Coupling mercury methylation rates to sulfate reduction rates in marine sediments

Anoxic slurry incubations were performed to examine the relationship betwee n mercury methylation rates (MMR) and sulfate reduction rates (SRR) in salt marsh sediments from the southeastern United States. The MMR and SRR were similarly affected by temperature, addition of low-weight molecular organic s, and microbial inhibitors. The MMR was at a maximum in the 12 h after ino rganic mercury addition. Initial rates of Hg methylation are correlated wit h SRR. A significantly slower MMR observed after the initial 12 h following inorganic mercury addition suggested that sorption or precipitation reduce d the availability of mercury for methylation. The MMR results for various concentrations of inorganic mercury indicate that saturation kinetics occur . Using this kinetic framework, a model for MMR based on SRR and inorganic Hg concentration was developed. This model was then used to predict the MMR reported in temperature and substrate/inhibitor experiments. The model pro vided a reasonable estimate of MMR observed in the initial 12 h of the slur ry incubations. However, the sequestering of inorganic mercury into less re active pools by sorption to surfaces or incorporation into other phases alt ers the amount of bioavailable Hg and hence MMR. Future models that assess the bioavailibility of Hg in the sediments must be developed to extend thes e observations to field sites.