Xylose metabolism in Debaryomyces hansenii UFV-170.: Effect of the specific oxygen uptake rate

The new yeast Debaryomyces hansenii UFV-170 was tested in this work in batch experiments under variable oxygenation conditions. To get additional information on its fermentative metabolism, a stoichiometric network was proposed and checked through a bioenergetic study performed using the experimental data of product and substrate concentrations. The yeast metabolism resulted to be practically inactive under strict oxygen-limited conditions (q(O2) = 12.0 mmol(O2) C-mol(DM)(-1) h(-1)), as expected by the impossibility of regenerating NADH(2)(+). Significant fractions of the carbon source were addressed to both respiration and biomass growth under excess oxygen levels (q(O2) greater than or equal to 55.0 mmol(O2) C-mol(DM)(-1) h(-1)), thus affecting xylitol yield (Y-P/S = 0.41-0.52 g g(-1)). Semi-aerobic conditions (q(O2) = 26.8 mmol(O2) C-mmol(DM)(-1) h(-1)) were able to ensure the best xylitol production performance (P-max = 76.6 g L-1), minimizing the fractions of the carbon source addressed either to respiration or biomass production and increasing Y-P/S up to 0.73 g g(-1). An average P/O ratio of about 1.0 mol(ATP) mol(O)(-1) allowed estimation of the main kinetic-bioenergetic parameters of the biosystem. The overall ATP requirements of biomass were found to be particularly high and dependent on the oxygen availability in the medium as well as on the physiological state of the culture. Under semi-aerobic and aerobic conditions, they varied in the ranges 13.5-15.4 and 9.74-10.2 MOl(ATP) C-mol(DM)(-1), respectively, whereas during the best semiaerobic bioconversion they progressively increased from 5.68 to 24.7 mol(ATP) C-mol(DM)(-1). After a starting phase of adaptation to the medium, the cell achieved a phase of decelerated growth during which its excellent xylose-to-xylitol capacity kept almost constant after 112 h up to the end of the run.