BIOENERGETICS AND END-PRODUCT REGULATION OF CLOSTRIDIUM-THERMOSACCHAROLYTICUM IN RESPONSE TO NUTRIENT LIMITATION

Fermentation of xylose by Clostridium thermosaccharolyticum was studie d in batch and continuous culture in which the limiting nutrient was e ither xylose, phosphate, or ammonia. Transient results obtained in con tinuous cultures with batch grown inoculum and progressively higher fe ed substrate concentrations exhibited ethanol selectivities (moles eth anol/moles other products) in excess of 11. The hypothesis that this h igh ethanol selectivity was a general response to mineral nutrient lim itation was tested but could not be supported. Growth and substrate co nsumption were related by the equation q(s)(1 - Y(x)c)G(ATP) = (mu/Y(A TP)max) + m, with q(s) the specific rate of xylose consumption (moles xylose/hour . g cells), Y(x)c the carbon based cell yield (g cell carb on/g substrate carbon), G(ATP) the ATP gain (moles ATP produces/mol su bstrate catabolized), mu the specific growth rate (1/h), Y(ATP)max the ATP-based cell yield (g cells/mol ATP), and m the maintenance coeffic ient (moles ATP/hour . g cells). Y(ATP)max was found to be 11.6 g cell s/mol ATP, and m 9.3 mol ATP/hour . g cells for growth on defined medi um. Different responses to nutrient limitation were observed depending on the mode of cultivation. Batch and immobilized cell continuous cul tures decreased G(ATP) by initiating production of the secondary metab olites, propanediol, and in some cases, D-lactate; in addition, batch cultures increased the fractional allocation of ATP to maintenance and /or wastage. Nitrogen-limited continuous free-cell cultures maintained a constant cell yield, whereas phosphate-limited continuous free-cell cultures did not. In the case of phosphate limitation, the decreased ATP demand associated with the lowered cell yield was accompanied by a n increased rate of ATP consumption for maintenance and/or wastage. Ne ither nitrogen or phosphorus-limited continuous free-cell cultures exh ibited an altered G(ATP) in response to mineral nutrient limitation, a nd neither produced secondary metabolites. (C) 1993 John Wiley & Sons, Inc.