Monooxygenase-Mediated 1,2-Dichloroethane Degradation by Pseudomonas sp. Strain DCA1

1,2-Dichloroethane (DCA) is a synthetic chemical which has no known natural sources. DCA is mainly used as an intermediate in the synthesis of vinyl chloride, but it is also used in the production of chlorinated solvents such as trichloroethene, tetrachloroethene, and 1,1,1-trichloroethane. Based on the low sorption coefficient (log Koc = 1.28) and the good solubility in water, leaching of DCA to groundwater is expected when DCA is released to the environment (23). Abiotic degradation of DCA does occur, however, very slowly. Under conditions similar to those in groundwater (15°C, pH 7, and in the presence of 1 mM total sulfide) the half-life of DCA is 23 years (3). Aerobic biodegradation of DCA was first demonstrated in 1983 by Stucki et al. (16). The isolated Pseudomonas sp. strain DE2, is able to grow on DCA as the sole source of carbon and energy with a growth rate of 0.08 h−1 at 30°C. A strain studied in more detail is Xanthobacter autotrophicus GJ10 (11, 12). This strain was isolated from a mixture of activated sludge and contaminated soil. The first step in DCA metabolism in X. autotrophicus GJ10 is a hydrolytic dehalogenation to 2-chloroethanol, which is subsequently converted to chloroacetaldehyde by an alcohol dehydrogenase (12). Chloroacetaldehyde is further dehydrogenated to form chloroacetic acid. The latter compound is presumably dehalogenated by chloroacetate dehalogenase to glycolic acid, which can enter the central metabolic pathways (12). X. autotrophicus GJ10 has been used in several studies aimed at biological treatment of synthetic waste waters containing DCA (5–7). In most of these studies, however, DCA concentrations were in the millimolar range whereas, for practical application, DCA in groundwater has to be removed at concentrations in the micromolar range. However, the kinetic parameters determined for suspended cells of strain GJ10 (20) indicate that the affinity of this strain for DCA is too low for the efficient removal of DCA at these low concentrations. In an attempt to isolate microorganisms with a higher affinity for DCA, van den Wijngaard et al. (19) isolated the bacterial strain Ancylobacter aquaticus AD25. They reported a Ks value of 24 μM for this strain compared to 260 μM for strain GJ10. Interestingly, strain AD25 expressed the same dehalogenase as strain GJ10, but at much higher levels, which explains the higher apparent affinity of strain AD25 for DCA (19, 20). However, A. aquaticus AD25, as well as X. autotrophicus GJ10 and Pseudomonas sp. strain DE2, require vitamins for optimal growth (16, 21). Therefore, we set out to isolate DCA-degrading bacteria with a high affinity for DCA but without the dependency on vitamins for optimal growth. In this report we describe the initial characterization of a new isolate, Pseudomonas sp. strain DCA1, which has a high affinity for DCA and utilizes a novel pathway for DCA degradation.