIR, EDX, and XPS analysis results, we can conclude that the biosyntheses of CdSe core and CdSe@ZnSe core–shell QDs using HSA form a layer on the respective QDs surface, having both functional groups (positive and negatively charged ) intake with the protein layer. The passivating layer (especially the -SH group) provides a robust biochemical affinity toward Hg 2+ . The excellent luminescence properties and the strong affinity toward Hg 2+ of the respective biosynthesized QDs lay the basis for a fluorescence probe for Hg 2+ ions.…”

“…The passivating layer (especially the -SH group) provides a robust biochemical affinity toward Hg 2+ . 63 The excellent luminescence properties and the strong affinity toward Hg 2+ of the respective biosynthesized QDs lay the basis for a fluorescence probe for Hg 2+ ions. Moreover, the extremely small size (TEM and XRD analyses) of the synthesized respective CdSe-HSA and CdSe@ ZnSe-HSA QDs and their unique surface properties also open a new window to treating the synthesized bio-QDs as antimicrobial agents.…”

Biochemical characterization and mercury methylation capacity of Geobacter sulfurreducens biofilms grown in media containing iron hydroxide or fumarate

“…A possible role of glycogen as a long-term electron storage in electroactive G. sulfurreducens biofilms has been addressed [ 60 ]. The evidence of glycogen accumulation in G. sulfurreducens obtained using ATR-FTIR spectroscopy has recently been reported [ 61 ]. It was found that upon the transfer of planktonic cells into a minimal nutritive medium used for Hg(II) methylation assays the content of glycogen in bacteria varies as a function of time.…”