Discovery of Organohalide-Respiring Processes and the Bacteria Involved

Abstract

Hazardous halogenated organic compounds are produced industrially for a variety of uses, are highly resistant to degradation by aerobic organisms, and are now widely distributed throughout the natural environment. Discovered in the 1960s were, anaerobic organisms that can transform chlorinated pesticides such as DDT and lindane. In the 1980s, other halogenated organics, the chlorinated solvents, were found to be major contaminants of groundwaters, and were found degradable by anaerobic organisms as well. While reductive dehalogenation, the process involved, was believed initially to be a fortuitous enzymatic or cometabolic process, organisms were found in the 1980s that could use halogenated compounds as electron acceptors in an energy-yielding process. Numerous species, both facultative and anaerobic, were then found capable of obtaining energy from reductive dehalogenation, but generally were very restricted in the particular halogens and particular halogenated compounds that they could dehalogenate. Some could dehalogenate tetrachloroethene (PCE) to trichloroethene (TCE) and cis-dichloroethene, and some even to vinyl chloride. An organism found in the late 1980s could even anaerobically convert PCE and TCE all the way to ethene. This organism was isolated in the late 1990s and named Dehalococcoides. Several strains of Dehalococcoides have now been isolated and their genomes have been sequenced. Each has different dehalogenases and dechlorinating abilities, but collectively they are capable of dehalogenating a broad variety of halogenated organic compounds. These organisms are now finding wide application in the engineered remediation of natural environments contaminated with halogenated compounds.