GROWTH AND PHOSPHORUS UPTAKE BY THE TOXIC DINOFLAGELLATE ALEXANDRIUM CATENELLA (DINOPHYCEAE) IN RESPONSE TO PHOSPHATE LIMITATION1

Abstract

Alexandrium catenella (Whedon et Kof.) Balech has exhibited seasonal recurrent blooms in the Thau lagoon (South of France) since first reported in 1995. Its appearance followed a strong decrease (90%) in phosphate (PO43−) concentrations in this environment over the 1970–1995 period. To determine if this dinoflagellate species has a competitive advantage in PO43−-limited conditions in terms of nutrient acquisition, semicontinuous cultures were carried out to characterize phosphorus (P) uptake by A. catenella cells along a P-limitation gradient using different dilution rates (DRs). Use of both inorganic and organic P was investigated from measurements of 33PO43− uptake and alkaline phosphatase activity (APA), respectively. P status was estimated from cellular P and carbon contents (QP and QC). Shifts in trends of QP/QC and QP per cell (QP·cell−1) along the DR gradient allowed the definition of successive P-stress thresholds for A. catenella cells. The maximal uptake rate of 33PO43− increased strongly with the decrease in DR and the decrease in QP/QC, displaying physiological acclimations to PO43− limitation. Concerning maximal APA per cell, the observation of an all-or-nothing pattern along the dilution gradient suggests that synthesis of AP was induced and maximized at the cellular scale as soon as PO43− limitation set in. APA variations revealed that the synthesis of AP was repressed over a PO43− threshold between 0.4 and 1 μM. As lower PO43− concentrations are regularly observed during A. catenella blooms in Thau lagoon, a significant portion of P uptake by A. catenella cells in the field may come from organic compounds.