Extreme dust deposition and ocean acidification are intensifying under climate change, yet their combined impacts on marine carbon cycling remain poorly constrained. Here, we examined the physiological and biogeochemical responses of the diatom Skeletonema costatum to current and elevated CO₂ (1000 ppm) conditions combined with low (2 mg L^-1) and high (10 mg L^-1) dust additions. Elevated CO₂ alone promoted sustained growth by reducing investment in carbon-concentrating mechanisms and enhancing dissolved organic nitrogen utilization. In contrast, high dust addition under elevated CO₂ maintained high cell densities but reduced cellular particulate organic carbon and nitrogen quotas by more than 50% relative to the elevated CO₂ alone. Consequently, enhanced cell proliferation was no longer accompanied by proportional biomass accumulation, leading to a decline in per-cell carbon uptake efficiency to 34.8% of the elevated CO₂ baseline. Transcriptomic analyses revealed that a dust-derived ammonium pulse reconfigured nitrogen acquisition pathways and redirected cellular resources toward rapid proliferation at the expense of lipid biosynthesis and carbon storage. The resulting carbon-depleted diatom biomass may be more prone to upper-ocean remineralization, thereby weakening the biological carbon pump. These findings suggest that extreme dust deposition in future acidified oceans does not necessarily enhance, and may even diminish carbon sequestration.