Relationship Between Hg Speciation and Hg Methylation/Demethylation Processes in the Sulfate-Reducing Bacterium Pseudodesulfovibrio hydrargyri: Evidences From HERFD-XANES and Nano-XRF

Abstract

Microorganisms are key players in the transformation of mercury into neurotoxic methylmercury (MeHg). Nevertheless, this mechanism and the opposite MeHg demethylation remain poorly understood. Here, we explored the impact of inorganic mercury (IHg) and MeHg concentrations from 0.05 to 50 μM on the production and degradation of MeHg in two sulfate-reducing bacteria, Pseudodesulfovibrio hydrargyri BerOc1 able to methylate and demethylate mercury and Desulfovibrio desulfuricans G200 only able to demethylate MeHg. MeHg produced by BerOc1 increased with increasing IHg concentration with a maximum attained for 5 μM, and suggested a saturation of the process. MeHg was mainly found in the supernatant suggesting its export from the cell. Hg L 3 -edge High- Energy-Resolution-Fluorescence-Detected-X-ray-Absorption-Near-Edge-Structure spectroscopy (HERFD-XANES) identified MeHg produced by BerOc1 as MeHg-cysteine 2 form. A dominant tetracoordinated βHgS form was detected for BerOc1 exposed to the lowest IHg concentrations where methylation was detected. In contrast, at the highest exposure (50 μM) where Hg methylation was abolished, Hg species drastically changed suggesting a role of Hg speciation in the production of MeHg. The tetracoordinated βHgS was likely present as nano-particles as suggested by transmission electron microscopy combined to X-ray energy dispersive spectroscopy (TEM-X-EDS) and nano-X ray fluorescence (nano-XRF). When exposed to MeHg, the production of IHg, on the contrary, increased with the increase of MeHg exposure until 50 μM for both BerOc1 and G200 strains, suggesting that demethylation did not require intact biological activity. The formed IHg species were identified as various tetracoordinated Hg-S forms. These results highlight the important role of thiol ligands and Hg coordination in Hg methylation and demethylation processes.

Effect of exogenous and endogenous sulfide on the production and the export of methylmercury by sulfate-reducing bacteria

“…The chosen cysteine and sul de concentrations correspond to the minimal sulfur concentration needed for optimal growth without Hg(II) (Online Resource 2a). The Hg(II) concentrations were chosen from a previous study that showed a maximum of mercury methylation rate at 5 µM of Hg(II) (Isaure et al 2020). Sul de concentrations measured at the end of the growth in the presence of cysteine (0.010 ± 0.003 -0.020 ± 0.005 mM, Interestingly, the Hg(II) methylation potential under non-sul dogenic growth with 0.1 mM sul de was higher than in the other tested conditions, regardless of the Hg(II) concentration (Figure 1a, Online Resource Table ).…”

“…For this purpose, we used P. hydrargyri BerOc1 under three different conditions regarding sul de contents: grown i) under fumarate respiration (non sul dogenic metabolism) in the presence of a wide range of exogenous sul de concentrations, ii) under fumarate respiration in the presence of cysteine and, iii) under sulfate reduction (sul dogenic metabolism). Because Hg methylation depends on Hg(II) concentration (Gilmour et al 2011;Isaure et al 2020), the effect of sul des (added or produced) was also examined at three different Hg(II) concentrations (from nM to µM). Our study provides new insights into the major role of sul des on Hg methylation process at sul de concentrations of environmental relevance, which is important to address the risks of Hg regarding the environmental and the health standpoints on this issue.…”

Bioinformatics Investigations of Universal Stress Proteins from Mercury-Methylating Desulfovibrionaceae

“…A yeast divalent cation transporter DMT1 of participates in the uptake of inorganic mercury [ 56 ]. Research publications on the uptake of inorganic mercury in mercury-methylating Desulfovibrionaceae species and related organisms could guide these future studies [ 57 , 58 , 59 , 60 ].…”

Recent developments of X‐ray absorption spectroscopy as analytical tool for biological and biomedical applications

“…[11][12][13][14] For significantly improved signal-to-noise ratio for studying biological systems, methods providing high energy resolution fluorescence detected XAS (HERFD-XAS) are constantly gaining importance. [15][16][17][18] In cases where the analyte concentration is extremely low, in nanogram or even picogram range, the use of total reflection X-ray fluorescence XAS (TXRF-XAS) is the variant of choice. 19 Also relevant in the field of biological systems is the advent of X-ray free electron lasers (XFELs) producing ultrashort intense X-ray pulses (pulse widths <100 fs).…”

“…For significantly improved signal‐to‐noise ratio for studying biological systems, methods providing high energy resolution fluorescence detected XAS (HERFD‐XAS) are constantly gaining importance 15–18 . In cases where the analyte concentration is extremely low, in nanogram or even picogram range, the use of total reflection X‐ray fluorescence XAS (TXRF‐XAS) is the variant of choice 19 …”