Phosphate limitation stress induces xylitol overproduction byDebaryomyces hansenii

Abstract

The physiological responses of xylose-grown Debaryomyces hansenii were studied under different nutritive stress conditions using continuous cultivation at a constant dilution rate of 0.055 h−1. Metabolic steady-state data were obtained for xylose, ammonium, potassium, phosphate and oxygen limitation. For xylose and potassium limitation, fully oxidative metabolism occurred leading to the production of biomass and CO2 as the only metabolic products. However, potassium-limiting cultivation was the most severe nutritional stress of all tested, exhibiting the highest xylose and O2 specific consumption rates along with the lowest biomass yield, 0.22 g g−1 xylose. It is suggested that carbon was mainly channelled to meet the cellular energy requirements for potassium uptake. For the other limiting nutritional conditions increasing amounts of extracellular xylitol were found for ammonium, phosphate and oxygen limitation. Although xylitol excretion is not significant for ammonium limitation, the same is not true for phosphate limitation where the xylitol productivity reached 0.10 g l−1 h−1, about half of that found under oxygen-limiting conditions, 0.21 g l−1 h−1. This work is the first evidence that xylitol production by D. hansenii might not only be a consequence of a redox imbalance usually attained under semi-aerobic conditions, but additional physiological mechanisms must be involved, especially under phosphate limitation. Cell yields changed drastically as a function of the limiting nutrient, being 0.22, 0.29, and 0.39 g g−1 xylose for potassium, oxygen and phosphate limitation, respectively, and are a good indicator of the severity of nutritive stress.