Comprehensive understanding of regulatory mechanisms, physiological models and key enzymes in microalgal cells based on various concentrations of CO2

• 3 physiological models were established to depict microalgal response to high CO 2. • Slightly-high reduced growth rate through regulators from tryptamine metabolism. • Ultra-high carbon induced D-galactosamine generation and cell integrity destruction. • Different high CO 2 induced different functional characteristics of cell subcategories. • Protein structure of 5 special unknown gene related to ultra-high carbon was predicted. Carbon fixation from industrial flue gas with high concentrations of carbon dioxide (CO 2 ) requires a comprehensive and systematic understanding of the physiological influences of CO 2 molecules on microalgal cells. Differing from other previous studies, this study tried to reveal the diverse effects of different high concentrations of CO 2 molecules on the physiological mechanisms of microalgae: Three physiological models were established to clarify the photosynthetic features, regulatory systems, metabolic mechanisms, and key enzymes of Nannochloropsis oceanica based solely on different high carbon concentrations in an unvarying environment. An independent influence (irrespective of pH changes) on microalgal cells was only found at high concentrations of CO 2 . The inhibition of auxin synthesis and an increase in sedatives and hallucinogens owing to the regulation of tryptamine metabolism induced stagnation of growth with slightly high carbon. However, cell division, and carbon and protein metabolism became too active at ultra-high concentrations of carbon, which induced multiple cell death factors, including the destruction of cell integrity from expansins, insulin resistance, and hepatotoxin D-galactosamine toxicity. Moderately high carbon achieved the optimal microalgal biomass yield (3.2 g/L) and fixed the carbon content (61%) in dried biomass for several reasons. It reduced cell division, provided an active carboxylic acid cycle to store abundant CO 2 , initiated an effective urea cycle and ammonia transport to inhibit the generation of D-galactosamine, and induced DNA repair and antioxidant flavonoids to eliminate the damage from high concentrations of CO 2 . This study provides new theoretical support to improve the tolerance of microalgae to high CO 2 from industrial flue gas.