METABOLIC FLUXES IN CHEMOSTAT CULTURES OF SCHIZOSACCHAROMYCES-POMBE GROWN ON MIXTURES OF GLUCOSE AND ETHANOL

Simultaneous utilization of glucose and ethanol by the yeast Schizosac charomyces 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 e thanol accounted for more than 30% of the substrate carbon in the mixe d feed, it was incompletely utilized. In mixed-substrate cultures with a saturating ethanol fraction in the feed, the increase of the biomas s yield as a result of ethanol consumption was highest at low dilution rates. This was not due to an increased specific rate of ethanol cons umption 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 cel l-free extracts of any of the cultures. Activities of acetaldehyde deh ydrogenase and acetyl-CoA synthetase were low and of the same order of magnitude as the in vivo rates of acetate activation to acetyl-CoA. T he results show that ethanol is a poor substrate for Sch. pombe, even as an auxiliary energy source.