Bacterial demethylation of methylmercury in the South Florida Ecosystem

Methylmercury (MeHg) degradation was investigated along an eutrophication gradient in the Florida Everglades by quantifying 14CH4 and 14CO2 production after incubation of anaerobic sediments with 14C-MeHg. Degradation rate constants (k) were consistently <=0.1 per day, and decreased with sediment depth. Higher k values were observed when shorter incubation times and lower MeHg amendment levels were used, and k increased two-fold as in-situ MeHg concentrations were approached. The average floc layer k was 0.046 +/- 0.023/ d (n=17) for 1-2 day incubations. In-situ degradation rates were estimated to be 0.02 to 0.5 ng MeHg/g dry sed/d, increasing 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 PO4-3 addition. 14CO2 production in all samples indicated that oxidative demethylation (OD) was an important degradation mechanism. OD occurred over five orders of magnitude of applied MeHg concentration, with lowest limits (1-18 ng MeHg/g dry sediment) 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.

The objective of this research is to provide ecosystem managers with MeHg degradation rate data from a number of study sites that represent a diversity of hydrologic and nutrient regimes common to the Everglades, and to forge a better understanding of the microbial and geochemical controls regulating MeHg degradation in this system.