Complete chloroform dechlorination by organochlorine respiration and fermentation

Abstract

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.

Overview of organohalide-respiring bacteria and a proposal for a classification system for reductive dehalogenases

“…It has recently been shown that some Dehalobacter spp. are able to ferment dichloromethane [10,11]. There is little to no correlation between phylogenetic affiliation and chlorinated substrate specificities: aliphatic and aromatic substrates are used by taxonomically diverse organisms [12].…”

The restricted metabolism of the obligate organohalide respiring bacterium Dehalobacter restrictus: lessons from tiered functional genomics

“…[11][12][13][14], suggesting that the degradation potential of the genus Dehalobacter is largely beyond PCE and TCE. Finally, fermentation of dichloromethane by members of Dehalobacter has been shown [15,16], suggesting that not necessarily all members of this genus are obligate OHR bacteria (OHRB).…”

Refined experimental annotation reveals conserved corrinoid autotrophy in chloroform-respiring Dehalobacter isolates

“…Dehalobacter are specialized in organohalide respiration (Holliger et al, 1998) or organohalide fermentation (JusticiaLeon et al, 2012;Lee et al, 2012), and are active participants in the global halogen cycle. Based on 16S ribosomal RNA (rRNA) gene sequences (499% average nucleotide identity), all characterized strains described to date belong to the type species Dehalobacter restrictus and use H 2 or formate as an electron donor to reductively dechlorinate a variety of chlorinated and brominated hydrocarbons (Holliger et al, 1998;Sun et al, 2002;Grostern and Edwards, 2009;Yoshida et al, 2009;Grostern et al, 2010;Deshpande et al, 2013;Tang and Edwards, 2013;Nelson et al, 2014;Wang et al, 2014).…”