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 <i>Thalassiosira pseudonana</i> (<i>T. pseudonana</i>) and the coccolithophore <i>Emiliania huxleyi</i> (<i>E. huxleyi</i>;calcifying and noncalcifying strains). Olivine addition increased total alkalinity across all cultures, although the increase was less pronounced in calcifying <i>E. huxleyi</i>. Notably, olivine stimulated growth and particulate organic carbon production in <i>T. pseudonana</i> and calcifying <i>E. huxleyi</i>, while noncalcifying strains showed no significant response. Olivine addition dramatically accelerated sinking rates, which increased 9.36-fold for <i>T. pseudonana</i> and 2.39-fold for calcifying <i>E. huxleyi</i>. 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.