Environmental Science and Engineering Seminar
Reductive dehalogenation of chlorinated organic compounds is thermodynamically favorable, and microbes can use these compounds, including the groundwater pollutants tetrachloroethene (PCE) and trichloroethene (TCE), as electron acceptors for anaerobic respiration and growth. Our laboratory isolated Dehalococcoides ethenogenes (now mccartyi) strain 195, the first organism known to reduce PCE and TCE to the nontoxic product ethene. The genome sequences of Dehalococcoides strains show that they are highly adapted to using organohalides as electron acceptors, possessing 10-36 reductive dehalogenase (RDH) genes each. More recently we've found that members of the genus Dehalobacter can dehalogenate dichlorobenzenes and monochlorobenzene, and their genomes contain up to 40 RDH genes each. The phylogenetic depth of RDH genes indicates that reductive dehalogenation is ancient process, greatly preceding human release of organohalides. Contrary to common belief, natural organohalides are diverse and widespread in marine and terrestrial environments, and are produced by bacteria, plants, animals, and fungi. Haloperoxidase enzymes produce organohalides, and aerobic metabolism or reductive dehalogenation can convert carbon-bonded chlorine back to the chloride ion. These reactions, along with oxidation of chloride to (per)chlorate, and its subsequent reduction to chloride, form the chlorine cycle.