Acetate-mediated two-stage microbial production of poly[(R)-3-hydroxybutyrate] (PHB) from CO2 and H2 using a synthetic medium free of vitamins and cysteine

The objective of this study is to use the thermophilic acetogen Thermoanaerobacter kivui and a vitamin- and cysteine-free synthetic medium for an acetate-mediated two-stage microbial production of the biodegradable polymer poly[(R)-3-hydroxybutyrate] (PHB) from CO₂ and H₂. In the first stage, T. kivui could produce 10 g/L acetate within 14 days during anaerobic chemoautotrophic culture with substrate gas (H₂:CO₂ = 80:20) using cysteine-free DSMZ 171 medium (originally vitamin-free) in the presence of the pH neutralizer CaCO₃. In the second stage, the acetate-containing spent medium obtained from the vitamin- and cysteine-free chemoautotrophic culture of T. kivui could be used for the aerobic culture of Cupriavidus necator for PHB production. Specifically, C. necator consumed all of the 5 g/L acetate in the spent medium of T. kivui for 36 h. The cell concentration of C. necator reached 1.2 g/L after 30 h of culture. The PHB content of C. necator reached 38 % (=0.46 g-PHB/L). The PHB yield was 0.11 g-PHB/g-acetate used. The present study can contribute to a sustainable production of bioplastic PHB from CO₂ and H₂ without vitamins and amino acids and without the risk of flammability problems caused by combustible gases (mixture of H₂, O₂, CO₂).