Changes in the growth and enzyme level of Zymomonas mobilis under oxygen-limited conditions at low glucose concentration

Zymomonas mobilis growing aerobically with 20 g glucose-1 (carbon-limited) in a chemostat exhibited an increase in both the molar growth yield (Yx/s) and the maximum molar growth yield (Yx/smax) and a decrease in both the specific substrate consumption rate (qs) and the maintenance energy consumption rate (me). Stepwise increase in the input oxygen partial pressure showed that anaerobic-to-aerobic transitional adaptation occurred in four stages: anaerobic (0 mm HgO2), oxygen-limited (7.6- 230 mm HgO2), intermediate (273 mm HgO2), and oxygen excess (290 mm HgO2). The steady-state biomass concentration, Yx/s, and intracellular ATP content increased between oxygen partial pressures of 7.6 and 120 mm HgO2, accompanied by a decrease in the qs and the specific acid production rate. The membrane ATPase activity decreased with increasing oxygen partial pressure and reached its lowest levels at 273 mm HgO2, which was the highest input oxygen partial pressure where steady-state conditions were possible. Glucokinase, glucose-6-phosphate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, and alcohol dehydrogenase activities also decreased when the oxygen partial pressure was increased above 15 mm Hg, whereas pyruvate decarboxylase was unaffected by aeration. Growth inhibition at 290 mm HgO2 was characterised by a drastic reduction in the pyruvate kinase activity and a collapse in the intracellular ATP pool. The growth and enzyme data suggest that at low glucose concentrations and oxygen-limited conditions, the increase in biomass yields is a reflection of a redirection of ATP usage rather than a net increase in energy production.