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.

Division of Plant Sciences University of Dundee at the James Hutton Institute Invergowrie Dundee DD2 5DA UK

Functional Plant Biology and Climate Change Cluster University of Technology Sydney Ultimo NSW 2007 Australia

Institute of Marine Science National Research Council Arsenale Castello 2737 F 30122 Venezia Italy

Institute of Microbiology Academy of Sciences of the Czech Republic Trěboň 379 01 Czech Republic

Istituto di Biologia Agro Ambientale e Forestale National Research Council Via G Marconi n 2 Porano 05010 Terni Italy

Laboratorio di Fisiologia delle Alghe e delle Piante Dipartimento di Scienze della Vita e dell'Ambiente Università Politecnica delle Marche via Brecce Bianche Ancona 60131 Italy

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