HYPERBOLIC GROWTH OF THERMOANAEROBACTER-THERMOHYDROSULFURICUS (CLOSTRIDIUM-THERMOHYDROSULFURICUM) INCREASES ETHANOL-PRODUCTION IN PH-CONTROLLED BATCH CULTURE

Thermoanaerobacter thermohydrosulfuricus Rt8.B1 exhibited hyperbolic g rowth (i.e. a continuous rate of growth, without diauxie, during growt h and utilization of two carbon sources) on mixed carbohydrate substra tes when grown in pH-controlled batch culture. Hyperbolic growth was o bserved with xylose in combination with either glucose or cellobiose. Diauxic growth ways observed when T. thermohydrosulfuricus Rt8.B1 was grown on a glucose plus cellobiose substrate mix. The major fermentati on end-products under all substrate conditions were ethanol and acetat e. Ethanol production varied depending on the substrate supplied and w as always greatest on mixtures that included xylose (i.e. hyperbolic g rowth). High ethanol-to-acetate ratios could not be explained on the b asis of a greater substrate uptake and thus more ethanol production un der these conditions, or by variations in the levels of acetate kinase and NADP-linked alcohol dehydrogenase synthesis. The high ethanol-to- acetate ratio could not be increased by growing T. thermohydrosulfuric us Rt8.B1 under a partial pressure of hydrogen (1 atm) or by growth at different pH. Growth under these conditions decreased the ethanol-to- acetate ratio.