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Microbial adaptation to degradation of natural and synthetic organo halogens: Effects on ecosystem acclimation and natural bioremediation of polluted sites

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We have established that several halogenated aliphatic hydrocarbons can be effectively degraded in soil samples. The degredation of different halogenated aliphatic hydrocarbons was demonstrated using samples of untreated agricultural soil, 1.3-dichloropropene, and 1.2-dichloroethane-contaminated soil as an inoculum. It was found that 1.3-dichloropropene was best graded with soils that were treated with this nematocide. Degradation of 1.2-dichloroethane only occurred with soils that had been contaminated with this compound. Thes results indicate that organisms producing dehalogenating enzymes occur at higher frequency in soils that have a history of pollution with halogenated compounds. The dehalogenating enzymes cleave carbon-chlorine and carbon-bromine bonds and in this way detoxify the pollutants. Dehalogenating organisms are expected to play an important role in the natural bioremediation of these compounds (intrinsic biomediation). Enhanced biomediation rates may be obtained by simulating such organisms. Novel organisms were isolated that rapidly degrade halogenated compounds and even utilize natural or xenobiotic halohydrocarbons as a growth substrate. The degredation pathways for 1.3-dichloropropene and 1.2-dichloroethane, two compounds that were previously thought to be recalcitrant to biodegradation were established, and the dehalogenating enzymes have been characterized. Furthermore, an organism was isolated that grows on methylchloride, a compound that is produced in bulk quantities in the environment. The dehalogenating enzyme that is produced by this organism proved to be completely unrelated to the enzyme found in organisms growing on xenobiotic halohydrocarbons. It was found that the enzymes involved in dehalogenation reactions are very different and often have complementary activities allowing the degradation of a wide range of halogenated pollutants. The halocarboxylic acid dehalogenases constitute a diverse class of enzymes that can detoxify chlorinated compounds. The enzymes were reclassified according to a method that is based on sequence comparisons rather than on traditional methods such as activity profile, molecular weight and submit composition. PCR primers were developed that are highly specific for each class of these enzymes which provide a molecular tool to probe and/or quantify these genes in the environment. Previously, research on the formation of halogenated compounds focussed on the role of haloperoxidases which catalyze aspecific peroxide-dependent halogenation reactions. In halometabolide producing bacteria, novel halogenase were detected that catalyze the oxygen and NADH-dependent regiospecific halogenation of organic molecules. Insight in the molecular mechanisms underlying dehalogenation reactions should allow the development of molecular probes that can be used to assess the degradation potential of contaminated sites and monitor the proliferation of dehalogenating bacteria during bioremediation.