Microbial mercury resistance and potential methylation rates in the Upper Wisconsin River

Abstract

Microbial mercury resistance and potential mercury methylation rates were examined in water, sediment-floc, and sediment cores from the Upper Wisconsin River. Mercury (II) resistance was quantified using aerobic and anaerobic heterotrophic media containing 0-75 Hg/ml pg+2. Methylation activity was determined by measuring CH32O3Hg+ formation from 203Hg(N03)2. Sediment incubations were carried out under strict anaerobic conditions. Aerobic and anaerobic heterotrophic bacteria were highly resistant to 14 pg/ml Hg+2. Anaerobic heterotrophic bacteria were more resistant to higher concentrations of Hg+2 than aerobic heterotrophic bacteria. Mercury methylation activity was near background in the water, highest in surface sediments, and decreased with increasing sediment depth. More than 98% of the added 203Hg was bound to sediments within 4 hrs of inoculation, while more than 3% was methylated during a 10-day incubation. As much as 7% of the added 203Hg was methylated in other experiments. This suggests that bound Hg+2 was available for methylation. Organical1y enriched sediments exhibited higher methylation actiity than less eutrophic sediments. The addition of peptone to sediments caused highly significant (p < 0.01) increases in methylation activity, while vitamin B12 and sewage sludge caused significant (p < 0.05) increases. The presence of oxygen in sediments inhibited methylation activity. This indicates that mercury methylation in the Upper Wisconsin River is primarily an anaerobic process. The optimum temperature for methylation was 35degreesC, although the maximum in situ temperature was 24degreesC. A seasona1 summer peak in methylation activity was observed in water, f1oc, and sediments. These data suggest that the Upper Wisconsin River sediments have the potential to release large amounts of toxic methylmercury to the overlying water