Abstract
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, Cupriavidus necator, using vegetable oil as the carbon source. In our previous report, we investigated PHBHHx production from CO(2) via pH-stat jar cultivation of the newly created recombinants of C. necator under autotropic conditions, feeding the inorganic substrate gas mixture (H(2)/O(2)/CO(2) = 80:10:10 v/v%) into a recycled-gas closed-circuit (RGCC) culture system. The dry cell weight (DCW) and PHBHHx concentration with the best strain MF01/pBPP-ccr(Me)J(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 O(2) 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.