DICHLOROMETHANE AS THE SOLE CARBON SOURCE FOR AN ACETOGENIC MIXED CULTURE AND ISOLATION OF A FERMENTATIVE, DICHLOROMETHANE-DEGRADING BACTERIUM

Dichloromethane (DCM) is utilized by the strictly anaerobic, acetogeni c mixed culture DM as a sole source of carbon and energy for growth. G rowth with DCM was linear, and cell suspensions of the culture degrade d DCM with a specific activity of 0.47 mkat/kg of protein. A mass bala nce of 2 mol of chloride and 0.42 mol of acetate per mol of DCM was ob served. The dehalogenation reaction showed similar specific activities under both anaerobic and aerobic conditions. Radioactivity from [C-14 ]DCM in cell suspensions was recovered largely as (CO2)-C-14 (58%), [C -14]acetate (23%), and [C-14]formate (11%), which subsequently disappe ared. This suggested that formate is a major intermediate in the pathw ay from DCM to acetate. Efforts to isolate from culture DM a pure cult ure capable of anaerobic growth with DCM were unsuccessful, although o verall acetogenesis and the partial reactions are thermodynamically fa vorable. We then isolated bacterial strains DMA, a strictly anaerobic, gram-positive, endospore-forming rod, and DMB, a strictly anaerobic, gram-negative, endospore-forming homoacetogen, from culture DM. Both s train DMB and Methanospirillum hungatei utilized formate as a source o f carbon and energy. Coculture of strain DMA with either M. hungatei o r strain DMB in solid medium with DCM as the sole added source of carb on and energy was observed. These data support a tentative scheme for the acetogenic fermentation of DCM involving interspecies formate tran sfer from strain DMA to the acetogenic bacterium DMB or to the methano gen M. hungatei.