Transport and Utilization of Hexoses and Pentoses in the Halotolerant Yeast Debaryomyces hansenii

Debaryomyces hansenii is a yeast species that is known for its halotolerance. This organism has seldom been mentioned as a pentose consumer. In the present work, a strain of this species was investigated with respect to the utilization of pentoses and hexoses in mixtures and as single carbon sources. Growth parameters were calculated for batch aerobic cultures containing pentoses, hexoses, and mixtures of both types of sugars. Growth on pentoses was slower than growth on hexoses, but the values obtained for biomass yields were very similar with the two types of sugars. Furthermore, when mixtures of two sugars were used, a preference for one carbon source did not inhibit consumption of the other. Glucose and xylose were transported by cells grown on glucose via a specific low-affinity facilitated diffusion system. Cells derepressed by growth on xylose had two distinct high-affinity transport systems for glucose and xylose. The sensitivity of labeled glucose and xylose transport to dissipation of the transmembrane proton gradient by the protonophore carbonyl cyanide m-chlorophenylhydrazone allowed us to consider these transport systems as proton symports, although the cells displayed sugar-associated proton uptake exclusively in the presence of NaCl or KCl. When the V_max values of transport systems for glucose and xylose were compared with glucose- and xylose-specific consumption rates during growth on either sugar, it appeared that transport did not limit the growth rate.