Ocean Alkalinity Enhancement (OAE) is emerging as a promising Carbon Dioxide Removal (CDR) strategy. By adding alkaline minerals to elevate seawater alkalinity, OAE shifts the carbonate system to convert dissolved CO₂ into stable bicarbonates, thereby amplifying the ocean’s capacity to absorb atmospheric carbon. Recent studies demonstrate that marine microalgae can tolerate these engineered increases in alkalinity. However, how phytoplankton feedbacks regulate OAE’s carbon removal efficiency remains unclear. We investigated three widely distributed marine microalgae (diatoms Thalassiosira pseudonana, Skeletonema costatum and coccolithophore Emiliania huxleyi) under an alkalinity gradient (ΔTA < 2000 μmol kg^-1, pH < 8.5). All three algae survived and enhanced surface CO₂ uptake 5- to 25-fold over abiotic controls, yet feedbacks were highly species-specific. T. pseudonana increased atmospheric carbon removal and vertical transport. In E. huxleyi, OAE-stimulated calcification outpaced photosynthesis, causing a 35%–41% theoretical efficiency offset. Nevertheless, actual atmospheric CO₂ uptake remained stable, and enhanced ballasting raised its carbon burial by ~82%—showing that under nutrient‑replete and High OAE conditions, E. huxleyi achieves greater carbon removal. S. costatum maintained carbon fixation under Control and Moderate-OAE. Under High OAE, it induced calcification comparable to E. huxleyi, while its rapid photosynthesis uptake kept pCO₂ below 50 μatm, resulting in surface CO₂ uptake superior to that of E. huxleyi. This calcification increased its carbon burial seven‑fold relative to the non‑calcifying state. With biological intervention, OAE reshapes carbon removal through species-specific biological feedbacks, requiring accurate CDR accounting to incorporate dominant species’ responses to optimize the synergy between chemical uptake and biological fixation.

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