The relative availability of inorganic carbon and inorganic nitrogen influences the response of the dinoflagellate Protoceratium reticulatum to elevated CO 2 .

This work originates from three facts: (i) changes in CO2 availability influence metabolic processes in algal cells; (ii) Spatial and temporal variations of nitrogen availability cause repercussions on phytoplankton physiology; (iii) 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 conditions of N sufficiency. Our experiments used the dinoflagellate Protoceratium reticulatum, which we cultured under three CO2 regimes (400, 1,000, and 5,000 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.