Mercury (Hg) is a potent neurotoxin for humans, particularly if the metal is in the form of methylmercury. Mercury is widely distributed in aquatic ecosystems as a result of anthropogenic activities and natural earth processes. A first step towards bioaccumulation of methylmercury in aquatic food webs is the methylation of inorganic forms of the metal, a process that is primarily mediated by anaerobic microorganisms that are abundant in sediments. The production of methylmercury in the environment is controlled in part by the bioavailability of inorganic mercury Hg(II) to methylating microbes. In sediment porewater, mercury associates with sulfide and organic matter to form chemical species that include organic-coated mercury sulfide nanoparticles as reaction intermediates of heterogeneous mineral precipitation.
In this project, the geochemical processes that control the bioavailability of mercury to methylating bacteria in contaminated sediments were investigated. The research tested the hypothesis that kinetically-limited mercury sulfide mineralization reactions, rather than equilibrium porewater chemistry, controls the concentration of bioavailable mercury to sediment bacteria that convert it to methylmercury, the form that bioaccumulates in food webs. The relationship between mercury speciation and biouptake/methylation in sediments was studied, a relationship that remains poorly understood. The work focused specifically on the microbial methylation potential of nanoparticulate HgS in relation to bulk scale HgS and dissolved Hg-sulfide species. The aim was to establish a premise that links the ‘age’ and chemical form of Hg in sediment porewater to the rate of MeHg formation. The kinetic data was incorporated in a conceptual model describing the fate of mercury. The overall objectives were to assess the importance of nanoscale mercuric sulfides for methylation potential in sediments and to develop a conceptual model that links mercury geochemical speciation to methylation potential in sediments.
The research involved four major tasks. Tasks 1 and 2 involved pure culture studies in which the net production of MeHg was compared in bacterial cultures exposed to dissolved Hg and sulfide, nanoparticulate HgS, and bulk scale HgS.
Research for Task 3 involved kinetic modeling of the Hg speciation and net MeHg production rate of the pure culture experiments. The objective of the model calculation was to determine if the methylation of mercury originating from nanoparticles could be explained by dissolution of the particles and dissolved phase speciation. This model utilized kinetic expressions for complexation reactions involving dissolved Hg-ligand complexes and precipitation and dissolution reactions involving nanoparticles and microparticles of HgS. All dissolved forms of Hg were presumed to be bioavailable and rates of methylation and demethylation were fitted to previous methylmercury production experiments in which bacteria were exposed to dissolved mercury sulfides.
The final component of the research involved sediment slurry microcosm experiments in which the aim was to better capture the complexity of sediment settings in ways that could not be achieved with pure culture studies. The study involved sediment slurry microcosms that represented a spectrum of salinities in an estuary and were each amended with different forms of mercuric sulfides: dissolved Hg and sulfide, nanoparticulate HgS, and microparticulate HgS.
The results of the pure culture studies demonstrated that bacteria cultures exposed to HgS nanoparticles methylated mercury at a rate slower than cultures exposed to dissolved forms of mercury. However, methylation of the nanoparticles was considerably faster than larger microscale HgS particles, even when normalized to specific surface area. Furthermore, the methylation potential of HgS nanoparticles decreased with storage time of the nanoparticles in their original stock solution, suggesting that crystal ripening of the nanoparticles reduced their methylation potential. The methylation of mercury derived from nanoparticles (in contrast to the larger particles) would not be predicted by traditional models of mercury bioavailability and was probably caused by the disordered structure of nanoparticles that facilitated release of chemically labile mercury species immediately adjacent to cell surfaces. Overall these findings add new dimensions to the understanding of mercury methylation potential by demonstrating that bioavailability is related to the geochemical intermediates of rate-limited mercury sulfide precipitation reactions.
In kinetic models of the pure culture experiments, the enhancement of methylmercury production in cultures exposed to HgS nanoparticles relative to HgS microparticles could be simulated by assigning larger dissolution rates for the nanoparticles. However, the model showed that calculation of dissolved mercury through dissolution of HgS particles provided an incomplete picture of the overall bioavailability. The simulations were improved if a fraction of the nanoparticulate phase was assumed to be directly bioavailable, either through direct uptake of nanoparticles or the immediate uptake of Hg dissolving from the nanoparticles directly outside the cell. The results point to a new approach for modeling mercury speciation and bioavailability that considers the dynamic nature of mercury sulfide interactions in anaerobic environments.
In the sediment slurry microcosm experiments, the results indicated that net MeHg production was influenced by both the activity of sulfate-reducing microorganisms and the bioavailability of mercury. In the presence of abundant sulfate and carbon sources (resulting in relatively high microbial activity), net MeHg production in the slurries amended with dissolved Hg was greater than in slurries amended with nano-HgS, similar to previous experiments with pure bacterial cultures. However, in cases of minimal microbial activity (such as low sulfate reduction rate), the addition of either dissolved Hg or nano-HgS resulted in similar amounts of net MeHg production. For slurries receiving micro-HgS, MeHg production did not exceed abiotic controls. In slurries amended with dissolved and nano-HgS, mercury was mainly partitioned to bulk-scale mineral particles and colloids, such as iron sulfides, indicating that Hg bioavailability was not simply related to dissolved Hg concentration or speciation. Therefore, assessments of Hg bioavailability in sediments need to consider not only the dissolved phase speciation in pore water, but also the speciation of particle-bound Hg, including nanostructured species that may be weakly sorbed or more soluble than bulk mineral phases.
The results of this work demonstrated that dissolved phase speciation alone is inadequate for understanding and predicting Hg bioavailability to methylating microorganisms. The transformation reactions involving these mercury species, such as cluster formation, monomer aggregation and crystal ripening, are often times kinetically-hindered in the presence of DOM. Therefore, the bioavailability and methylation potential of mercury is most likely related to the ‘slow’ kinetics of these processes that control the relative abundance of various mercury species (i.e., those falling through a 0.2-mm filter), rather than the equilibrium chemistry. Future modeling efforts for predicting mercury bioavailability will need to consider the rate of transformations involving mercury species. Such an approach would require a series of rate constants for the geochemical reactions that dictate the concentration of the available forms of inorganic mercury for microbial methylation.