Microbial degradation of tetrachloromethane: mechanisms and perspectives for bioremediation

Toxic man-made compounds released into the environment represent potential nutrients for bacteria, and microorganisms growing with such compounds as carbon and energy sources can be used to clean up polluted sites. However, in some instances, microorganisms contribute to contaminant degradation without any apparent benefit for themselves. Such cometabolism plays an important part in bioremediation, but is often difficult to control. Microbial degradation of tetrachloromethane (carbon tetrachloride, CCl4), a toxic ozone-depleting organic solvent mainly of anthropogenic origin, is only known to occur by cometabolic reduction under anoxic conditions. Yet no microbial system capable of using CCl4 as the sole carbon source has been described. Microbial growth based on CCl4 as a terminal electron acceptor has not been reported, although corresponding degradation pathways would yield sufficient energy. Known modes for the biodegradation of CCl4 involve several microbial metabolites, mainly metal-bound coenzymes and siderophores, which are produced by facultative or strictly anaerobic bacteria and methanogenic Archaea. Recent reports have demonstrated that CCl4 dechlorination rates are enhanced by redox-active organic compounds such as humic acids and quinones, which act as shuttles between electron-providing microorganisms and CCl4 as a strong electron acceptor. The key factors underlying dechlorination of CCl4, the practical aspects and specific requirements for microorganism-associated degradation of CCl4 at contaminated sites and perspectives for future developments are discussed.