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

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.