ZnO nanoparticles (nZnO) are released into the coastal environment from industrial sources, textiles, and personal care products, yet their toxicity to marine organisms is not well understood. We investigated the interactive effects of salinity (normal 15, low 5, and fluctuating 5-15) and nZnO (100 µg l-1) on innate immunity of a keystone marine bivalve, the blue mussel Mytilus edulis. Exposure to Zn2+ (100 µg l-1) was used to test whether the toxic effects on nZnO can be attributed to Zn2+. Functional parameters (hemocyte abundance, viability, adhesion capacity, phagocytosis rate and lysosomal index) and the expression of key immune-related genes (the pathogen recognition receptors, complement system components, antimicrobial peptides and inflammatory signals) were investigated in the mussels exposed to nZnO or Zn2+ under different salinity regimes. ZnO exposures elevated hemocyte mortality, suppressed adhesion and stimulated phagocytosis and lysosomal enlargement/proliferation. At normal salinity (15), nZnO suppressed the mRNA expression of the Toll-like receptors TLRb and c, C-lectin, and the complement system component C3q indicating impaired ability for pathogen recognition. In contrast, the mRNA levels of an antimicrobial peptide defensin was stimulated by nZnO exposure at salinity 15. Exposure to the fluctuating salinity reversed the transcriptomic responses of M.edulis hemocytes to nZnO. Low salinity (5) had a strong immunosuppressive effects on the functional and molecular immune traits of M.edulis that overshadowed the effects of nZnO. The immunotoxic effects of nZnO were stronger than those of Zn2+ regardless of salinity. The salinity-dependent modulation of immune response to nZnO cannot be attributed to the differences in the aggregation or solubility of nZnO and likely reflects the interaction of the toxic effects of nanoparticles and physiological effects of the osmotic stress. These findings have implications for the environmental risk assessment of nanomaterials and the development of the context-specific biomarker baselines for coastal pollution monitoring.
