In France, oyster sites in the Mediterranean Sea are regularly confronted to high mortalities of Crassostrea gigas juveniles and to recurrent blooms of the dinoflagellate producer of Paralytic Shellfish Toxins (PSTs), Alexandrium catenella. Among the Pathogens associated to these mortalities, we found Vibrio strains belonging to Splendidus clade. We here focus on the interactions between A. catenella and the oyster C. gigas challenged with pathogenic vibrios. In the first part of this work, we have shown that, in vivo,A. catenella increased the susceptibility of the oyster C. gigas to the pathogen Vibrio tasmaniensis LGP32. In situ, we also Established the coincidence between oyster mortality in 2014 and the presence of PSTs in their tissues. In the second part of this work, we studied the interactions between the PSTs produced by A. catenella and oyster immune cells, the hemocytes. An important result of this thesis was that saxitoxin, a toxin produced by A. catenella, binds to granular structures in the cytoplasm of C. gigas hemocytes and induces their caspase-dependent cell death. This death Is independent of the production of reactive oxygen species. We also demonstrated that the major toxin of A. catenella cells, the gonyautoxin 5, is the most toxic on C. gigas hemocytes. Among affected hemocyte populations, the hyalinocytes are very sensitive to this toxic stress. As hemocytes are oyster immunocompetent cells and therefore play the central role in the defense against infections, we can presume that their cell death induced by the PSTs negatively affects the defense of bivalve mollusks and explains the increased susceptibility of oysters to the infection by Vibrio tasmaniensis LGP32 when exposed to A. catenella.