Structural incorporation of iron influences biomethylation potential of mercury sulfide

Mercury and ferrous iron commonly co-exist in anaerobic environments, the hotspots of microbial mercury methylation that leads to formation of neurotoxic methylmercury. However, the influence of ferrous iron on mercury bioavailability and methylation remains unclear. Here, we discover that microbial methylmercury production is inhibited by iron addition in a non-monotonical manner, reaching the minimal level at the initial molar ratio of dissolved Fe/Hg as 1/3, when highly aggregated Hg–S–Fe nanoparticles occur and prevent the nanoparticle–bacteria interactions. The dominant mercury species formed upon Hg–S–Fe co-precipitation is iron-incorporated nano-metacinnabar with short-range structural disorder. The methylation potential of this mercury species is inversely correlated with the aggregation state of nanoparticles. The non-monotonical trends of the aggregation state and subsequent methylation of nano-metacinnabar are due to the opposite effects of ferrous iron on the nucleation and particle attachment that determine the formation of nanoparticle aggregates. Increased Fe/Hg ratio hinders particle nucleation and thus decreases the density of particle monomers for aggregation. However, Fe substitution of sub-surface Hg weakens the inner-sphere coordination of surface Hg with natural thiol ligands and consequently enhances nano-metacinnabar aggregation. This new mechanistic insight is important for assessing and minimizing the risks of mercury methylation, and informs how intertwined iron and sulfur cycling may influence metal bioavailability in the changing environment.