Semi-biological domino catalysis using acetate- and propionate-producing microbes

Acetic acid and propionic acid are industrially relevant feedstock chemicals with applications in the production of polymers and food additives. The current production routes for both acetic acid and propionic acid in the chemical industry predominantly rely on fossil raw materials or agriculturally relevant crops as the carbon sources. Hence, sourcing these feedstocks from alternative sustainable carbon sources could contribute to the defossilisation of the chemical industry. Alternative carbon sources, such as carbon dioxide (CO2) and glycerol, both of which can be derived from waste streams, can be converted into acetate and propionate, respectively, via microbial fermentation. This thesis demonstrates new production routes to acetate and propionate utilising acetate-producing (acetogenic) and propionate-producing (propionic acid) bacteria. Acetogenic bacteria require hydrogen (H2) to be available during CO2 fermentation because H2 provides the electron and energy source necessary for CO2 reduction to acetate, in a process called acetogenesis. Given that H2 is also industrially produced from fossil raw materials, alternative production methods, namely photoelectrochemical and photocatalytic H2 production, were explored in this thesis and each were coupled with acetogenesis. Photoelectrochemical H2 production was coupled to acetogenesis by using a photocathode to generate H2 via the H2 evolution reaction (HER), which was then consumed by acetogenic bacteria Clostridium ljungdahlii (C. ljungdahlii) for acetate production. The photocathode consisted of an organic photovoltaic (OPV) device, which upon irradiation delivered photo-excited electrons to electrochemically active hydrogenase (H2ase) enzymes immobilised on the surface of the electrode. This approach of employing an OPV|H2ase photocathode to deliver H2 in situ to C. ljungdahlii has not previously been demonstrated, and resulted in a maximum acetate concentration of 1.9 ± 0.1 mM (0.11 ± 0.01 g L-1) with a rate of 0.5 mM d-1 (0.03 g L-1 d-1) at an applied potential of 0.6 V versus the reversible hydrogen electrode. Thus, this demonstration presents a novel route to light-driven H2-mediated photoelectrosynthesis of acetate from CO2 with C. ljungdahlii. C. ljungdahlii was also briefly explored for ethanol synthesis in order to expand the scope of feedstock production from CO2 fermentation. The effect of carbon monoxide as a substrate and lowering the pH of the fermentation media were both trialled to enhance ethanol production in C. ljungdahlii, following literature precedent. However, both methods were unsuccessful, and acetate remained the major end product of gas fermentation. Photocatalytic H2 production was coupled to acetogenesis by integrating photoreforming with acetogenic bacteria Moorella thermoacetica (M. thermoacetica). Photoreforming harnesses light energy absorbed by photocatalysts to oxidise organic waste substrates to value-added products and drive the HER. Photoreforming of pre-treated poly(ethylene terephthalate) was performed with a carbon nitride semiconductor, which upon irradiation delivered photo-excited electrons to a cobalt-promoted molybdenum disulfide HER co-catalyst. The H2 generated during photoreforming was then consumed by M. thermoacetica in situ for acetate production from CO2. The co-assembly of photoreforming and acetogenesis into a compact device has not previously been demonstrated, and thus this approach showcases a novel approach to photocatalytic H2-mediated acetogenesis. Propionate production was demonstrated using propionic acid bacteria Cutibacterium acnes (C. acnes). Glycerol, which was sourced from the transesterification of waste cooking oil, was fermented by C. acnes to produce propionate. The propionate was then extracted from the fermentation media and acidified. The resulting propionic acid was then used as a feedstock for photocatalytic decarboxylation to produce ethane. Hence, the production of propionic acid by fermenting waste derived substrates, and the use of the feedstock in downstream chemical synthesis was successfully demonstrated. Therefore, this thesis explores alternative production routes for acetate and propionate from sustainable carbon sources utilising microbial fermentation with photoelectro- and photo-catalysis.

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Short, Marion

Advisor dc:contributor.advisor

• Reisner, Erwin

• Microbial electrosynthesis
• Photoreforming

Licence

• https://www.repository.cam.ac.uk/bitstreams/0a574362-fe11-4822-93d0-16a6d3a199b9/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.125306

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/395981