Bio-succinic acid production from alternative feedstock

The use of fossil resources for the production of energy and chemicals is one major driver of global human-made emissions. A transition towards more sustainable processes and resources is necessary like the microbial conversion of crude glycerol into succinic acid. However, the economic feasibility of this process requires higher succinic acid yields and lower product recovery costs. Actinobacillus succinogenes has been demonstrated to achieve among the highest succinic acid yields from crude glycerol. However, challenges remain due to (i) its slow glycerol metabolism, (ii) insufficient knowledge of its growth kinetics, growth phenotypes, and product inhibitory effects, and (iii) a lack of suitable computational tools for rational process design and optimization.

During this Ph.D. thesis, some of those challenges were addressed by both experimental and computational methods. (i) An adaptive laboratory evolution approach was successful in improving glycerol consumption and succinic acid production. (ii) Glycerol-pulsed chemostat cultivations at increasing succinic acid concentrations demonstrated an adaptive ability of Actinobacillus succinogenes to succinic acid titers of 43 gl^-1. The results also indicated that succinic acid inhibitory effects on the growth kinetics followed either a linear or logistic decay-like trend. (iii) Chemostat cultivations at increasing dilution rates indicated a shift in the metabolite distribution and promoted attached growth. The maximum succinic acid volumetric productivity of 4.14 gl-1h^-1 was the highest reported value in the literature. (iv) A calibrated mechanistic kinetic model simulated the growth and the succinic acid inhibition kinetics. It estimated a maximum growth rate of 0.24 h^-1 and a critical succinic acid titer of 68 gl^-1. It predicted optimal process conditions at a succinic acid titer, yield, and volumetric productivity of 38 gl^-1, 0.73 gg^-1, and 3.8 gl^-1h^-1.

Despite the new insights, further interdisciplinary research will be necessary to utilize the full potential of Actinobacillus succinogenes. Key research fields will be (i) the genetic engineering of the strain to improve succinic acid production and tolerance to inhibitors, (ii) the combination of cultivation with in-situ product recovery, and (iii) the development of computational tools for rational process design of multiple unit operations.