EFFECT OF CO2 ON THE FERMENTATION CAPACITIES OF THE ACETOGEN PEPTOSTREPTOCOCCUS-PRODUCTUS U-1

The fermentative capacities of the acetogenic bacterium Peptostreptoco ccus productus U-1 (ATCC 35244) were examined. Although acetate was fo rmed from all the substrates tested, additional products were produced in response to CO2 limitation. Under CO2-limited conditions, fructose -dependent growth yielded high levels of lactate as a reduced end prod uct; lactate was also produced under CO2-enriched conditions when fruc tose concentrations were elevated. In the absence of supplemental CO2, xylose-dependent growth yielded lactate and succinate as major reduce d end products. Although supplemental CO2 and acetogenesis stimulated cell yields on fructose, xylose-dependent cell yields were decreased i n response to CO2 and acetogenesis. In contrast, glycerol-dependent gr owth yielded high levels of ethanol in the absence of supplemental CO2 , and pyruvate was subject to only acetogenic utilization independent of CO2. CO2 pulsing during the growth of CO2-limited fructose cultures stopped lactate synthesis immediately, indicating that CO2-limited ce lls were nonetheless metabolically poised to respond quickly to exogen ous CO2. Resting cells that were cultivated at the expense of fructose without supplemental CO2 readily consumed fructose in the absence of exogenous CO2 and formed only lactate. Although the specific activity of lactate dehydrogenase was not appreciably influenced by supplementa l CO2 during cultivation, cells cultivated on fructose under CO2-enric hed conditions displayed minimal capacities to consume fructose in the absence of exogenous CO2. These results demonstrate that the utilizat ion of alternative fermentations for the conservation of energy and gr owth of P. productus U-1 is augmented by the relative availability of CO2 and growth substrate.