Evaluating microalgal-induced carbonate precipitation for marine carbon sequestration using Chlorella species

Microalgae-induced carbonate precipitation (MAICP) is an emerging photosynthetic pathway for marine carbon sequestration, offering a sustainable alternative to traditional microbial-induced carbonate precipitation (MICP) that relies on ureolysis and produces undesirable ammonia byproducts. This study focuses on an unexplored potential of three Chlorella species-particularly Chlorella vulgaris-for MAICP under enriched-oceanic conditions. For this purpose, the bioengineered MAICP system was designed and developed in a bubble-column photobioreactor (PBR) and then systematically optimized the enriched-oceanic conditions (i.e. NaHCO₃ and CaCl₂ supplementation in modified artificial seawater) to enhance the MAICP performance including biomass productivity, pH regulation, CaCO₃ precipitation, and sedimentation efficacy. Among three tested strains, C. vulgaris demonstrated higher performance, promoting alkaline pH regulation, and achieving highest biomass production (449.71 ± 5.88 mg L^-1) and CO₂ fixation rate (97.23 ± 3.05 mg L^-1 under optimized NaHCO₃ levels. MAICP facilitated a 63 % reduction in alkalinity and 87.43 % calcium removal, leading to CaCO₃ precipitation (316.42 ± 3.12 mg L^-1), self-sedimentation (94.57 %), and total carbon capture (1139.39 ± 11.52 mg L^-1). Further enrichment of CaCl₂ supplementation increased the CaCO₃ precipitation yield up to 1355.42 ± 17.95 mg L^-1 and total carbon capture to 1943.37 ± 12.53 mg L^-1, indicating the MAICP potential for large-scale carbon sequestration in bioengineered PBRs. XRD and FTIR confirmed aragonite and calcite polymorphs, while SEM revealed distinct needle-like and rhombohedral morphologies. This work is the first to systematically evaluate fast-growing Chlorella species for MAICP, revealing their capacity for rapid pH modulation, crystal formation, and self-sedimentation-positioning C. vulgaris-derived MAICP as a scalable and eco-friendly strategy for long-term carbon storage and climate change mitigation.