Bacterial methylmercury degradation in Florida Everglades peat sediment

Methylmercury (MeHg) degradation was investigated along an eutrophication gradient in the Florida Everglades by quantifying (CH4)-C-14 and (CO2)-C-14 production after incubation of anaerobic sediments with [C-14]MeHg. Degradation rate constants (k) were consistently less than or equal to 0.1 d(-1) and decreased with sediment depth. Higher k values were observed when shorter incubation times and lower MeHg amendment levels were used, and k increased 2-fold as in-situ MeHg concentrations were approached. The average flee layer k was 0.046 +/- 0.023 d(-1) (n = 17) for 1-2 day incubations. In-situ degradation rates were estimated to be 0.02-0.5 ng of MeHg (g of dry sediment)(-1) d(-1) increasing I from eutrophied to pristine areas. Nitrate-respiring bacteria did not demethylate MeHg, and NO3- addition partially inhibited degradation in some cases. MeHg degradation rates were not affected by PO43- addition. (CO2)-C-14 production in all samples indicated that oxidative demethylation (OD)was an important degradation mechanism. OD occurred over 5 orders of magnitude of applied MeHg concentration, with lowest limits [1-18 ng of MeHg (g of dry sediment)(-1)] in the range of in-situ MeHg levels. Sulfate reducers and methanogens were the primary agents of anaerobic OD, although it is suggested that methanogens dominate degradation at in-situ MeHg concentrations. Specific pathways of OD by these two microbial groups are proposed.