Improved CO₂-derived polyhydroxybutyrate (PHB) production by engineering fast-growing cyanobacterium Synechococcus elongatus UTEX 2973 for potential utilization of flue gas

Industrial application of cyanobacterial poly-β-hydroxybutyrate (PHB) production from CO₂ is currently challenged by slow growth rate and low photoautotrophic PHB productivity of existing cyanobacteria species. Herein, a novel PHB-producing cyanobacterial strain was developed by harnessing fast-growing cyanobacteria Synechococcus elongatus UTEX 2973 with introduction of heterologous phaCAB genes. Under photoautotrophic condition, the engineered strain produced 420 mg L⁻¹ (16.7% of dry cell weight) with the highest specific productivity of 75.2 mg L⁻¹ d⁻¹. When compared with a native PHB producer Synechocystis PCC 6803 under nitrogen deprivation, the engineered strain exhibited 2.4-fold higher PHB productivity. The performance of the engineered strain was further demonstrated in large scale cultivation using photobioreactor and outdoor cultivation employing industrial flue gas as the sole carbon source. This study can provide a promising solution to address petroleum-based plastic waste and contribute to CO₂ mitigation.