Substrate Gas Utilization and C3/C4 Metabolic Analysis of Actinobacillus succinogenes: Integration into a Model for Fermentation Prediction in BES

The aim of this work is to study the metabolism of Actinobacillus succinogenes in greater detail with the aim of optimizing succinate production and creating a metabolic model. The inhibitory properties of various substances were first investigated. It was found that the nature and availability of the gas can have a strong influence on metabolism. By studying the effects of different gas sources, it was found that when A. succinogenes lacks a CO2 source, the metabolism completely switches to the C3 pathway. This also completely changes the path within the pathway. In the presence of CO2, significantly more formate (2.44 ± 0.04 g L−1) and significantly less acetate (1.63 ± 0.03 g L−1) was produced. In contrast, in the absence of CO2, the formate concentration was 1.94 ± 0.12 g L−1, and the acetate concentration was 2.73 ± 0.15 g L−1. In addition, larger amounts of ethanol (1.34 ± 0.28 g L−1) were produced in the absence of CO2, whereas hardly any ethanol was produced otherwise. All these results show that, in the absence of a CO2 source, the organism has to regenerate much more NADH to NAD+ via the C3 pathway. In the subsequent investigation of the CO2 source, an increase in product concentration from 1.55 ± 0.13 g L−1 to 6.11 ± 0.09 g L−1 was achieved by combining gaseous CO2 with NaHCO3. It was shown that a microaerobic environment is not sufficient to influence the metabolism of the organism towards lactate formation. Using the model, it was possible to verify the main metabolic pathways observed during experimental bioreactor runs on a 2-L scale. By conducting further modification, it is now possible to use the model to predict the effects of an external electron supply on the redox metabolism.