METHYLATION AND DEMETHYLATION OF SOLID AND ADSORBED MERCURY SPECIES IN CONTRASTING MARINE ENVIRONMENTS

Methylmercury (MeHg) is a neurotoxin produced from inorganic mercury (Hg^II) by, for example, sulfate and iron reducing bacteria in aquatic sediments. The bacteria take up dissolved neutral Hg species by passive uptake, or by other mechanisms, making the rate of MeHg production dependent on the bacterial activity as well as the concentration of bioavailable species. Since mercury has a strong affinity to the solid/adsorbed phase, as shown by its high partitioning coefficient (K_D), and is mostly in the solid phase in sediments, the rate of methylation is also dependent on the rate of mercury dissolution/desorption of bioavailable Hg from the solid/adsorbed phase. The bioavailability of different forms of solid/adsorbed Hg species is largely unknown. A recent study has shown, using multi isotope dilution with solid or adsorbed Hg tracers, the availability to be in the order of Hg^II (aq) > Hg bound to organic matter (OM) or FeS (s) > α- and β-HgS(s) (Jonsson et al., 2012). This data is however limited to one single site. We have conducted a follow-up study using this approach examining sediments from multiple estuaries on the US east coast (sampled in summer of 2015) representing contrasting environments with respect to temperature, salinity, sediment organic matter content and sulfide concentration. Methylation assays were performed by amending sediment with isotope-labelled ionic Hg in the following forms: inorganic Hg bound to dissolved and particulate OM of marine origin; nanoparticulate β-HgS(s) coated with DOM and aggregated in the absence of DOM (both aged 1, 3 or 7 days); and aqueous Hg(OH)₂. Furthermore, we investigated the demethylation potential of MeHg using a similar approach with MeHg adsorbed onto marine DOM and POM. We will present the data from these experiments and discuss the results in terms of the kinetics and thermodynamics of the Hg within each assay, the relative importance of these factors in controlling both methylation and demethylation in sediments. These results are critical for understanding and modelling the impact of changing anthropogenic inputs and climate change on the concentrations of Hg and MeHg in the environment.

Jonsson, S.; Skyllberg, U.; Nilsson, M.B.; Westlund, P.-.; Shchukarev, A.; Lundberg, E.; Björn, E. ES&T 2012, 46 (21), 11653-11659.