Abstract
Fermentations were performed in an external recycle bioreactor using CO2 and d-glucose at feed concentrations of 20 and 40gL−1. Severe biofilm formation prevented kinetic analysis of suspended cell (‘chemostat’) fermentation, while perlite packing enhanced the volumetric productivity by increasing the amount of immobilised cells. The highest productivity of 6.35gL−1h−1 was achieved at a dilution rate of 0.56h−1. A constant succinic acid yield of 0.69±0.02g/(g of glucose consumed) was obtained and found to be independent of the dilution rate, transient state and extent of biofilm build-up – approximately 56% of the carbon that formed phosphoenolpyruvate ended up as succinate. Byproduct analysis indicated that pyruvate oxidation proceeded solely via the formate-lyase pathway. Cell growth and corresponding biofilm formation were rapid at dilution rates higher than 0.35h−1 when the product concentrations were low (succinic acid<10gL−1), while minimal growth was observed at succinic acid concentrations above this threshold.
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