Census - Chlorinated Methanes Remediation

The mechanisms for degradation of chlorinated methanes and consequently the assessment approaches differ considerably between compounds and depend on redox conditions. Below provides a brief summary of degradation mechanisms and applicable assessment tools.

Carbon Tetrachloride: Under anaerobic conditions, cometabolic degradation of carbon tetrachloride and chloroform by methanogens, acetogens, fermenters, sulfate-reducing bacteria, and iron-reducing bacteria has been widely reported (see Field and Sierra-Alvarez 2004 for a review). In these studies, CT is sequentially reduced to CF and DCM but CO2 and CS2 have also been reported as byproducts. The biotransformation of CT and CF is believed to be a result of reduced enzyme co-factors common in anaerobic microorganisms such as vitamin B12. CT is also susceptible to abiotic degradation by magnetite and other iron-bearing minerals including iron sulfides (He et al. 2009) formed as a result of iron- and sulfate-reducing bacteria.

Chloroform: Unlike CT, chloroform can serve as a growth supporting electron acceptor by some Dehalobacter and Desulfitobacterium spp. (Grostern et al. 2010; Tang et al. 2013; Ding et al. 2014). In these strains, reductive dechlorination of chloroform to DCM was linked to the cfrA and ctrA genes also implicated in reductive dechlorination of 1,1,1-trichloroethane. As with carbon tetrachloride, chloroform is susceptible to cometabolic degradation by methanogens and other anaerobic microorganisms. Under aerobic conditions, methanotrophs and other microorganisms producing oxygenase enzymes can co-oxidize chloroform.

Dichloromethane: Anaerobic degradation of DCM as a sole carbon and energy source has been demonstrated for methanogenic and aceotgenic cultures. For example, Dehalobacterium formicoaceticum isolated from an acetogenic enrichment culture converts DCM and CO2 into formate and acetate (Magli et al. 1996). More recently, a Dehalobacter isolate which had been thought to be an obligate halorespiring bacterium, has also been shown to ferment DCM producing acetate (Justicia-Leon et al. 2012). Many aerobic methylotrophic bacteria, belonging to diverse genera (Hyphomicrobium, Methylobacterium, Methylophilus, Pseudomonas, Paracoccus, and Albibacter) have been isolated which are capable of utilizing DCM as a growth substrate. The DCM metabolic pathway in methylotrophic bacteria is initiated by a DCM dehalogenase (dcmA) producing formaldehyde which is further oxidized (La Roche et al. 1990). Finally, methanotrophs utilizing methane as a primary substrate can co-oxidize DCM (Oldenhuis et al. 1989).