Cytotoxicity unleashed: how Prymnesium parvum extract drives RTgill-W1 rainbow trout cells into cell death

Massive algal blooms, including the 2022 Oder disaster linked to the toxic haptophyte Prymnesium parvum, highlight critical gaps in our understanding of waterborne ecotoxicants. Prymnesins are ladder-framed polyether-type compounds produced by P. parvum with well-documented ichthyotoxic properties; however, the cellular mechanisms underlying prymnesin-induced fish cell injury have yet to be resolved at the cellular level. Here, we establish and optimize molecular and cellular bioassays using fluorescence microscopy and flow cytometry to characterize early cell-damage pathways in rainbow trout gill cells (RTgill-W1) following a 3-hour exposure to a B-type prymnesin-containing algal extract. We specifically focused on cell death-associated events induced by P. parvum extract. Our endpoint analyses revealed a concentration-dependent shift in the dominant mode of cell death, with lower toxin levels predominantly triggering apoptosis, as indicated by nuclear morphological alterations, decreased mitochondrial mass, and phosphatidylserine externalization (Annexin V staining). In contrast, higher prymnesin concentrations induced necrotic cell death, characterized by membrane rupture and loss of cellular integrity as detected by propidium iodide uptake. By resolving concentration-dependent transitions between apoptotic and necrotic cell death, this study positions prymnesin-containing extracts as drivers of progressive intracellular injury rather than as agents exhibiting exclusively rapid membrane-lytic toxicity within the spectrum of macrocyclic polyether algal toxins. These findings highlight the dual nature of prymnesin-mediated cytotoxicity and demonstrate that exposure concentration is a critical determinant of cellular outcome, thereby advancing the mechanistic understanding of P. parvum-associated ichthyotoxicity. Beyond defining prymnesin-associated cytotoxicity during harmful algal blooms, the results indicate that fish gill epithelial cells initially engage in regulated stress responses at sublethal exposure levels, which may reflect early compensatory processes that modulate cellular tolerance.