The role of microorganisms in elemental mercury formation in natural waters

Gas evasion of elemental Hg (Hg°) from the open ocean plays a prominent role in the global mercury cycle. Elemental Hg is formed primarily by reduction of ionic Hg in the mixed layer of aquatic systems. By culturing phytoplankton in defined media, and by incubating natural seawater and freshwater samples, we have demonstrated that Hg° is produced by microorganisms, with formation rates (0.5 to 10% d^-1) similar to those estimated from mass balance studies. Our results also suggest that <3 μm microorganisms are the primary Hg reducers in natural waters. Eucaryotic phytoplankton are capable of reducing ionic Hg to Hg° but the rate of reduction is insufficient to account for the observed reduction rates found in incubated field samples. Bacteria are thus the more likely Hg reducers. In seawater, cyanobacteria such as Synecococcus may account for much of the mercury reduction, while in the eutrophic, polluted Upper Mystic Lake north of Boston other procaryotic microorganisms are contributing to the overall Hg reductive capacity of the medium. By reducing ionic Hg, microorganisms play a pivotal role in the aquatic biogeochemistry of Hg, not only by enabling evasion to the atmosphere, but by directly decreasing the amount of ionic Hg available for methylation.