Efficient Production of High-Concentration Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) from CO2 Employing the Recombinant of Cupriavidus necator

A copolymer of 3-hydroxybutyrate (3HB) and 3-hydoxyhexanoate (3HHx), PHBHHx, is a practical biodegradable plastic, and at present, the copolymer is produced at commercial scale via heterotrophic cultivation of an engineered strain of a facultative hydrogen-oxidizing bacterium, <i>Cupriavidus necator</i>, using vegetable oil as the carbon source. In our previous report, we investigated PHBHHx production from CO₂ via pH-stat jar cultivation of the newly created recombinants of <i>C. necator</i> under autotropic conditions, feeding the inorganic substrate gas mixture (H₂/O₂/CO₂ = 80:10:10 <i>v</i>/<i>v</i>%) into a recycled-gas closed-circuit (RGCC) culture system. The dry cell weight (DCW) and PHBHHx concentration with the best strain MF01/pBPP-ccr_MeJ_Ac-emd increased to 59.62 ± 3.18 g·L^-1 and 49.31 ± 3.14 g·L^-1, respectively, after 216 h. In this study, we investigated the high-concentration production of PHBHHx with a shorter cultivation time by using a jar fermenter equipped with a basket-shaped agitator to enhance oxygen transfer in the culture medium and by continuously supplying the gases with higher O₂ concentrations to maintain the gas composition within the reservoir at a constant ratio. The concentrations of ammonium and phosphate in the culture medium were maintained at low levels. As a result, the DCW and PHBHHx concentrations increased to 109.5 ± 0.30 g·L^-1 and 85.2 ± 0.62 g·L^-1 after 148 h, respectively. The 3HHx composition was 10.1 ± 0.693 mol%, which is suitable for practical applications.