Investigating physiology and metabolism of Thermoanaerobacter kivui in continuous gas fermentation

Abstract

Facing climate change and the need to enhance the development of a global carbon economy, highlights the need to use alternative feedstocks and sustainable technologies for the production of indispensable chemicals and fuels.This thesis investigates the use of the thermophilic acetogen Thermoanaerobacter kivui as a biocatalyst to set up a platform for syngas fermentation, which couples the use of exhaust gases with the production of acetate. Gas fermentation plants are already operated at industrial scale using mesophilic acetogens, bearing several disadvantages, such as high contamination risks, high cooling costs, and slower conversion rates. Therefore, it is investigated whether the use of thermophiles is favoured over to the use of mesophilic acetogens for gas fermentation.Evaluating the potential of thermophilic acetogens was achieved through the development of a continuous cultivation process to characterize the physiology and metabolism of T. kivui. This was achieved through the investigation of differences, of a wild type strain and a strain adapted to grow on CO, regarding gas uptake rates and product formation using different synthetic gas mixtures. To gain further insights to the metabolism of T. kivui, steady state data was used to perform a flux balance analysis, which allows a deeper understanding of the intracellular flux distributions. The calculation of the volumetric gas transfer coefficient was used to assess possible limitations of gas solubility at high temperatures and influences of gas compositions on mass transfer. The gained data was then used to compare it with the reported literature on the cultivation of mesophiles.The conducted experiments showed no differences regarding the use of different strains of T. kivui but pointed out that a CO content of 30% in the gas mixture positively influenced the acetate productivity and increased biomass formation. Leading to a maximum acetate productivity of r_ac = 41 mmol/g/h and a acetate per biomass yield of Y_ac/x = 20.47 g/g. It was possible to operate continuous cultivations with a dilution rate up to D = 0.2 1/h. Comparing cultivation results of T. kivui with literature data of mesophiles showed that T. kivui uses the majority of the substrate for product formation in contrast to mesophiles.This work successfully demonstrated the efficiency of T. kivui in metabolizing diverse gas mixtures into acetate, highlighting the advantages of employing thermophilic acetogens over the use of mesophilic acetogens. Future directions include optimizing process parameters for maximized product formation and the use of metabolic engineering to expand the product range, utilizing real syngas, and exploring the potential for the integration of gas fermentation in biomass gasification plants.This thesis contributes to the development of sustainable methods to counteract climate change, presenting gas fermentation as a promising technology for carbon capture and utilization. Through the innovative application of T. kivui, it could lead the way towards a circular economy less reliant on fossil fuels.