Process-level optimization of light-driven CO₂ fixation and PHB production by Synechocystis salina in scalable...

Photoautotrophic cultivation of cyanobacteria represents a sustainable platform for biomass-based carbon utilization and bioproduction, yet its scalability remains limited by light utilization, nutrient balance, and reactor design. In this study, an integrated photoautotrophic cultivation framework was established for Synechocystis salina through systematic optimization of light regime, nutrient availability, and photobioreactor configuration. Highly reproducible cultivation under inorganic BG-11 medium achieved a maximum dry cell weight of approximately 2 g L^-1 within 9 days, corresponding to a biomass productivity of approximately 0.22 g L^-1 d^-1 and providing a robust baseline for process evaluation. Light wavelength primarily affected biomass formation rather than intracellular carbon storage, with red-light illumination maximizing photosynthetic growth and nutrient removal efficiency. Although intracellular polyhydroxybutyrate (PHB) content remained nearly constant at ∼24 wt% across different light conditions, the highest volumetric PHB concentration reached approximately 0.505 g L^-1 under red-light cultivation, indicating that volumetric PHB production was primarily governed by biomass accumulation. Moderate light intensity (200 µmol m^-2 s^-1) supported stable growth, whereas excessive illumination induced partial photoinhibition. Nitrogen limitation acted as a relatively tolerable stress condition that maintained cellular viability, while phosphorus limitation severely impaired growth. Scale-up validation in a 100-L column photobioreactor and a 350-L tubular photobioreactor (300-L working volume) showed that a two-step light intensity increase strategy enhanced biomass productivity while minimizing apparent photodamage, with the tubular configuration outperforming the column configuration. Taken together, these results provide practical guidelines for scalable and energy-efficient photoautotrophic cultivation, with PHB serving as a representative carbon-storage product.