Defining ‘Reactive' Inorganic Mercury and Its Relationship to Microbial Methylmercury Formation in Aquatic Ecosystem Studies

Mercury (Hg) contamination of aquatic ecosystems has long been recognized as a persistent environmental problem that threatens both wildlife and human health. The conversion of inorganic divalent mercury (Hg(II)) to the more toxic methylmercury (MeHg) is a process largely facilitated by sulfate reducing bacteria in anoxic sediments. Recent advances in the study of Hg biogeochemistry have begun to unravel the most relevant environmental factors that control MeHg formation, both in terms of those that mediate the activity of the Hg(II)-methylating bacteria and those that mediate Hg(II) availability to those bacteria. Current evidence suggests that only a small fraction of the total Hg(II) may be available for Hg(II)-methylation; termed the ‘reactive' inorganic mercury (Hg(II)R) fraction. An operationally defined measure of Hg(II)R has been developed and is being used across a wide range of mercury ecosystem studies. Results from an international study comparing multiple methods used to quantify specific mercury fractions indicate that the Hg(II)R method provides a good proxy for the pool of Hg(II) truly available for Hg(II)-methylation. Deep core profiles from San Francisco Bay demonstrate that Hg(II)R concentrations are highest in surface sediment and decreases with depth. Geochemical data indicates that the size of this pool is controlled by sediment redox conditions and the association of Hg(II) with solid phase reduced sulfur minerals. Laboratory oxic/anoxic slurry experiments simulating sediment perturbations demonstrate that the oxidation of deeply buried sediment can increase Hg(II)R concentration up to 60-fold. This result is presumably related to the reoxidation of the reduced sulfur minerals that non-reactive Hg(II) is strongly bound to, and the subsequent liberation of Hg(II) (as Hg(II)R). These findings have significant implications for dredging operations and for the reconstruction of wetland habitat with dredge material.