Two-stage nutrient-assisted CO2 modulation enhances microalgal carbon capture and bioenergy efficiency with auto-sedimented biomass harvesting

Microalgal CO2 sequestration offers a promising approach for mitigating climate change while producing renewable bioenergy, but large-scale application is constrained by culture acidification, carbon saturation, and high harvesting costs. In this study, Micractinium pusillum was cultivated under three CO2 supply regimes (ambient air, 5%, and 15%), demonstrating that prolonged exposure to elevated CO2 suppressed photosynthesis through nutrient depletion and acidification. To overcome these barriers, a two-stage nutrient-assisted strategy with calcium and phosphorus supplementation was implemented. Growth was initiated under 15% CO2 to maximize productivity and subsequently transitioned to 5% CO2 with nutrient enrichment, which stabilized pH, improved carbonic anhydrase activity, and enhanced nutrient assimilation. This approach increased CO2 fixation by 35.4% (287.13 mg/L/day), biomass yield by 25.7% (3.23 g/L), and lipid productivity by 1.86-fold. A 9% rise in higher heating value (21.48 to 23.4 kJ/g) and a 37.65% improvement in total intracellular bioenergy efficiency (47.32 to 65.14 kJ) were also achieved. Importantly, biodiesel derived from the process met Indian, US, and European standards, while calcium–phosphorus-assisted auto-sedimentation ensured 98.46% biomass recovery efficiency on a dry cell weight basis. Collectively, this two-stage nutrient-assisted CO2 modulation strategy enhanced microalgal carbon capture and bioenergy efficiency, offering a scalable and sustainable pathway for microalgal biorefineries.