Although the dissolved inorganic carbon concentration, pH, and nutrient regimes of seawater dramatically change in coastal regions, the synergistic effects of changes in the CO₂ and nutrient levels on the elemental dynamics of the particulate and dissolved organic matters (DOMs) produced by diatoms are rarely investigated. Here, we investigated the impacts of four different CO₂ levels (180, 380, 600, and 1000 μ atm partial pressure of CO₂ : p CO₂) on the allocation of carbon, nitrogen, phosphorus, and silicon between the particulate matter (PM) and DOM in two cosmopolitan coastal diatoms, Chaetoceros affinis and Ditylum brightwellii , under nutrient-replete and nitrate-depleted conditions. Under nutrient-replete conditions, the specific growth rates of both species were positively correlated with p CO₂ levels. The elemental compositions of the exponentially growing diatoms were stable under the different p CO₂ conditions. After nitrate depletion, the particulate organic carbon to particulate nitrogen ratio and biogenic silica content per unit biomass in both species were positively correlated with the p CO₂ value. Factors affecting the p CO₂ dependent change in elemental composition were the variations in the partitioning of organic carbon between PM and DOM in C. affinis , and the physiological uncoupling of intracellular carbon and nitrogen and the intracellular silicon and nitrogen, as well as resting spore formation in D. brightwellii . Under high-CO₂ conditions, the faster growth rates of both diatom species could lead to their dominance in a phytoplankton community; their blooms could modify the first-order processes in the biogeochemical cycling of bioelements after nitrate depletion.