Abstract
Redox reactions play a critical role in determining the availability of mercury species, Hg(II) and Hg(0), 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, Hg(0). 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 Hg(0) 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.