The fermentative capacities of the acetogenic bacterium Peptostreptococcus productus U-1 (ATCC 35244) were examined. Although acetate was formed from all the substrates tested, additional products were produced in response to CO 2 limitation. Under CO 2 -limited conditions, fructose-dependent growth yielded high levels of lactate as a reduced end product; lactate was also produced under CO 2 -enriched conditions when fructose concentrations were elevated. In the absence of supplemental CO 2 , xylose-dependent growth yielded lactate and succinate as major reduced end products. Although supplemental CO 2 and acetogenesis stimulated cell yields on fructose, xylose-dependent cell yields were decreased in response to CO 2 and acetogenesis. In contrast, glycerol-dependent growth yielded high levels of ethanol in the absence of supplemental CO 2 , and pyruvate was subject to only acetogenic utilization independent of CO 2 . CO 2 pulsing during the growth of CO 2 -limited fructose cultures stopped lactate synthesis immediately, indicating that CO 2 -limited cells were nonetheless metabolically poised to respond quickly to exogenous CO 2 . Resting cells that were cultivated at the expense of fructose without supplemental CO 2 readily consumed fructose in the absence of exogenous CO 2 and formed only lactate. Although the specific activity of lactate dehydrogenase was not appreciably influenced by supplemental CO 2 during cultivation, cells cultivated on fructose under CO 2 -enriched conditions displayed minimal capacities to consume fructose in the absence of exogenous CO 2 . These results demonstrate that the utilization of alternative fermentations for the conservation of energy and growth of P. productus U-1 is augmented by the relative availability of CO 2 and growth substrate.Acetogenic bacteria utilize CO 2 as a terminal electron acceptor and synthesize acetate by the acetyl coenzyme A (acetyl-CoA) Wood/Ljungdahl pathway (12,16,35,36,44). The reduction of CO 2 to acetate is coupled to respiratory conservation of energy via either membranous electron transport or sodium pumping systems (11,27,33). Although supplemental CO 2 can be essential or greatly stimulatory to acetogenesis under certain conditions, acetogens can use other respiratory reductant sinks (15,17,18), including aromatic acrylate groups (2,22,32,40), fumarate (13, 31), dimethyl sulfoxide (3), and nitrate (20,37).In contrast to the numerous studies that have evaluated the respiratory capacities of acetogens, the fermentation capacities of this bacteriological group have been minimally examined. Indeed, although certain acetogens can form fermentative end products such as ethanol (8, 39), acetogens are commonly thought to route the reductant derived from glycolysis toward the respiration of CO 2 to acetate (12,16,44). The acetogenic sewage isolate Peptostreptococcus productus U-1 has been reported to form lactate (28). Although lactate production by P. productus U-1 has not been quantitatively examined, preliminary studies in our laboratory indicated th...

The influence of CO, on the ability of Pepfostrepfococcus) productus U-1 (ATCC 35244) t o use an aromatic acrylate group as an energy-conserving electron acceptor during 0-methyl-dependent growth was examined. Ferulate (a methoxylated phenylacrylate), unlike hydroferulate (a methoxylated phenylpropionate), supported growth under C0,-limited conditions. Two phases occurred during ferulate utilization in C0,-limited cultures. In phase I (maximum growth), 0-methyl-derived reductant was coupled mainly t o acrylate group reduction, and acetate synthesis (CO, as reductant sink) was minimal. In phase II, acetate synthesis increased, but cell yields in this phase were much less than in phase 1. In C0,-enriched cultures, distinct phases were not observed; reductant was coupled equally t o CO, and acrylate group reduction. Under C0,-enriched conditions, 0-methyl and acrylate groups were incompletely metabolized, and molar growth yields were significantly lower compared to C0,-limited conditions. Resting cell studies indicated that 0-demethylase and aromatic acrylate oxidoreductase activities were induced by ferulate. These findings demonstrated that P. productus U-1 can use the aromatic acrylate oxidoreductase system as a sole, energy-conserving, electron-accepting process, but is not able to prevent the simultaneous use of the bioenergetically less favourable acetyl-CoA pathway during 0-methyldependent growth.Keywords : ace t og eni c bacteria, Pep tostrep tococcw p roductw , p hen y lac r y late s , 0-demethylation, reductant sinks