Rhodospirillum rubrum as a biocatalyst for the conversion of carbon monoxide to polyhydroxyalkanoates

Plastics have become essential over the past century thanks to their versatility and ease of production. However, most are fossil-based, making their manufacture environmentally unsustainable. In addition, growing macro- and microplastic pollution threatens ecosystems and human health, highlighting the need for biodegradable plastics produced from sustainable sources. One promising avenue is the use of C1 resources, which can be derived from waste streams such as agricultural by-products and municipal or plastic waste. Yet converting C1 compounds into plastics has proven difficult.This thesis explores a novel synthetic co-culture using Rhodospirillum rubrum and an acetogen to convert carbon monoxide (CO) into polyhydroxyalkanoates (PHA), a biodegradable plastic. In this system, R. rubrum converts CO and water into H₂ and CO₂, fueling its metabolism. The resulting H₂ and CO₂ are consumed by Acetobacterium woodii to produce acetate, which R. rubrum then takes up in situ to grow and accumulate poly-3-hydroxybutyrate (poly-3HB). Batch cultivations confirmed PHA production from CO as the sole substrate.During chemostat operation, a mismatch emerged between acetate production by A. woodii and acetate consumption by R. rubrum. Sodium limitation was identified as necessary to slow A. woodii growth and prevent acetate buildup. The study then examined whether ethanol could replace acetate as a carbon source for R. rubrum. Ethanol not only improved growth and PHA yield but also shifted the polymer composition from a poly-3HB homopolymer to a copolymer containing 29 mol% poly-3-hydroxyvalerate (poly-3HV). Physiological experiments, proteomics, and flux balance analysis revealed the mechanism behind 3HV formation. During ethanol metabolism, reverse electron flow was suppressed, redirecting electrons in the quinone pool toward the tricarboxylic acid cycle. The resulting partially reductive activity generated intracellular propionyl-CoA, which was ultimately polymerized into polyhydroxyvalerate. These findings clarify aspects of R. rubrum redox homeostasis and indicate that replacing acetate with ethanol in the co-culture could improve PHA yield and diversify its composition.A second strategy assessed the addition of light as an auxiliary energy source to improve co-culture performance. The energy obtained by R. rubrum from converting CO and water into H₂ and CO₂ was identified as a key bottleneck, as its acetate consumption could not match the acetate production of A. woodii if all H₂ was used to generate acetate. Rather than solely restricting acetate production, stimulating R. rubrum consumption was evaluated by combining CO and light. Carboxydotrophic and phototrophic acetate uptake rates were determined and applied to an in-silico co-culture model to predict the light intensity needed to balance energy demands. This requirement was compared to energy inputs used for microalgae cultivation, a more established technology.