The effect of glucose on high-level xylose fermentations by recombinant Zymomonas in batch and fed-batch fermentations

Xylose-fermenting recombinant Zymomonas mobilis has been proposed as a candidate biocatalyst for the production of fuel ethanol from cellulosic biomass and wastes. This study documents the effect of glucose on xylose utilization by recombinant Z. mobilis CP4:pZB5 using a nutrient-rich synthetic (pure sugar) hardwood dilute-acid prehydrolyzate medium containing 0.8% (w/v) glucose and 4% (w/v) xylose that was enriched with respect to xylose concentration within the range 6-10% (w/v) xylose. Supplementation with glucose to a final concentration of 2% (w/v) resulted in faster xylose utilization of both 6% and 8% xylose; however, higher levels of glucose supplementation (>2%) did not result in a decrease in the time required for fermentation of either 6% or 8% xylose. An improvement in the rate of 8% xylose utilization was also achieved through continuous glucose feeding in which the total glucose concentration was about 1.3% (w/v). This fed-batch experiment was designed to mimic the continuous supply of glucose provided by the cellulose saccharifying enzymes in a simultaneous saccharifying and cofermentation process. The upper limit ethanol concentration at which xylose utilization by recombinant Z. mobilis CP4:pZB5 is completely inhibited is about 5.5% (w/v) at pH 5 and >6% at pH 5.75. At pH 5.75, this level of ethanol was achieved with the following media of pure sugar mixtures (each containing the same sugar loading of 12% (w/v): 1. 6% xylose + 6% glucose; 2. 8% xylose + 4% glucose; and 3. 4% xylose + 8% glucose. At the level of inoculum used in this study, complete fermentation of the 12% sugar mixtures required 2-3 d (equivalent to a volumetric ethanol productivity of 0.83-1.25 g ethanol/L.h). The sugar-to-ethanol conversion efficiency was 94-96% of theoretical maximum.