Olivine-Induced Alkalinity Enhancement Amplifies Phytoplankton Carbon Export Efficiency

Ocean alkalinity enhancement (OAE) via olivine addition is a promising carbon dioxide (CO₂) removal strategy, yet its impact on phytoplankton-driven biogeochemical processes remains unclear. We investigated the effects of olivine on the diatom Thalassiosira pseudonana (T. pseudonana) and the coccolithophore Emiliania huxleyi (E. huxleyi;calcifying and noncalcifying strains). Olivine addition increased total alkalinity across all cultures, although the increase was less pronounced in calcifying E. huxleyi. Notably, olivine stimulated growth and particulate organic carbon production in T. pseudonana and calcifying E. huxleyi, while noncalcifying strains showed no significant response. Olivine addition dramatically accelerated sinking rates, which increased 9.36-fold for T. pseudonana and 2.39-fold for calcifying E. huxleyi. This enhancement was driven by distinct mechanisms: silicon-mediated ballasting in diatoms and extracellular polysaccharide-induced cell-olivine aggregation in coccolithophores, alongside minor silicon deposition on coccoliths. These results indicate that olivine-based OAE could increase CO₂ drawdown through two independent mechanisms and may potentially strengthen the biological pump by accelerating organic matter export. Our findings suggest that olivine addition could serve as a potent approach for enhancing carbon export efficiency, with diatoms exhibiting a stronger response than coccolithophores, although its ecological impacts will require further investigation.