Methylation and demethylation of mercury by sulphate-reducing bacteria: input from the combination of HERFD-Xanes, nano-XRF and GC-ICP MS

Marie-Pierre Isaure^a, M. Albertelli^a, I. Kieffer^b, R. Tucoulou^b, M. Monperrus^a and M. Goñi-Urriza^a

^a IPREM, CNRS and Université de Pau et des Pays de l’Adour, France

^b European Synchrotron Radiation Facility (ESRF), France

marie-pierre.isaure@univ-pau.fr

Mercury (Hg) is one of the most concerning pollutants at the global scale. It is persistent, highly volatile and is able to convert into methylmercury (MeHg), a strong human neurotoxic. MeHg is mainly released in the environment by sulfate-reducing bacteria (SRB), and is then biomagnified and bioaccumulated in the aquatic food web. Understanding the biotransformation processes of Hg by these microorganisms is thus a key for risk assessment in ecosystems and human health.

Despite the discovery of genes hgcA and hgcB involved in bacterial Hg methylation, the way Hg is taken up, methylated and released by SRB is still unknown. Our aim is to clarify these mechanisms at the bacterial cell level to progress in the understanding of Hg metabolic pathways. For that, we have been studying an original SRB strain Pseudodesulfovibrio hydrargyri BerOc1, able to methylate and demethylate Hg, and the Desulfovibrio alaskensis G20 SRB strain, only able to demethylate MeHg. Importantly, we used a novel approach combining High Energy Resolution Fluorescence Detected – X-ray Absorption Near Edge Structure spectroscopy (HERFD-XANES) at the Hg L~III~-edge to speciate Hg with high sensitivity and synchrotron nano-X-ray Fluorescence imaging (nano-XRF) to locate Hg and other elements at the bacterial cell level. Hg methylation/demethylation rates were also determined using multiple stable isotopic tracers (Me²⁰¹Hg and ¹⁹⁹Hg(II)) and Gaz Chromatography-ICP-MS (GC-ICP MS) after exposure to various inorganic Hg (IHg) and MeHg concentrations (from 10 ppb to 10 ppm) and various times.

Results showed that mercury methylation rates decreased with increasing Hg concentration in contrast to MeHg demethylation, suggesting a saturation of the Hg methylation process. HERFD-XANES identified a tetracoordinated βHgS form as dominant Hg species for both strains exposed to IHg while MeHg-cysteine was also detected in BerOc1 for the lower IHg exposure in agreement with Hg methylation rates. During the demethylation process, various Hg- and MeHg-thiols were also identified for both strains. These results highlight the important role of S ligands in Hg biotransformation processes, even when bacteria are grown in non-sulfidogenic conditions. Interestingly, nano-XRF and kinetics studies also suggest excretion of (Hg, S)- containing biomolecules by the bacterial cell.