Production of ethanol by Clostridium thermosaccharolyticum: II. A quantitative model describing product distributions

The study of the continuous culture of Clostridium thermosaccharolyticum on xylose showed multiple steady states and hysteresis. A quantitative model based on the biochemistry and physiology of xylose fermentation by C. thermosaccharolyticum was developed. The objective in developing this model was to bring together the observations both of this study and of other researchers on the fermentation of xylose. The model equations were written based on the metabolic pathway for xylose utilization by C. thermosaccharolyticum and the requirement that the carbon, ATP, and NADH within the cell be balanced. Given the specific growth rate mu and the specific xylose utilization rate q(s), a set of product distributions (ethanol, acetate, and lactate) satisfying these balances was obtained. This set was plotted on a triangular plot and named the permitted region. The product distributions within this permitted region were shown to be affected by the environmental parameters such as iron concentration and hydrogen partial pressure. The model predicted trends in product distribution which correlate with experimentally observed phenomena. The model was also used to analyze the continuous-culture data from our experimental work.