Improved polyhydroxybutyrate production by Cupriavidus necator and the photocatalyst graphitic carbon nitride from fructose under low light intensity

The photocatalyst graphitic carbon nitride (g-C3N4) is known to photostimulate the production of the bioplastic polyhydroxybutyrate (PHB) by Cupriavidus necator. In previous studies, the combination of C. necator and g-C3N4 increased PHB yield from either an organic or inorganic carbon substrate under a light intensity of 4200 lx. Here, different parameters including light intensity, pH, temperature, nitrogen and carbon concentrations, aeration, and inoculum size were explored to maximize PHB production by hybrid photosynthesis from fructose and visible light. A g-C3N4/C. necator culture grown with a lower light intensity of 2100 lx, an inoculum size of 128.30 × 106 CFU ml-1, and constant aeration produced 7.16 g l-1 d-1 PHB with a product yield from fructose of 60.94%. Furthermore, the ratio of incident photons harvested by g-C3N4 converted into NADPH+H+ by C. necator for PHB production was improved to 19.74% after the process optimization. In comparison, the PHB production rate of a non-optimized g-C3N4/C. necator system exposed to 4200 lx was only 2.94 g l-1 d-1 with a product yield from fructose of 33.29%. These results demonstrate that hybrid photosynthesis productivity can be significantly augmented by decreasing light intensity and adjusting other parameters, which is promising for future bioproduction applications.