Interactions entre un dinoflagellé toxique et sa microflore bactérienne associée: rôle des bactéries dans la toxicité de Prorocentrum lima Ehrenberg (Dodge)

Interactions between a toxic strain of the dinoflagellate Prorocentrum lima Ehrenberg (Dodge), producing okadaic acid (OA) and dinophysistoxin 1 (DTX1), and its associated bacteria were studied in culture conditions. Electron microscopy showed a typical dinoflagellate ultrastructure, though with certain peculiarities, and the presence of an abundant associàted bacterial microflora free-living or bound to P. lima cells. A few intracellular (endosymbiotic or infectious) bacteria were also detected within P. lima cells. Double-staining with anti-OA antibody and DNA staining revealed the presence of OA mainly in peripheral cytoplasm. OA was not detected in extranuclear DNA, indicating that these bacteria do not contain the toxin. High-pressure liquid chromatography (HPLC) assays and immunochemical labelling tended to show that P. lima was the main source of toxin in the culture. Moreover, a Proteobacterium of the alpha group which is non-photosynthetic was isolated from the culture. This bacterium, belonging to an unknown species, appeared to produce low quantities of OA and DTX1. HPLC assays also showed that a large quantity of toxin was released into the extracellular environment and weakly adsorbed by free-living bacteria. Bacterial microflora increased toxin production in the culture when P. lima was senescent. Toxin production by toxinogenic bacteria might also take place during this culture period. It would seem likely that a synergistic action occurs between P. lima and one or more of its associated bacteria for toxin synthesis.