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

Abstract

Continuous anaerobic fermentations were performed in a novel external-recycle, biofilm reactor using D-glucose and CO 2 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 -1respectively. 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. KeywordsFermentation, biofilms, bioreactors, succinic acid, metabolic flux analysis.Continuous succinic acid production by Actinobacillus succinogenes in a biofilm reactor: Steady-state metabolic flux variation 2

Succinic acid-producing biofilms of Actinobacillus succinogenes: reproducibility, stability and productivity

“…Additional details on the reactor system (incl. pH and temperature control, and data acquisition) and the feed preparation method can be found in Bradfield and Nicol (2014). in all fermentations to maintain similar shear conditions.…”

“…However, both Van Heerden and Nicol (2013b) and Bradfield and Nicol (2014) report a major variation in bioreactor performance given a controlled dilution rate. Although steady-states were achieved in both studies, repeatability of the steady-states per dilution rate could not be achieved.…”

“…succinogenes is by far the most reported SA producing bacterial strain, although only four studies report on the continuous culture (Bradfield and Nicol 2014;Van Heerden and Nicol 2013b;Kim et al 2009;Urbance et al 2004).…”

“…succinogenes (Bradfield and Nicol 2014;Van Heerden and Nicol 2013b) and is a possible reason why Kim et al (2009) experienced severe membrane fouling within 50 h of continuous operation in a cell-recycle fermenter. Attempts to operate in a chemostat mode by removing internal packing were unsuccessful due to biofilm attachment on the glass and metal surfaces of the reactor (Van Heerden and Nicol 2013b).…”

The pentose phosphate pathway leads to enhanced succinic acid flux in biofilms of wild-type Actinobacillus succinogenes

“…The parity plot, that reflects incomplete closure of the redox balance ( Figure 3B), reveals that more NADH is being consumed in succinic acid production than is being generated by the studies, and together with the limited central metabolic network of A. succinogenes, led to the hypothesis that the oxidative pentose phosphate pathway (OPPP) is a plausible source of the unaccounted reduction power (Bradfield et al, 2015;Bradfield and Nicol, 2014;Bradfield and Nicol, 2016).…”