AME 53:271-280 (2008)  -  DOI: https://doi.org/10.3354/ame01249

ABSTRACT: 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 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 (0 to 10 µgatN l^–1). Simultaneous N uptake rates of the distinct nitrogen sources were obtained from ¹⁵N-NH₄⁺ and ¹⁵N-urea incorporation measurements. A strong inhibition of urea uptake by NH₄⁺ (maximum inhibition, I_max > 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 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.

KEY WORDS: Alexandrium catenella · Ammonium · Urea · Interactions · Strain variability