The biodegradation of hydroxybenzoate isomers was investigated with samples obtained from two sites within a shallow anoxic aquifer. The metabolic fates of the substrates were compared in denitrifying, sulfate-reducing, and methanogenic incubations. Under the latter two conditions, phenol was detected as a major intermediate of p-hydroxybenzoate, but no metabolites were initially found with m- or o-hydroxybenzoate. However, benzoate accumulation was noted when metabolic inhibitors were used with these samples. About 9 to 17 days was required for >95% removal of the parent isomers under these conditions. When aquifer slurries were amended with nitrate, the equivalent removal of the hydroxybenzoates occurred within 4 days. In the denitrifying incubations, phenol was formed from all three hydroxybenzoates and accounted for about 30% of the initial substrate amendment. No benzoate was measured in these samples. All metabolites were identified by chromatographic mobility, mass spectral profiles, or both. Autoclaved controls were uniformly incapable of transforming the parent substrates. These results suggest that the anaerobic fate of hydroxybenzoate isomers depends on the relative substitution pattern and the prevailing ecological conditions. Furthermore, since these compounds are central metabolites formed during the breakdown of many aromatic chemicals, our findings may help provide guidelines for the reliable extrapolation of metabolic fate information from diverse anaerobic environments
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