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

This work originates from three facts: (i) changes in CO 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 pCO 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 CO regimes (400, 1,000, and 5,000 ppmv, pH of 8.1) and either variable (the NO concentration was always 2.5 mmol · L ) or constant (NO concentration varied to maintain the same C /NO ratio at all pCO ) C /NO ratio. Regardless of N availability, cells had higher specific growth rates, but lower cell dry weight and C and N quotas, at elevated CO . The carbohydrate pool size and the C/N was unaltered in all treatments. The lipid content only decreased at high pCO at constant C /NO ratio. In the variable C /NO conditions, the relative abundance of Rubisco (and other proteins) also changed; this did not occur at constant C /NO . Thus, the biomass quality of P. reticulatum for grazers was affected by the C /NO ratio in the environment and not only by the pCO , both with respect to the size of the main organic pools and the composition of the expressed proteome.