Hemolytic toxicity of Prymnesium parvum (B-type) reveals species-specific differences in freshwater fishes

The toxin-producing flagellated alga Prymnesium parvum threatens aquatic ecosystems by causing mass die-offs of aquatic species when harmful algal blooms (HABs) occur. In the absence of a sensitive analytical method to quantify the prymnesin toxins, hemolysis assays are commonly used to assess P. parvum toxicity. This study demonstrates that assay outcomes vary significantly depending on the species from which the red blood cells (RBCs) originate. RBCs from five freshwater fish species - rainbow trout (Oncorhynchus mykiss), sterlet (Acipenser ruthenus), perch (Perca fluviatilis), bream (Abramis brama), and carp (Cyprinus carpio) - were exposed to P. parvum in five biological replicates per species. Hemolytic activity was quantified as the percent decline of intact RBCs compared to the control after 15, 18, 21, and 24 h. A dose-dependent response was observed across all species, with species-specific effects emerging after 18 h of incubation and becoming most evident after 21 h. Rainbow trout RBCs exhibited the highest 21 h median effect concentration (EC₅₀ = 41348 cells ml^-1), indicating the lowest sensitivity, while perch RBCs showed the lowest EC₅₀ (10237 cells ml^-1), indicating significantly higher sensitivity (p < 0.001). Carp RBCs also displayed a low EC₅₀ (12390 cells ml^-1) and exhibited the steepest dose-response curve (p < 0.01). These findings reveal significant interspecies variability in sensitivity to the hemolytic effects of P. parvum. This study highlights the importance of species-specific assessments for HAB risk evaluation, providing critical insights to inform future research and management of freshwater ecosystems.