Enhancing ocean alkalinity and CO₂ sequestration via microalgae-driven carbonate precipitation and biomimetic...

Microalgae-induced carbonate precipitation (MAICP) offers a promising approach for CO₂ capture and ocean alkalinity enhancement, but its efficiency in seawater is limited by the slow hydration of CO₂, resulting low production of bicarbonate (HCO₃^-), and reduces algal growth and carbonate (CaCO₃) mineralization. In this study, a bio-hybrid biomimetic-MAICP system was developed that integrates the microalga Chlorella vulgaris with carbonic anhydrase-functionalized Zn-based metal-organic framework-5 (CA@fZnMOF-5), encapsulated in alginate beads, to accelerate CO₂ hydration and MAICP mineralization. Experiments were conducted under CO₂ concentrations of 0.04-15 % and catalyst dosages of 0.5-2.0 g L^-1 to evaluate algal growth, CaCO₃ precipitation, alkalinity, and total carbon capture. At 5 % CO₂, the system produced 1438.48 mg L^-1 of biomass, 330.74 mg L^-1 of CaCO₃, 1978 mg L^-1 of alkalinity, and 4941.15 mg L^-1 of total carbon capture. CO₂ levels above 10 % induced acidification that suppressed mineralization, while optimized catalyst loading helped stabilize pH and support carbonate formation. Increasing the catalyst dosage from 1.0 to 1.5 g L^-1 (PBR-2 to PBR-3) yielded only modest improvements, with biomass, CaCO₃ precipitation, alkalinity, and total carbon capture increasing by 1.03, 1.08, 1.14, and 1.16 times, respectively. Hence, the CA@fZnMOF-5 beads enhanced CO₂ hydration and increased HCO₃^- availability, promoting photosynthesis and mineral precipitation. Mineral analysis (XRD, FTIR) also confirmed the formation of calcite and aragonite polymorphs. This bio-hybrid approach couples biomimetic catalysis with MAICP and demonstrates a pathway toward scalable ocean-based CO₂ sequestration and alkalinity enhancement.