Interactions between ammonium and urea uptake by five strains of Alexandrium catenella (Dinophyceae) in culture

10 pages, 3 figures, 2 tablesShort-term experiments were carried out to investigate whether interactions between ammonium (NH4+) and urea uptake regulate the total nitrogen assimilation of the toxic dinoflagellate Alexandrium catenella. To test for strain variability, 5 strains of A. catenellafrom the NW Mediterranean were used: 3 strains from the Thau lagoon (southern France) and 2 strains from the Catalonia basin (Spain). For each strain, the uptake rate of 1 nutrient (NH4+or urea) at a reference concentration (10 μgatN l–1) was measured as a function of the increasing concentration of the other nutrient (0to 10 μgatN l–1). Simultaneous N uptake rates of the distinct nitrogen sources were obtained from 15 N-NH4+ and 15N-urea incorporation measurements. A strong inhibition of urea uptake by NH4+ (maximum inhibition, Imax> 55%) was observed exclusively for the French strains. No influence of urea on the NH4+-uptake rate was noted for any strain. Estimation of total N uptake rates revealed that the N-urea uptake inhibition was not a competitive disadvantage for A. catenellacells considering that the reduced N-urea uptake was more than compensated for by NH4+uptake. Furthermore, the computation of composite kinetic parameters from total N uptake data suggested that French strains were more competitive than the Spanish ones in an environment characterized by low NH4+ concentrations (≤5 μgatN l–1) and high urea concentrations (as 10 μgatN l–1). These N uptake characteristics may reflect particular metabolic adaptations by the strains to their respective environment.We thank R. Ventosa for the nutrient analyses. This study was financed by the Centre National de la Recherche Scientifique (CNRS) and Institut Français pour l’Exploitation de la Mer (Ifremer) ALTOX program, as well as the EU-funded Research Project SEED (Life cycle transformations among HAB species, and the environmental and physiological factors that regulate them, GOCE-CT 2005-003875). S.L. was supported by a Fundação para a Ciência e para a Tecnologia, Portugal (FCT) grant within the III Quadro Comunitário de Apoio by the Fundo Social Europeu. E.G. was supported by the Ramon y Cajal contract of the Spanish Ministry of Education and Science. C.J. was supported by a scholarship from the Région Languedoc-Roussillon and IfremPeer reviewe