Production of Poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) from CO2 by pH-Stat Jar Cultivation of Engineered Hydrogen-Oxidizing Bacterium Cupriavidus necator

The copolyester of 3-hydroxybutyrate (3HB) and 3-hydoxyhexanoate (3HHx), PHBHHx, is a bi-odegradable plastic that has a high flexibility, softness, wide process window and marine bio-degradability. PHBHHx is usually produced from structurally related carbon sources such as vegetable oils or fatty acids but not from inexpensive carbon sources like sugars. In the previous studies, we demonstrated that engineered strains of hydrogen-oxidizing bacterium, Cupriavidus necator synthesized PHBHHx with high cellular content from not only sugars but also from CO2 as the sole carbon source in the flask culture. In this study, highly efficient production of PHBHHx from CO2 was investigated by pH-stat jar cultivation of the recombinant C. necator strains with feeding the substrate gas mixture (H2/O2/CO2=8:1:1) to complete mineral medium in recycled-gas closed-circuit culture system. As the result with the strain MF01/pBPP-ccrMeJAc-emd, the dry cell mass and PHBHHx concentration reached up to 59.62 ± 3.18 g･L-1 and 49.31 ± 3.14 g･L-1, respectively after 212 h of jar cultivation with limited addition of ammonia and phosphate solutions. The 3HHx composition was close to 10 mol% which is suitable for practical applica-tions. It is expected that autotrophic cultivation of the recombinant C. necator can be a feasible process for mass production of PHBHHx from CO2.