BACKGROUND : Bio-manufacturing of high-value chemicals in parallel to renewable biofuels has the potential to
dramatically improve the overall economic landscape of integrated lignocellulosic biorefineries. However, this will
require the generation of carbohydrate streams from lignocellulose in a form suitable for efficient microbial conversion
and downstream processing appropriate to the desired end use, making overall process development, along
with selection of appropriate target molecules, crucial to the integrated biorefinery. Succinic acid (SA), a high-value
target molecule, can be biologically produced from sugars and has the potential to serve as a platform chemical for
various chemical and polymer applications. However, the feasibility of microbial SA production at industrially relevant
productivities and yields from lignocellulosic biorefinery streams has not yet been reported.
RESULTS : Actinobacillus succinogenes 130Z was immobilised in a custom continuous fermentation setup to produce
SA on the xylose-enriched fraction of a non-detoxified, xylose-rich corn stover hydrolysate stream produced from
deacetylation and dilute acid pretreatment. Effective biofilm attachment, which serves as a natural cell retention
strategy to increase cell densities, productivities and resistance to toxicity, was accomplished by means of a novel
agitator fitting. A maximum SA titre, yield and productivity of 39.6 g L−1, 0.78 g g−1 and 1.77 g L−1 h−1 were achieved,
respectively. Steady states were obtained at dilution rates of 0.02, 0.03, 0.04, and 0.05 h−1 and the stirred biofilm reactor
was stable over prolonged periods of operation with a combined fermentation time of 1550 h. Furthermore, it was
found that a gradual increase in the dilution rate was required to facilitate adaptation of the culture to the hydrolysate,
suggesting a strong evolutionary response to the toxic compounds in the hydrolysate. Moreover, the two primary
suspected fermentation inhibitors, furfural and HMF, were metabolised during fermentation with the concentration of
each remaining at zero across all steady states.
CONCLUSIONS : The results demonstrate that immobilised A. succinogenes has the potential for effective conversion
of an industrially relevant, biomass-derived feed stream to succinic acid. Furthermore, due to the attractive yields,
productivities and titres achieved in this study, the process has the potential to serve as a means for value-added
chemical manufacturing in the integrated biorefinery.