Mineralization versus fermentation: evidence for two distinct anaerobic bacterial degradation pathways for dichloromethane

Dichloromethane (DCM) is an anthropogenic pollutant with ozone destruction potential that is also formed naturally. Under anoxic conditions, fermentation of DCM to acetate and formate has been reported in axenic culture Dehalobacterium formicoaceticum, and to acetate, H-2 and CO2 in mixed culture RM, which harbors the DCM degrader 'Candidatus Dichloromethanomonas elyunquensis'. RM cultures produced 28.1 +/- 2.3 mu mol of acetate from 155.6 +/- 9.3 mu mol DCM, far less than the one third (i.e., about 51.9 mu mol) predicted based on the assumed fermentation model, and observed in cultures of Dehalobacterium formicoaceticum. Temporal metabolite analyses using gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy revealed that no C-13-labeled acetate was formed in C-13-DCM-grown RM cultures, indicating acetate was not a direct product of DCM metabolism. The data were reconciled with DCM mineralization and H-2 consumption via CO2 reduction to acetate and methane by homoacetogenic and methanogenic partner populations, respectively. In contrast, Dehalobacterium formicoaceticum produced C-13-labeled acetate and formate from C-13-DCM, consistent with a fermentation pathway. Free energy change calculations predicted that organisms with the mineralization pathway are the dominant DCM consumers in environments with H-2 <100 ppmv. These findings have implications for carbon and electron flow in environments where DCM is introduced through natural production processes or anthropogenic activities.