<xhtml:span xmlns:xhtml="http://www.w3.org/1999/xhtml" xml:lang="en">Anaerobic HgII&#160;reduction is driven by cellular HgII-thiol interactions </xhtml:span>

Redox reactions play a critical role in determining the availability of mercury species, HgII 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 HgII 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 HgII reduction in the versatile anoxygenic photoheterotroph and fermenter Heliomicrobium modesticaldum Ice1. Given HgII strong electrophilic affinity for thiol groups, we hypothesized that cellular thiols are key interaction sites mediating HgII 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 HgII reduction is dependent on intracellular thiol interactions. These findings highlight the importance of thiol-mediated pathways in Heliobacterial HgII reduction. Although the exact cellular components remain unidentified, we discuss potential thiol-containing coupling sites that warrant further investigation.