Unraveling the mechanism of high‐efficiency CO ₂ fixation in the high CO ₂ tolerant Oedocladium carolinianum : Synergistic enhancement of photosynthetic light conversion and carbon assimilation

The filamentous microalga Oedocladium carolinianum , capable of synthesizing high‐value compounds like astaxanthin, represents a promising candidate for integrated carbon capture and biorefinery. This study investigated its physiological and molecular responses to a wide range of CO 2 concentrations (0.04%–20%) at an aeration rate of 500 mL min −1 , without CO 2 addition during the night, to assess carbon fixation efficiency and application potential. Results demonstrated that O. carolinianum possesses remarkable tolerance to high CO 2 levels. The optimal 5% CO 2 treatment yielded the highest average carbon fixation rate of 243.51 mg L −1 d −1 . Physiologically, CO 2 enrichment markedly enhanced photosynthetic performance: it boosted the efficiency of Photosystem II, accelerated electron transport rates, and concurrently reduced respiratory energy loss. Furthermore, CO 2 aeration stabilized the culture pH within a neutral range by counteracting photosynthetic alkalinization, thereby promoting algal growth. Transcriptomic analysis revealed that optimal CO 2 concentrations (5%–10%) systematically upregulated genes encoding key components of the photosynthetic apparatus—including antenna proteins, both photosystems, and the Calvin cycle enzymes—thereby orchestrating a coordinated enhancement of carbon assimilation metabolism. This study elucidates the integrated physiological and molecular basis of efficient carbon fixation in O. carolinianum under elevated CO 2 , supporting its potential application in industrial flue gas remediation and sustainable natural astaxanthin coproduction.