Anaerobic HgII reduction is driven by cellular HgII-thiol interactions

Redox reactions play a critical role in determining the availability of mercury species, Hg^II and Hg⁰, to anaerobic microbes responsible for methylating inorganic mercury into toxic monomethylmercury. Some anaerobes also contribute to Hg cycling in methylation hotspots by reducing Hg^II to its gaseous elemental form, Hg⁰. However, their contributions remain poorly quantified due to limited mechanistic insights and the absence of genetic targets. In this study, we investigated the mechanisms of anaerobic Hg^II reduction in the versatile anoxygenic photoheterotroph and fermenter Heliomicrobium modesticaldum Ice1. Given Hg^II strong electrophilic affinity for thiol groups, we hypothesized that cellular thiols are key interaction sites mediating Hg^II reduction. Exposure of H. modesticaldum to the thiol-alkylating agent N-ethylmaleimide (NEM), which irreversibly binds thiols, resulted in a concentration-dependent inhibition of Hg⁰ production during both photoheterotrophy and fermentation. Hg partitioning assays with Escherichia coli cells revealed no significant differences in Hg-cell partitioning in the presence or absence of NEM, suggesting that Hg^II reduction is dependent on intracellular thiol interactions. These findings highlight the importance of thiol-mediated pathways in Heliobacterial Hg^II reduction. Although the exact cellular components remain unidentified, we discuss potential thiol-containing coupling sites that warrant further investigation.