The anaerobic degradation of 3-chloro-4-hydroxybenzoate in freshwater sediment proceeds via either chlorophenol or hydroxybenzoate to phenol and subsequently to benzoate

Abstract

To study the anaerobic degradation of the chimera 3-chloro-4- hydroxybenzoate (3-Cl,4-OHB), anaerobic freshwater sediment samples from the vicinity of Athens, Ga., were adapted for the transformation of 4- hydroxybenzoate (4-OHB), 3-chlorobenzoate (3-CB), 2-chlorophenol (2-CP), and 2,4-dichlorophenol (2,4-DCP). In nonadapted samples, both 4-OHB (product of aryl dechlorination) and 2-CP (product of aryl decarboxylation) were observed as intermediates in the transformation of 3-Cl,4-OHB to phenol. The accumulated phenol was subsequently transformed to benzoate, an intermediate in the conversion to methane and CO2. In 4-OHB-adapted samples (i.e., samples adapted for aryl decarboxylation), 2-CP was the first intermediate which was subsequently dechlorinated to phenol. In 3-CB-adapted samples (i.e., samples adapted for meta-chlorobenzoate dehalogenation), 3-Cl,4-OHB was stoichiometrically dechlorinated to 4-OHB. In 2-CP-adapted samples (i.e., samples adapted for ortho-chlorophenol dehalogenation), 4-OHB was the first major intermediate. Furthermore, 3-CB was not dechlorinated in 2-CP-adapted sediment samples, suggesting the possibility that different 3-Cl,4-OHB dechlorinating systems were induced in the 2-CP- and 3-CB-adapted sediments. Adaptation of sediment samples for dechlorination of 2,4-DCP did not lead to adaptation for dechlorination of 3-Cl,4-OHB. However, 3-Cl,4-OHB was dechlorinated to 4-OHB in our stable, sediment-free 2,4-DCP-dechlorinating enrichment, isolated previously from the same environment. In summary, these results suggest that the history of the sediment determines whether the first step in the anaerobic degradation of 3-Cl,4-OHB is a decarboxylation to 2-CP or a dechlorination to 4-OHB or whether both reactions occur simultaneously. But in all instances, the hydroxyl-substituted benzoic acid was converted via either route to phenol, which was then again transformed to benzoic acid before ring fission occurred.