Determining seawater mercury methylation and demethylation rates by the seawater incubation approach: A critique

The discovery of a pervasive subsurface methylmercury maximum in the world's oceans highlights the importance of understanding the processes and rates that produce and destroy this potent neurotoxin in seawater. Two approaches have been published in the literature, one based on the spatial distribution of mercury species in seawater and, more recently, a second approach based on seawater incubation studies using additions of isotopically enriched mercury species. The reaction rates determined by these two approaches differ by several orders of magnitude. Using Arctic Ocean seawater and the incubation approach to determine mercury methylation and demethylation rates, we observed unexplainable methylation and demethylation at time zero and poor fitting of the data to first-order kinetics. A critical analysis of previous incubation studies reveals similar deficiencies, especially when the mercury was added at low concentrations to simulate ambient seawater conditions. These findings cast doubt on the validity of mercury methylation and demethylation rates determined by the seawater incubation approach, and call for the development of alternative methods to determine these crucially important rate constants in ambient seawater.