Abstract

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 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 two-fold as in-situ MeHg concentrations were approached. The average floc 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 to 0.5 ng MeHg*g dry sed^-1*d^-1, 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^-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.

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