ABSTRACT: Historical records show a link between CO₂ and silicon-aluminum (Si-Al) during periods of global climate change, and their biogeochemical cycles are closely intertwined with the thriving of diatoms in the world’s oceans. However, our understanding of how diatoms respond to varying levels of CO₂ and Si-Al remains limited. In this study, we investigated the effects of different CO₂ concentrations, specifically 400 ppm and 1200 ppm, in combination with varying levels of Si-Al, on a model diatom species, Phaeodactylum tricornutum. Both elevated CO₂ and Si-Al supply significantly enhance the growth, photosynthesis, and frustule carbon-to-nitrogen (C/N) ratio of the diatom cells. However, the impact of CO₂ on the diatom was found to be more pronounced than that of Si-Al addition. Interestingly, the modifications in frustule composition induced by CO₂ and Si-Al exhibited a synergistic effect on the heterogeneous distribution of surface potential and adhesion, attributing to an accelerated sinking rate of the diatom cells. Metabolomics analysis revealed alterations in lipids profiles under different CO₂ and Si-Al conditions, and these metabolic shifts in lipid metabolism appeared to be linked to the cellular adaptation to nutrient depletion, particularly nitrogen limitation. Overall, our findings indicate that under elevated CO₂ levels, specific species of diatoms may play an enhanced feedback role in carbon sequestration, with Si-Al acting as a facilitating regulator.