title: SHORT-TERM TEMPORAL VARIABILITY OF AMMONIUM AND UREA UPTAKE BY ALEXANDRIUM CATENELLA (DINOPHYTA) IN CULTURES(1).
abstract: In batch cultures of four Mediterranean strains (from France, Italy, and Spain) of Alexandrium catenella (Whedon et Kof.) Balech growing on a daily light cycle, ammonium and urea uptake were estimated by the (15) N tracer technique. Ammonium uptake could be described by Michaelis-Menten kinetics along a substrate gradient of 0.1-10 μgat N · L(-1) for the four strains, while two different patterns were observed for urea uptake with Michaelis-Menten kinetics for one strain and linear kinetics for the others. In all cases, an increase in uptake rates with time was noted over the daylight period. This trend led to a net increase in the maximum uptake rate (Vmax ; for saturable kinetics) and in the initial slope α. For ammonium, Vmax increased by a factor of 2-10 depending on the strain, and, for urea, the maximal uptake rates measured increased by a factor of 2-18. Temporal variations of half-saturation constants (Ks ) for both nutrients did not show a clear trend. Increases in Vmax and α showed an acclimation of the cells' uptake system over time to a N pulse, which may be explained by the light periodicity. For two strains, extensive ammonium release was observed during urea assimilation. This mechanism removes urea from the medium, so it is no longer available to other potential competitors, but supplies N back to the medium in the form of ammonium. From a methodological point of view, the phenomenon leads to considerable underestimates of the contribution of urea to phytoplankton growth.
authors.affiliation: Université Montpellier II, CNRS, Ifremer, Laboratoire Ecosystèmes Lagunaires (UMR 5119), CC093, 34095 Montpellier Cedex 5, France Laboratoire Environnement Ressources/LR, Ifremer, B.P. 171, 34203 Sète, FranceInstitut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta, 37-49, E08003 Barcelona, Catalunya, Spain.
authors.forename: Cécile
authors.initials: C
authors.lastname: Jauzein
authors.affiliation: Université Montpellier II, CNRS, Ifremer, Laboratoire Ecosystèmes Lagunaires (UMR 5119), CC093, 34095 Montpellier Cedex 5, France Laboratoire Environnement Ressources/LR, Ifremer, B.P. 171, 34203 Sète, FranceInstitut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta, 37-49, E08003 Barcelona, Catalunya, Spain.
authors.forename: Yves
authors.initials: Y
authors.lastname: Collos
authors.affiliation: Université Montpellier II, CNRS, Ifremer, Laboratoire Ecosystèmes Lagunaires (UMR 5119), CC093, 34095 Montpellier Cedex 5, France Laboratoire Environnement Ressources/LR, Ifremer, B.P. 171, 34203 Sète, FranceInstitut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta, 37-49, E08003 Barcelona, Catalunya, Spain.
authors.forename: Esther
authors.initials: E
authors.lastname: Garcés
authors.affiliation: Université Montpellier II, CNRS, Ifremer, Laboratoire Ecosystèmes Lagunaires (UMR 5119), CC093, 34095 Montpellier Cedex 5, France Laboratoire Environnement Ressources/LR, Ifremer, B.P. 171, 34203 Sète, FranceInstitut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta, 37-49, E08003 Barcelona, Catalunya, Spain.
authors.forename: Magda
authors.initials: M
authors.lastname: Vila
authors.affiliation: Université Montpellier II, CNRS, Ifremer, Laboratoire Ecosystèmes Lagunaires (UMR 5119), CC093, 34095 Montpellier Cedex 5, France Laboratoire Environnement Ressources/LR, Ifremer, B.P. 171, 34203 Sète, FranceInstitut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta, 37-49, E08003 Barcelona, Catalunya, Spain.
authors.forename: Mercedes
authors.initials: M
authors.lastname: Maso
keywords: Alexandrium catenella; ammonium; ammonium release; temporal variability; uptake kinetics; urea