SULFATE STIMULATION OF MERCURY METHYLATION IN FRESH-WATER SEDIMENTS

The relationship between bacterial sulfate reduction and mercury methylation, as well as the in situ distribution of methylmercury in sediments, was studied in Quabbin Reservoir, MA. Fish methylmercury levels in Quabbin and other lakes affected by acid deposition are often elevated. However, the cause of acceleration of net methylmercury production or bioaccumulation in these lakes is poorly understood. Experimental additions of sulfate to either anoxic sediment slurries or lake water above intact sediment cores resulted in increased microbial production of methylmercury from added inorganic mercury. Sediment depth profiles of bacterial sulfate reduction and mercury methylation were similar, and specific inhibition of sulfate-reducing bacteria blocked methylmercury production at all depths. In situ methylmercury concentrations, like sulfate-reduction rates, were highest near the sediment-water interface and in shallow sediments. These data suggest that sulfate-reducing bacteria are important mediators of mercury methylation in lacustrine sediments and provide a possible mechanism for increased methylmercury bioaccumulation in water bodies affected by increased sulfate deposition.