Reductive Dehalogenations of Halobenzoates by Anaerobic Lake Sediment Microorganisms

Abstract

CH4-producing freshwater lake sediment dehalogenated chloro-, bromo- and iodobenzoates by a reductive reaction in which the halogen was replaced by a H atom. The identity of the dehalogenated products was confirmed by mass spectrometry, NMR 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 wk - > 6 mo., 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; 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 findings of the absence of dehalogenation in sterile sediments and by sediments previously incubated at .gtoreq. 39.degree. 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.