Stimulating bioplastic production with light energy by coupling Ralstonia eutropha with the photocatalyst graphitic carbon nitride

Abstract

Bioproduction processes relying on natural photosynthesis have low solar energy-to-specific product conversion efficiency. A possible solution is the development of hybrid photosynthesis systems where sunlight is harvested by more efficient inorganic devices, which then generate energy used by microbial catalysts for the production of valuable chemicals from carbon dioxide. Microbial processes employing organic carbon molecules as a source of energy could also benefit from being coupled with inorganic photocatalysts to drive their metabolism toward a higher production yield. Here, the photocatalyst graphitic carbon nitride (g-C₃N₄) was employed to improve polyhydroxybutyrate (PHB) production from fructose by Ralstonia eutropha H16. PHB is a bioplastic generated by R. eutropha for energy and carbon storage purposes. Its synthesis requires reducing power in the form of NADPH. In this hybrid photosynthesis strategy, g-C₃N₄ harvested light energy and generated reducing power, which were then consumed by R. eutropha for the conversion of acetyl-CoA into PHB. Coupling illuminated R. eutropha with g-C₃N₄ augmented PHB production by 1.2 times in the absence of a sacrificial electron donor and by 1.4 times when triethanolamine was supplied. The formation of large aggregates composed of cells and photocatalyst particles was observed with SEM, indicating that direct physical contact between g-C₃N₄ and R. eutropha is required for the effective transfer of reducing power. The significantly increased ratio of intracellular NADPH/NADP⁺ further demonstrated the positive impact of g-C₃N₄ on R. eutropha energy metabolism. This proof-of-concept study illustrates that inorganic photocatalysts can augment the productivity of white biotechnologies aiming at converting organic carbon substrates into useful chemicals.