High PHA density fed-batch cultivation strategies for 4HB-rich P(3HB-co-4HB) copolymer production by transformant Cupriavidus malaysiensis USMAA1020

P(3HB-co-4HB) with a high 4HB monomer composition was previously successfully produced using the transformant Cupriavidus malaysiensis USMAA1020 containing an additional copy of the PHA synthase gene. In this study, high PHA density fed-batch cultivation strategies were developed for such 4HB-rich P(3HB-co-4HB). The pulse, constant and mixed feeding strategies resulted in high PHA accumulation, with a PHA content of 74-92 wt% and 4HB monomer composition of 92-99 mol%. The pulse-feed of carbon and nitrogen resulted in higher PHA concentration (30.7 g/L) than carbon alone (22.3 g/L), suggesting that a trace amount of nitrogen is essential to support cell density for PHA accumulation. Constant feeding was found to be a more feasible strategy than mixed feeding, since the latter caused a drastic fluctuation in the C/N ratio, as evidenced by higher biomass formation indicating more carbon flux towards the competitive TCA pathway. A two-times carbon and nitrogen pulse feeding was the most optimal strategy achieving 92 wt% accommodation of the total biomass, with the highest PHA concentration (46 g/L) and yield (Yp/x) of 11.5 g/g. The strategy has kept the C/N at optimal ratio during the active PHA-producing phase. This is the first report of the production of high PHA density for 4HB-rich P(3HB-co-4HB).