Chloroform (CF, CHCl(3)) is a recalcitrant and toxic environmental pollutant. In this communication we report for the first time a microbial community capable of complete CF dechlorination by metabolic processes. Cultures derived from subsurface soil (3.5 m) could sustain complete dechlorination of CF at levels of least 360 µM at a rate of 40 µM per day. Scrutiny of CF dechlorination revealed two metabolic processes at work. First, CF was respired to dichloromethane (DCM, CH(2) Cl(2)), which was then fermented to acetate, hydrogen and carbon dioxide. Elevated hydrogen partial pressures were found to inhibit the fermentation process. Interspecies hydrogen transfer was observed in the form of methanogenesis and acetogenesis. This suggests that the dechlorination process required syntrophic partners to maintain low hydrogen partial pressures. (13)C-labelled DCM was employed to help elucidate the chemistry of the process and identify bacterial community members involved. CF respiring cultures, where emulsified vegetable oil was supplied as the electron donor and DCM fermenting cultures, where DCM was supplied as the sole organic carbon source were studied separately. Pyrosequencing of these cultures revealed Dehalobacter lineages as a predominant community member in both. Subsequent growth experiments confirmed that the proliferation of Dehalobacter was linked directly to both the dehalorespiration and dehalofermentation processes.

The role of bacteria and zerovalent iron (Fe(0)) in the degradation of chlorinated solvents in subsurface environments is of interest to researchers and remediation practitioners alike. Fe(0) used in reactive iron barriers for groundwater remediation positively interacted with enrichment cultures containing Dehalobacter strains in the transformation of halogenated methanes. Chloroform transformation and dichloromethane formation was up to 8-fold faster and 14 times higher, respectively, when a Dehalobacter-containing enrichment culture was combined with Fe(0) compared with Fe(0) alone. The dichloromethane-fermenting culture transformed dichloromethane up to three times faster with Fe(0) compared to without. Compound-specific isotope analysis was employed to compare abiotic and biotic chloroform and dichloromethane degradation. The isotope enrichment factor for the abiotic chloroform/Fe(0) reaction was large at -29.4 ± 2.1‰, while that for chloroform respiration by Dehalobacter was minimal at -4.3 ± 0.45‰. The combined abiotic/biotic dechlorination was -8.3 ± 0.7‰, confirming the predominance of biotic dechlorination. The enrichment factor for dichloromethane fermentation was -15.5 ± 1.5‰; however, in the presence of Fe(0) the factor increased to -23.5 ± 2.1‰, suggesting multiple mechanisms were contributing to dichloromethane degradation. Together the results show that chlorinated methane-metabolizing organisms introduced into reactive iron barriers can have a significant impact on trichloromethane and dichloromethane degradation and that compound-specific isotope analysis can be employed to distinguish between the biotic and abiotic reactions involved.

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