Butyrate Production from Methanol and CO2 in Microbial Electrosynthesis

Abstract

Microbial electrosynthesis (MES) is a promising technology for producing value-added chemicals from carbon dioxide (CO<SUB>2</SUB>). In MES, microorganisms serve as the biocatalyst and are cultivated in a cathode chamber of an electrochemical cell and use the electrons or H<SUB>2</SUB> derived from cathode for the reduction of CO<SUB>2</SUB> to short chain fatty acids. Acetate has been a common product in MES. Various studies have indicated that adding extra electron donors is an effective strategy to shift the product spectrum towards butyrate, which has a higher economic value compared to acetate. Methanol is a promising electron donor. Methanol can be produced from organic waste or synthesized directly from CO<SUB>2</SUB>. In fermentation process, methanol has been used both with pure and mixed cultures as the electron donor for chain elongation, which elongates CO<SUB>2</SUB> and acetate to other carboxylates like butyrate. However, the utilization of methanol in MES has never been reported.

This doctoral dissertation investigated the feasibility of methanol addition in MES for butyrate production. First, a mixed culture microbial community was enriched in MES reactors fed with methanol and CO<SUB>2</SUB>. Butyrate was the predominant product with methanol acting both as carbon and electron donor. Subsequently, operational parameters were optimized, including the cathode pH, temperature, methanol/CO<SUB>2</SUB> ratio, pressure, and CO<SUB>2</SUB> feeding mode. With optimized conditions, butyrate production was obtained with 87% selectivity (carbon basis) and production rate of 0.6 g L<SUP>-1</SUP> d<SUP>-1</SUP> (107.4 g m<SUP>-2</SUP> d<SUP>-1</SUP>). Additionally, 16S ribosomal RNA gene sequencing and shotgun metagenomic sequencing revealed that <i>Eubacterium callanderi</i> was the responsible genus for methanol and CO<SUB>2</SUB> assimilation via the Wood-Ljungdahl pathway as well as the butyrate production via the reverse β-oxidation pathway in methanol assisted MES. These findings highlight the potential of methanol assisted MES for efficient carbon utilization and contribute to the development of bioprocesses for platform chemical production.