This work originates from three facts: 1) changes in CO2 availability influence metabolic processes in algal cells. 2) Spatial and temporal variations of nitrogen availability cause repercussions on phytoplankton physiology. 3) Growth and cell composition are dependent on the stoichiometry of nutritional resources. In this study we assess whether the impact of rising pCO2 is influenced by N availability, through the impact that it would have on the C/N stoichiometry, in condition of N sufficiency. Our experiments used the dinoflagellate Protoceratium reticulatum, which we cultured under three CO2 regimes (400, 1000 and 5000 ppmv, pH of 8.1) and either variable (the NO3 (-) concentration was always 2.5 mmol • L(-1) ) or constant (NO3 (-) concentration varied to maintain the same Ci /NO3 (-) ratio at all pCO2 ) Ci /NO3 (-) ratio. Regardless of N availability, cells had higher specific growth rates, but lower cell dry weight and C and N quotas, at elevated CO2 . The carbohydrate pool size and the C/N was unaltered in all treatments. The lipid content only decreased at high pCO2 at constant Ci /NO3 (-) ratio. In the variable Ci /NO3 (-) conditions, the relative abundance of Rubisco (and other proteins) also changed; this did not occur at constant Ci /NO3 (-) . Thus, the biomass quality of P. reticulatum for grazers was affected by the Ci /NO3 (-) ratio in the environment and not only by the pCO2 , both with respect to the size of the main organic pools and the composition of the expressed proteome. This article is protected by copyright. All rights reserved.