Abstract
Simultaneous utilization of glucose and ethanol by the yeast Schizosaccharomyces pombe CBS 356 was studied in aerobic chemostat cultures. In glucose-limited cultures, respirofermentative metabolism occurred at growth rates above 0.16 h-1. Although Sch. pombe lacks a functional glyoxylate cycle and therefore cannot utilize ethanol as a sole carbon source, ethanol was co-consumed by glucose-limited chemostat cultures. As a result, biomass yields increased, but not up to the theoretical value [0.92 g biomass (g glucose)-1] expected if all of the acetyl-CoA produced from glucose was instead synthesized from ethanol. When ethanol accounted for more than 30% of the substrate carbon in the mixed feed, it was incompletely utilized. In mixed- substrate cultures with a saturating ethanol fraction in the feed, the increase of the biomass yield as a result of ethanol consumption was highest at low dilution rates. This was not due to an increased specific rate of ethanol consumption at low growth rates; rather, the longer residence times at low dilution rates allowed Sch. pombe to utilize a larger fraction of the available ethanol, part of which was oxidized to acetate. Activities of gluconeogenic and glyoxylate-cycle enzymes were not detected in cell-free extracts of any of the cultures. Activities of acetaldehyde dehydrogenase and acetyl-CoA synthetase were low and of the same order of magnitude as the in vivo rates of acetate activation to acetyl-CoA. The results show that ethanol is a poor substrate for Sch. pombe, even as an auxiliary energy source.