Advanced PHB fermentation strategies with CO2-derived organic acids

Over the past decade, formic acid and acetic acid have gained increasing attention as alternative feedstocks for poly-3-hydroxybutyrate (PHB) production as these potentially CO₂-derived molecules are naturally assimilated by Cupriavidus necator. Both organic acids were individually evaluated in fed-batch fermentations at bioreactor scale. Acetic acid was revealed as the most promising carbon source yielding 42.3 g L^-1 PHB, whereas no significant amount of PHB was produced from formic acid. Hence, acetic acid was further used as the substrate during process intensification. Key performance characteristics, including process stability, PHB titer, and productivity were optimized by introducing NH₄-acetate as the nitrogen source, extending the growth phase, and implementing a repeated fed-batch procedure, respectively. These advanced fermentation strategies resulted in the establishment of a stable fermentation process reaching 58.5 g L^-1 PHB, while doubling the productivity to 0.93 g L^-1 h^-1 PHB.