The relative availability of inorganic carbon and inorganic nitrogen influences the response of the dinoflagellate protoceratium reticulatum to elevated CO2.

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.