Histological and transcriptional responses of the Chilean mussel Mytilus chilensis to the toxic dinoflagellates Alexandrium catenella and Protoceratium reticulatum.

Harmful algal blooms (HABs) are natural phenomena caused by toxin-producing microalgae whose occurrence appears to have increased worldwide. Although microalgae constitute a natural food source for bivalve molluscs, certain species can exert negative effects. The present study assessed the molecular and histological effects of exposure to the toxic dinoflagellates Alexandrium catenella and Protoceratium reticulatum on adult Chilean mussels (Mytilus chilensis). Mussels were exposed for 15 days to 500 cells mL-1 of A. catenella and for 10 days to 100 cells mL-1 of P. reticulatum, with 50,000 cells mL-1 of the non-toxic microalgae Isochrysis galbana added to both treatments. Exposure to both dinoflagellates induced changes in gene expression and histopathological alterations. Tissue-specific responses were observed: gills were primarily affected by A. catenella, whereas digestive glands showed greater alterations following exposure to P. reticulatum. At the molecular level, A. catenella activated antioxidant and immune-related responses in both tissues, likely associated with oxidative stress, as indicated by the overexpression of def, myt A, myt B, pgrp, sod1, mt, and fer. Conversely, P. reticulatum appeared to weaken immune defence, as characterized by increased expression of pgrp, myt B, and fer after 5 days, followed by reduced expression of def and myt A after 10 days and in myt A after 5 days. Histopathological alterations were limited, although hemocytic infiltrations in the gills increased following exposure to both species, together with abnormalities in digestive tissues, particularly in mussels exposed to P. reticulatum. These findings provide new insights into the effects of HABs on bivalve molluscs.