Extreme dust deposition disrupts diatom carbon allocation under CO2-elevated ocean conditions.

Abstract

Extreme dust deposition and ocean acidification are intensifying environmental stressors under climate change, yet their combined impacts on marine phytoplankton physiology and carbon cycling remain poorly understood. Here, we explored the responses of the cosmopolitan diatom Skeletonema costatum to present-day (420 ppm) and future (1000 ppm) CO2 conditions under low (2 mg L-1) and high (10 mg L-1) dust deposition scenarios. 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 triggered a metabolic reallocation: although cell density remained high, cellular particulate organic carbon and nitrogen quotas declined by more than 50% relative to the elevated CO2 alone. Transcriptomic analyses revealed that a dust-derived ammonium pulse disrupted coordinated nitrogen metabolism, redirecting cellular resources toward rapid proliferation at the expense of lipid biosynthesis and carbon storage. This shift decoupled population growth from carbon accumulation, resulting in a 65.2% reduction in per-cell carbon uptake efficiency relative to the elevated CO2 control. Although transient dissolved inorganic carbon drawdown and pH elevation occurred during dust-stimulated growth, the production of carbon-depleted biomass may reduce carbon export efficiency by increasing the likelihood of upper-ocean remineralization. These findings suggest that extreme dust deposition under future ocean acidification may reduce the efficiency of the biological carbon pump rather than enhance carbon sequestration, representing a potential environmental risk to this critical marine ecosystem service.

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