Effects of gas bubble sizes on CO2 fixation, biomass synthesis, and metabolic responses in Chlorella pyrenoidosa

Photosynthetic microbial CO 2 fixation represents an efficient strategy for industrial flue gas mitigation. However, system performance is critically dependent on the gas bubble size within the bioreactor. While large bubbles limit gas-liquid mass transfer, leaving substantial carbon unabsorbed and unconverted, excessively small bubbles can potentially impair microbial viability and activity. In this case, this study systematically investigates the trade-off in flue gas bubble size for carbon fixation by Chlorella pyrenoidosa , specifically elucidating the mechanisms by which fine bubbles impact cellular functions. The use of flue gas bubble with mean diameters of 259, 485, and 653 μm resulted in carbon fixation rates of 0.8651, 1.0175, and 0.7956 g/L/h, respectively. Notably, the peak performance at 485 μm was also associated with the highest levels of protein (59.36 %) and chlorophyll (3.483 %). Mechanistic analysis indicated that 485 μm bubbles provided an optimal balance, enhancing mass transfer compared to larger bubbles while avoiding the severe cell membrane damage and lipid peroxidation induced by exposure to 259 μm bubbles. At this optimal size, carbon metabolism was accelerated through the upregulation of key enzymes (e.g., carbonic anhydrase and RuBisCO) and enhanced energy supply (e.g., NADPH and ATP), whereas 259 μm bubbles caused metabolic suppression. These findings establish bubble size as a decisive factor governing carbon fixation efficiency in photosynthetic biohybrid systems, providing a mechanistic basis for enhancing their efficacy in flue gas mitigation.