Physiological responses of Chlorella sp. To high CO₂ stress and effective alleviation strategies

Industrial flue gas with high CO₂ concentrations severely inhibits microalgal growth, restricting the practical application of microalgae-based carbon capture. This study systematically investigates the physiological responses of Chlorella sp. to elevated CO₂ levels (6-90%) using pH-stat culture systems, confirming a significant pH-independent growth inhibition under hypercapnic conditions (≥50% CO₂). Based on distinct physiological alterations in photosynthetic efficiency, key enzyme activities, and elemental composition, a metabolic model is proposed to illustrate the trade-off mechanisms between carbon assimilation and stress adaptation in Chlorella sp. Three practical mitigation strategies were evaluated: (1) Phased CO₂ regulation (6%→10%→50% or 50%→6%→50%) increased final biomass by 47.5% compared to constant 50% CO₂ exposure; (2) Maintaining stable pH (7.5-8.5) enhanced carbonic anhydrase (CA) activity, facilitating HCO₃^- utilization and carbon fixation; (3) Intermittent aeration (10 min aeration/30 min cessation) balanced CO₂ supply and dissolved oxygen (DO) accumulation, sustaining a high carbon sequestration rate. These findings provide critical theoretical and technical support for optimizing microalgal CO₂ sequestration from industrial flue gas.