Reductive Dehalogenations of Halobenzoates by Anaerobic Lake Sediment Microorganisms

Methane-producing freshwater lake sediment was found to dehalogenate chloro-, bromo-, and iodobenzoates by a reductive reaction in which the halogen was replaced by a hydrogen atom. The identity of the dehalogenated products was confirmed by mass spectrometry, nuclear magnetic resonance, or cochromatography. Removal of the halogens to produce benzoate was necessary before mineralization to CH₄ + CO₂ could occur. The dehalogenation occurred after a lag period which lasted from 1 week to more than 6 months, depending on the chemical. Dehalogenation was not observed in the absence of CH₄ production, and it was inhibited by the addition of 20% O₂. Once sediment was acclimated to halobenzoate dehalogenation, new additions of the halobenzoate were degraded without lag. Acclimation was observed regardless of whether the parent substrates were eventually mineralized to CH₄ + CO₂. Sediment acclimated to bromo- and chlorobenzoate degradation generally metabolized bromo- and chlorobenzoates, but sediment acclimated to iodobenzoate degradation only metabolized iodobenzoate. Prior acclimation of sediment to benzoate decomposition did not alter the pattern of dehalogenation, and sediment acclimated to dehalogenation was not concurrently acclimated to benzoate degradation. The presence of this apparent specificity, the lag period, and subsequent acclimation, together with our findings of the absence of dehalogenation in sterile sediments and by sediments previously incubated at ≥39°C, suggests that this reaction was biologically catalyzed. Apparently, a pathway for the reductive dehalogenation of aryl halides is present in anaerobic microorganisms of this methanogenic sediment.