Experimental evidence for recovery of mercury-contaminated fish populations

Anthropogenic releases of mercury (Hg)1–3 are a human health issue4 because the potent toxicant methylmercury (MeHg), formed primarily by microbial methylation of inorganic Hg in aquatic ecosystems, bioaccumulates to high concentrations in fish consumed by humans5,6. Predicting the efficacy of Hg pollution controls on fish MeHg concentrations is complex because many factors influence the production and bioaccumulation of MeHg7–9. Here we conducted a 15-year whole-ecosystem, single-factor experiment to determine the magnitude and timing of reductions in fish MeHg concentrations following reductions in Hg additions to a boreal lake and its watershed. During the seven-year addition phase, we applied enriched Hg isotopes to increase local Hg wet deposition rates fivefold. The Hg isotopes became increasingly incorporated into the food web as MeHg, predominantly from additions to the lake because most of those in the watershed remained there. Thereafter, isotopic additions were stopped, resulting in an approximately 100% reduction in Hg loading to the lake. The concentration of labelled MeHg quickly decreased by up to 91% in lower trophic level organisms, initiating rapid decreases of 38–76% of MeHg concentration in large-bodied fish populations in eight years. Although Hg loading from watersheds may not decline in step with lowering deposition rates, this experiment clearly demonstrates that any reduction in Hg loadings to lakes, whether from direct deposition or runoff, will have immediate benefits to fish consumers.

Natural organic matter facilitates formation and microbial methylation of mercury selenide nanoparticles

Tiemannite nanoparticles available for microbial mercury methylation are formed during the co-precipitation of natural organic matter, divalent mercury and selenium.

Exposed facets dictate microbial methylation potential of mercury sulfide nanoparticles

Methylmercury formation is the major concern of global mercury contamination. Accurate prediction of methylmercury production remains elusive due in part to the lack of mechanistic understanding of microbial methylation potential of particulate-phase mercury. Here we show that the methylation potential of nanoparticulate metacinnabar, which is formed during the early stage of mercury mineralization and is ubiquitous in contaminated soils and sediments, is determined by its exposed facets. Nanoparticulate metacinnabar with higher (111) content exhibits significantly greater affinity to the methylating bacterium Desulfovibrio desulfuricans ND132, leading to higher methylmercury production. This is likely attributable to the favored binding between the (111) facet and the protein transporter responsible for mercury cellular uptake prior to methylation. The (111) facet of metacinnabar tends to diminish during nanocrystal growth, but natural ligands alleviate this process by preferentially adsorbing to the (111) facet (verified with adsorption experiments using facet-engineered model materials coupled with theoretical calculations). This facet evolution of metacinnabar and its subsequent effect on mercury bioavailability explain the intriguing observation that methylation potential of nanoparticulate mercury is surface-area-independent. Our discovery provides new mechanistic insights for interfacial processes involved in nanoparticle−microorganism interactions that have important implications for understanding the environmental behavior of mercury and other nutrient or toxic elements associated with widely present crystalline nanoparticles.