Abstract:
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.
