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 C_i/NO₃ ⁻ ratio at all pCO₂) C_i/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 C_i/NO₃ ⁻ ratio. In the variable C_i/NO₃ ⁻ conditions, the relative abundance of Rubisco (and other proteins) also changed; this did not occur at constant C_i/NO₃ ⁻. Thus, the biomass quality of P. reticulatum for grazers was affected by the C_i/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.