Microbial methylation of mercury in upper Wisconsin river sediments

Abstract

The microbial methylation of Hg was studied in water and sediments from the Upper Wisconsin River by quantifying the methylation of radioactive Hg(II) (203Hg[NO3]2). Methylation activity was near detection limits in the water, highest in surface sediments (0 to 4 cm), and decreased with increasing sediment depth. Methylation had a temperature optimum of 35 °C. Organically enriched sediments exhibited higher methylation activity than less eutrophic sediments. Methylation activity in sediments was stimulated by the addition of peptone but not by glucose or starch. Oxygenation of sediments inhibited methylation activity. A summertime peak in methylation activity, observed in water, floc, and sediments, was related to factors other than temperature. More than 98% of the added 203Hg(II) was bound to sediments within 4 hr of inoculation, while more than 3% was methylated during a 10-day incubation. As much as 7% of the added 203Hg(II) was methylated in other experiments, suggesting that bound Hg is available for methylation. These data suggest that organic-rich surficial sediments in the Upper Wisconsin River have the potential to produce significant amounts of toxic methylmercury during late summer months.