Acetate as a source of reducing equivalents in the reductive dechlorination of 2,5-dichlorobenzoate |
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Authors: | Jan Dolfing James M Tiedje |
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Institution: | (1) Department of Crop and Soil Sciences, Michigan State University, 48824 East Lansing, MI, USA;(2) Department of Microbiology and Public Health, Michigan State University, 48824 East Lansing, MI, USA;(3) Present address: Department of Biochemistry, Groningen Biotechnology Center, University of Groningen, Nijenborgh 16, NL-9747 AG Groningen, The Netherlands |
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Abstract: | Desulfomonile tiedjei is the key dechlorinating organism in a three-tiered bacterial consortium that grows on the methanogenic degradation of 3-chlorobenzoate. 2,5-Dichlorobenzoate, however, is only converted to 2-chlorobenzoate and is not a methanogenic substrate for the consortium. The dechlorinator uses hydrogen produced from benzoate by the benzoate degrading member of consortium as its source of reducing equivalents for the dechlorination reaction. Incubation of 3-chlorobenzoate grown consortium cells with 2,5-dichlorobenzoate resulted in the consumption of acetate concurrent with the formation of 2-chlorobenzoate indicating that acetate can serve as an alternative source of reducing equivalents for reductive dechlorination. This interpretation was confirmed by the finding that the formation of 14CO2 from 2-14C-labeled acetate was stoichiometric. The addition of hydrogen to 2,5-dichlorobenzoate metabolizing cells resulted in (i) an 2.7-fold increase in the rate of dechlorination, and (ii) a drop in the amount of label recovered as CO2+CH4 from methyl
14C-labeled acetate, indicating that hydrogen was the preferred source of reducing equivalents for reductive dechlorination. Benzoate, an indirect source of H2 in the consortium, also inhibited the oxidation of acetate, while glucose, methanol, and butyrate did not affect labeled gas production and therefore were not suitable electron donors. Concomittant to dechlorination of 2,5-dichlorobenzoate 3- and 4-methoxybenzoate were converted to 3- and 4-hydroxybenzoate respectively. These conversions stimulated the rate of dechlorination 2-fold. Demethylation of 4-methoxybenzoate stimulated, but demethylation of 3-methoxybenzoate inhibited the oxidation of benzoate during the dechlorination of 2,5-dichlorobenzoate, suggesting that these isomers are metabolized through different pathways. Experiments with benzoate, 3-chlorobenzoate and 2,5-dichlorobenzoate metabolizing cells amended with 14CO2 showed that actively dechlorinating cells catalyzed an exchange reaction between CO2 and acetate. |
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Keywords: | Acetate oxidation Reduction equivalents Dehalogenation Dechlorination Demethoxylation Desulfomonile tiedjei |
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