Continuous succinic acid production by Actinobacillus succinogenes in a biofilm reactor: Steady-state metabolic flux variation

Continuous anaerobic fermentations were performed in a novel external-recycle, biofilm reactor using d-glucose and CO2 as carbon substrates. Succinic acid (SA) yields were found to be an increasing function of glucose consumption with the succinic acid to acetic acid ratio increasing from 2.4 g g−1 at a glucose consumption of 10 g L−1, to 5.7 g g−1 at a glucose consumption of 50 g L−1. The formic acid to acetic acid ratio decreased from an equimolar value (0.77 g g−1) at a glucose consumption of 10 g L−1 to a value close to zero at 50 g L−1. The highest SA yield on glucose and highest SA titre obtained were 0.91 g g−1 and 48.5 g L−1 respectively. Metabolic flux analysis based on the established C3 and C4 metabolic pathways of Actinobacillus succinogenes revealed that the increase in the succinate to acetate ratio could not be attributed to the decrease in formic acid and that an additional source of NADH was present. The fraction of unaccounted NADH increased with glucose consumption, suggesting that additional reducing power is present in the medium or is provided by the activation of an alternative metabolic pathway.