P36: Metabolic flux variation of continuous Actinobacillus succinogenes: Towards optimum yields and production conditions

Of all the wild succinic acid (SA) producers, A. succinogenes is by far the most reported organism. Almost all studies employ batch or fed-batch fermentation, in which the analysis of the flux distribution is complicated by the non-steadiness of the process. In this work the continuous culture of A. succinogenes is studied in order to analyse the metabolic flux under various steady conditions. Clear evidence is provided that the flux shifts towards SA at higher glucose conversions or SA titres where the non-growth-related production of metabolites dominates. In addition, the production of formic acid decreases to zero with increasing SA titre where additional reduction power is generated via the pyruvate dehydrogenase or formate dehydrogenase route. Redox closures reveal that an additional source of reduction power is present when the accepted metabolic map is considered, and that the formic acid decrease is not the sole reason for the increase in SA flux. It is postulated that the generation of the additional reducing power occurs via the pentose phosphate pathway in non-growing cells. SA yields on glucose of 0.91 g g^-1 were obtained at an SA titre of 48.5 g L^-1 where the mass ratio of SA to acetic acid was 5.7. Unavoidable cell immobilisation enhanced the volumetric productivity, with maximum values exceeding 10 g L^-1 h^-1. The results are promising from a commercial perspective since high yields are obtained at high titres, furthermore large productivities are possible due to natural cell retainment.