Effect of aeration strategy on the metabolic flux of Klebsiella pneumoniae producing 1,3-propanediol in continuous cultures at different glycerol concentrations

The microbial production of 1,3-propaneidol (1,3-PD) by Klebsiella pneumoniae in continuous fermentation was investigated under low, medium and high glycerol concentrations in the absence and presence of oxygen. The production of 1,3-PD increased with increasing glycerol concentrations, reaching a maximum (266 mmol l⁻¹) under high glycerol concentration (760 mmol l⁻¹) with air sparging at 0.04 vvm. The yield of 1,3-PD, however, decreased gradually with increasing glycerol concentrations, with the highest yield (0.52 mol mol⁻¹) obtained for low glycerol concentration (270 mmol l⁻¹) under anaerobic condition. Enzyme activity assays showed that the specific activity of glycerol dehydratase was highest (0.04 U mg⁻¹) for culture sparged with 0.04 vvm air under high glycerol concentration. The specific activities of glycerol dehydrogenase and 1,3-propanediol oxidoreductase were also improved for all glycerol concentrations and in the presence of oxygen, implying that the dha operon was not repressed under microaerobic conditions. Analysis of metabolic fluxes showed that more carbon flux was shifted to the oxidative pathway with increasing glycerol concentrations, resulting in a reduced flux to 1,3-PD formation. However, the increases in carbon fluxes were not evenly distributed among the oxidative branches of the pathway. Furthermore, ethanol and acetic acid levels were slightly increased whereas 2,3-butanediol and lactic levels were greatly enhanced.