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

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Short-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. catenella from the NW Mediter- ranean 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 concentra- tion (10 µgatN l -1 ) was measured as a function of the increasing concentration of the other nutrient (0 to 10 µgatN l -1 ). Simultaneous N uptake rates of the distinct nitrogen sources were obtained from 15 N-NH 4 + and 15 N-urea incorporation measurements. A strong inhibition of urea uptake by NH 4 + (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. catenella cells consider- ing 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 charac- teristics may reflect particular metabolic adaptations by the strains to their respective environment.