Effect of phosphate and oxygen concentrations on alginate production and stoichiometry of metabolism of Azotobacter vinelandii under microaerobic conditions

Alginate production by Azotobacter vinelandii was studied in batch and cont inuous cultures under microaerobic conditions. In batch culture at a pO(2) of 2-3% (air saturation) alginate production was enhanced by decreasing the PO43- level in the medium. Alginate yield from biomass (Y-P/X) reached the highest value of 0.66 g/g at the lowest phosphate level (100 mg/l), compar ed to 0.40 g/g and 0.25 g/g at higher phosphate levels (200 mg/l and 400 mg /l, respectively). In contrast, biomass formation behaved differently and t he growth yield (Y-X/S) decreased with decreasing PO43- concentrations. Mor eover, the respiratory quotient (RQ) of the culture was dependent on the in itial phosphate concentration, especially in the phosphate-limited phase of growth. As the initial phosphate level decreased from 400 mg/l to 100 mg/l , the average RQ value of the culture declined from 1.46 to 0.89. The low R Q value is very close to the theoretical optimum RQ, calculated to be 0.8 o n the basis of the stoichiometry of the metabolic pathways for alginate for mation from sucrose. This optimum RQ was also confirmed in continuous cultu re at different dilution rates. Independent of the dilution rate, a pO(2) v alue of 2-5% (air saturation) was found to be optimal for alginate producti on, the corresponding RQ values being 0.80-0.84. In addition, the molecular mass and composition of alginate were also found to be affected by both ph osphate and oxygen concentrations. In conclusion, the RQ appears to be a us eful parameter for optimum control of alginate production with this microor ganism.