Activités métaboliques et toxicité du dinoflagellé Alexandrium tamarense en culture à dialyse exposée à un milieu estuarien

Abstract

Metabolic activities and toxicity of the dinoflagellate Alexandrium tamarense were studied for a dialysis culture exposed to unenriched natural seawater and a batch culture grown in an enriched medium. The following observations were made on the dialysis culture compared with the batch culture: (i) Depending on the season period, the cell densities per litre were similar (≈ 10 × 106 – 14 × 106) or greater (66 × 106). (ii) During growth and stationary phases, cellular consumption of the nutrients (NO3− + NO2−) was much greater (5–50 times), (iii) During growth, the composition of the intracellular (IFAA) and extracellular dissolved free amino acid (EFAA) was characterized by the presence of ORN and LYS in the dialysis culture, which represent a substantial fraction of the IFAA pool of the dinoflagellate; a significant depletion of the IFAA was observed, but the recovery of Gln, Glu, and Arg cell concentration levels at the stationary phase was more important. (iv) During the growth phase, the maximum excretion rate of amino acids (fmol∙cell−1∙d−1) was 263 (in decreasing order of importance: Glu, Gin, Ser, Asp, Leu, Ala, G+T, and Orn) compared with 14,5. (v) During stationary phase, the reduction in excretion activity was twofold with Gln, Glu, Leu, and Orn while 9,2 fmol∙cell−1∙d−1 were observed in batch culture, (vi) Total toxin content was about 4 to 5 times higher (means: 95 and 75 fmol∙cell−1 during growth and stationary phases respectively), with greater concentrations in more potent derivates like saxitoxin and neosaxitoxin. This higher cell toxicity may be attributed to the particular growth conditions of the dialysis culture, which more closely approximate the natural environment, through diffusion processes involving a constant elimination of low molecular weight autoinhibitory metabolites and a more efficient nutrient uptake.Key words: Alexandrium tamarense, dinoflagellate, dialysis culture, phycotoxins, nitrogen metabolism.