In vitro effects of the harmful benthic dinoflagellates Prorocentrum hoffmannianum and Ostreopsis cf. ovata on immune responses of the farmed oyster Crassostrea gasar.

Oyster culture is a sustainable solution to food production. However, this activity can be severely impacted by the presence and proliferation of harmful microalgae such as the benthic dinoflagellates Prorocentrum hoffmannianum and Ostreopsis cf. ovata. This study aimed to evaluate the in vitro effects of P. hoffmannianum and O. cf. ovata on immune system cells (hemocytes) of the native cultured oyster Crassostrea gasar. The direct toxicity of both dinoflagellates was first evaluated assessing hemocyte viability exposed to eight concentrations of each HAB species. No reduction in hemocyte viability was found with the exposure to cell culture or the crude extract of P. hoffmannianum, but O. cf. ovata culture induced hemocyte death in a concentration-dependent manner. Ostreopsis cf. ovata concentration that promoted half of maximal reduction in hemocyte viability (EC) was 779 cells mL. Posteriorly, hemocytes were exposed to both dinoflagellate cells and crude extracts to investigate their effects on hemocyte functional parameters. Despite no direct toxicity of the dinoflagellate cells, P. hoffmannianum extract caused a threefold increase in ROS production and decreased the phagocytosis rate by less than half. Ostreopsis cf. ovata cells and crude extracts also triggered an increase in ROS production (two-fold), but the phagocytosis rate was reduced (by half) only in response to the two lower cell concentrations. These results indicate a harmful potential of both dinoflagellates through a direct toxicity (only for O. cf. ovata) and functional impairment of hemocytes (both species) which could expose C. gasar oyster to opportunistic infections.