Physiological and enzymatic study of Debaryomyces hansenii growth on xylose and oxygen limited chemostats

The effect of the growth rate and oxygen transfer rate on Debaryomyces hansenii physiology was studied using xylose-limited and oxygen-limited chemostat cultures respectively, and complemented with enzymatic assays. Under xylose-limited chemostat (oxygen-excess) neither ethanol nor xylitol was produced over the entire range of dilution rate (D). The maximal volumetric biomass productivity was 2.5 glh at dilution rate of 0.25 h and cell yield was constant with D. The respiratory rates and xylose consumption rate show a linear increase with growth rate but above 0.17 h oxygen consumption rate has a steeper increase compared to carbon dioxide production rate. An enzymatic analysis of the xylose metabolism suggests that internal fluxes redirection must occur as function of growth rate. For dilution rates up to 0.17 h xylose reductase (XR) titer is lower than xylitol dehydrogenase (XDH) titer, whereas above 0.17 h XR activity is about twice of XDH and the NADPH-producing enzymes sharply increase their titres indicating an internal metabolic flux shift to meet higher NADPH metabolic requirements. Moreover, the enzymes around pyruvate node also exhibited different patterns with D below or above 0.17 h. Under oxygen-limited chemostat (xylose-excess) the metabolism changed drastically and due to oxidative phosphorylation limitation, cell yield decreased to 0.16 gg for the oxygen transfer rate (OTR) of 1.4 mmol l h and xylitol becomes the major extracellular product along with minor amounts of glycerol. The enzymatic analysis shows that isocitrate dehydrogenase is not regulated by oxygen, whereas XR and XDH and the NADPH-producing enzymes decreased and increased their titres with OTR, respectively.