Automatic Control of Chemolithotrophic Cultivation of Cupriavidus necator: Optimization of Oxygen Supply for Enhanced Bioplastic Production

Gas fermentation is an upcoming technology to convert gaseous substrates into value-added products using autotrophic microorganisms. The hydrogen-oxidizing bacteria Cupriavidus necator efficiently uses CO₂ as its sole carbon source, H₂ as electron donor and O₂ as electron acceptor. Surplus CO₂ is stored in microbial storage material poly-(R)-3-hydroxybutyrate. O₂ supply is the most critical parameter for growth and poly-(R)-3-hydroxybutyrate formation. A narrow O₂ optimum between ~0.2 and ~4 mg/L was previously reported. Here, a standard benchtop bioreactor was redesigned for autotrophic growth of C. necator on explosive mixtures of CO₂, H₂ and O₂. The bioreactor was equipped with mass flow control units and O₂ and CO₂ sensors. A controller for automated gas dosage based on a mathematical model including gas mass transfer, gas consumption and sensor response time was developed. Dissolved O₂ concentrations were adjusted with high precision to 1, 2 and 4% O₂ saturation (0.4, 0.8 and 1.5 mg/L dissolved O₂, respectively). In total, up to 15 g/L cell dry weight were produced. Residual biomass formation was 3.6 ± 0.2 g/L under all three O₂ concentrations. However, poly-(R)-3-hydroxybutyrate content was 71, 77 and 58% of the cell dry weight with 1, 2 and 4% dissolved O₂, respectively.