Optimization of polyhydroxybutyrate production from a wild type and two mutant strains of Rhodobacter sphaeroides using statistical method

Interaction studies using central composite design (CCD) gave the optimum concentrations of acetate at 4 g l−1 and (NH4)2SO4 at 0.01 g l−1 with an optimum temperature of 35 °C. Rhodobacter sphaeroides N20 gave the highest PHB (7.8 g l−1) and biomass (DCW) (8.2 g l−1) values compared to the wild type strain and the mutant strain U7. The CCD results predicted that the optimum medium for the mutant strain N20 consisted of 3.90 g l−1 acetate, 0.01 g l−1 (NH4)2SO4 at 33.5 °C (R2 = 0.985). Validation of this model by culturing the mutant strain in this optimum medium exhibited similar values of PHB (7.76 g l−1), biomass (8.32 g l−1) and the PHB content in the cell 93.2% of DCW. Similar amounts of PHB were also obtained in batch fermentations using a 5-l bioreactor. The effect of pH and aeration rate was also studied and the optimum values were found to be pH 7.0 with an aeration rate of 1.0 vvm. Under these optimal conditions, strain N20 produced the highest amount of PHB production (8.76 g l−1), PHB content (95.4% of DCW) as well as the product yield (Yp/x) (0.72). These results are the highest values ever obtained from photosynthetic bacteria reported so far.