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, bioflim 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 gg(-1) at a glucose consumption of log L-1, to 5.7 gg(-1) at a glucose consumption of 50g L-1. The formic acid to acetic acid ratio decreased from an equimolar value (0.77 gg(-1)) at a glucose consumption of 10g 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 gg(-1) and 48.5 g L-1 respectively. Metabolic flux analysis based on the established C-3 and C-4 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. (C) 2014 Elsevier B.V. All rights reserved.