Insights into gas fermentation optimisation for enhanced acetate production

Abstract

Conversion of CO2 utilising gas fermenting acetogens is a feasible and environmentally beneficial solution to the global problem of greenhouse gas emissions. Homoacetogens are an especially intriguing type of microorganisms since they can yield acetate as their primary metabolic product via the Wood–Ljungdahl pathway. A product inhibition mechanism is observed when acetate accumulates in high concentrations, adversely impacting bacterial growth and acetate production. The tolerance of these microorganisms towards varying concentrations of acetate and under identical conditions has not previously been investigated. However, it has been defined as crucial for selecting the most robust acetogen for its use at industrial scale. The designed experimental setup for acetate tolerance studies, including positive and negative controls, was intended to answer this scientific question. Following the pre-screening test, it was observed that the carbon source affected the microbial ability to tolerate acetate; bacteria performed substantially better when a more energy-rich carbon source, glucose, was supplemented. The latter finding was accounted for when designing the main screening setup for acetate tolerance in four well-known homoacetogenic strains, where Moorella thermoacetica and Thermoanaerobacter kivui performed the best. Then, T. kivui was utilised to perform pH-controlled fermentations in pressurised gas bioreactors. Furthermore, the outcomes of this master's thesis support the hypothesis that gram-positive anaerobes undergo variation in their Gram staining due to oxygen exposure. This thesis provides valuable information for the future selection and screening of homoacetogenic strains and the effect of pH-controlled cultivation in a pressurised bioreactor.