Effects of the dinoflagellate Alexandrium minutum and its toxin (saxitoxin) on the functional activity and gene expression of Crassostrea gigas hemocytes

International audience; Blood cells or hemocytes are the major component of the cellular immune responses in bivalves. Recent evidences have demonstrated that the immune functions of these cells can be impaired by harmful algae and their toxins. In this study, we compared the in vitro effect of the toxic dinoflagellate Alexandrium minutum and its purified paralytic toxin, saxitoxin (STX) on the oyster Crassostrea gigas hemocytes. Oyster hemocytes were incubated with A. minutum or STX, and their size, complexity, viability, phagocytic activity and reactive oxygen species (ROS) production were analyzed through flow cytometry. Also, expression of eleven defense-related genes was evaluated through real-time quantitative PCR. Cell incubation with both, the noxious microalga and STX did not alter hemocyte viability neither complexity. However, their phagocytic capacity and ROS production were significantly affected by both treatments. On the other hand, only A. minutum exposure resulted in increase of hemocyte size. Transcript levels of the gene encoding the cytokine Interleukin-17 was significantly higher (4.5-fold) after hemocytes were exposed to A. minutum, which contrasted with a decreased (7-fold) number of transcripts, when hemocytes were treated with STX. The purified toxin also led to lower transcript levels of a cytochrome P450 gene, the CYP356A1 (3.5-fold). On the other hand, hemocytes incubated with STX presented higher mRNA levels of antimicrobial peptide defensin-2 (7-fold) and heat shock protein Hsp70 (2-fold) genes. Altogether, these results suggest that the noxious alga A. minutum and STX can indeed negatively affect immunocompetence of C. gigas hemocytes, although in distinct manners. These differences could be due to a lower STX bioavailability in living microalgal cultures or alternatively to the presence of allelopathic compounds produced by living microalgae. This is the first report comparing the transcription of defense-related genes in oysters exposed to a harmful alga and its purified toxin.