Influences of light quality on the flashing light effect to enhance growth and CO

Microalgal CO₂ fixation is a promising strategy for industrial carbon capture, yet its application remains limited by inefficient light utilization. Although the flashing light effect has been explored as a light-management strategy for improving photon utilization in microalgal cultivation, its effectiveness depends on spectral composition, flashing frequency, duty cycle, and cultivation conditions. In this study, light quality, including red (R), blue (B), white (W), and mixed red-blue (RB) light, and PWM flashing parameters, including frequency (10-1000 Hz) and duty cycle (30%-90%), were manipulated to evaluate their effects on the growth, chlorophyll fluorescence characteristics, biomass-based apparent CO₂ fixation, biochemical composition, and antioxidant responses of Chlorella pyrenoidosa. Under the cultivation conditions, mixed red-blue light (R: B = 1:5) combined with 200 Hz flashing and a 50% duty cycle increased biomass yield by 58%, biomass-based apparent CO₂ fixation rate by 60%, and lipid yield by 41.9% relative to the continuous white-light reference. These responses were accompanied by improved PSII photochemical performance and coordinated antioxidant responses, suggesting that the optimized red-blue PWM regime helped maintain a more favorable balance between photochemical energy conversion and downstream biomass formation under the tested conditions. Overall, these results provide evidence that spectral optimization combined with PWM flashing illumination can improve the cultivation performance of Chlorella pyrenoidosa, offering physiological insights for light-management strategies and high-performance photobioreactor design.