EFFECTS OF OXYGEN LIMITATION ON SUGAR METABOLISM IN YEASTS - A CONTINUOUS-CULTURE STUDY OF THE KLUYVER EFFECT

Growth and metabolite formation were studied in oxygen-limited chemost at cultures of Saccharomyces cerevisiae CBS 8066 and Candida utilis CB S 621 growing on glucose or maltose at a dilution rate of 0.1 h(-1). W ith either glucose or maltose S. cerevisiae could be grown under dual limitation of oxygen and sugar. Respiration and alcoholic fermentation occurred simultaneously and the catabolite fluxes through these proce sses were dependent on the magnitude of the oxygen feed. C. utilis cou ld also be grown under dual limitation of glucose and oxygen. However, at very low oxygen feed rates (i.e. below 4 mmol I-1 h(-1)) growth wa s limited by oxygen only, as indicated by the high residual glucose co ncentration in the culture. In contrast to S. cerevisiae, C. utilis co uld not be grown anaerobically at a dilution rate of 0.1 h(-1). With C . utilis absence of oxygen resulted in wash-out, despite the presence of ergosterol and Tween-80 in the growth medium. The behaviour of C. u tilis with respect to maltose utilization in oxygen-limited cultures w as remarkable: alcoholic fermentation did not occur and the amount of maltose metabolized was dependent on the oxygen supply. Oxygen-limited cultures of C. utilis growing on maltose always contained high residu al sugar concentrations. These observations throw new light on the so- called Kluyver effect. Apparently, maltose is a non-fermentable sugar for C. utilis CBS 621, despite the fact that it can serve as a substra te for growth of this facultatively fermentative yeast. This is not du e to the absence of key enzymes of alcoholic fermentation. Pyruvate de carboxylase and alcohol dehydrogenase were present at high levels in m altose-utilizing cells of C. utilis grown under oxygen limitation. It is concluded that the Kluyver effect, in C. utilis growing on maltose, results from a regulatory mechanism that prevents the sugar from bein g fermented. Oxygen is not a key factor in this phenomenon since under oxygen limitation alcoholic fermentation of maltose was not triggered .