Microbial methylation potential of mercury sulfide particles dictated by surface structure

Environmental contamination by mercury in its organometallic form, methylmercury, remains a major global concern due to its neurotoxicity, environmental persistence and biomagnification through the food chain. Accurate prediction of mercury methylation cannot be achieved based on aqueous speciation alone, and there remains limited mechanistic understanding of microbial methylation of particulate-phase mercury. Here we assess the time-dependent changes in structural properties and methylation potential of nanoparticulate mercury using microscopic and spectroscopic analyses, microcosm bioassays and theoretical calculations. We show that the methylation potential of a mercury sulfide mineral ubiquitous in contaminated soils and sediments (nanoparticulate metacinnabar) is determined by its crystal structure. Methylmercury production increases when more of nano-metacinnabar’s exposed surfaces occur as the (111) facet, due to its large binding affinity to methylating bacteria, likely via the protein transporter responsible for mercury cellular uptake prior to methylation. During nanocrystal growth, the (111) facet diminishes, lessening methylation of nano-metacinnabar. However, natural ligands alleviate this process by preferentially adsorbing to the (111) facet, and consequently hinder natural attenuation of mercury methylation. We show that the methylation potential of nanoparticulate mercury is independent of surface area. Instead, the nano-scale surface structure of nanoparticulate mercury is crucial for understanding the environmental behaviour of mercury and other nutrient or toxic soft elements. The environmental behaviour of mercury and other toxic soft elements is in part dictated by the surface structure of nanoparticulates, according to a combination of microcosm bioassays and theoretical calculations.