Toxic effects of the emerging Alexandrium pseudogonyaulax (Dinophyceae) on multiple trophic levels of the pelagic food web

The dinoflagellate Alexandrium pseudogonyaulax, a harmful algal bloom species, is currently appearing in increasing frequency and abundance across Northern European waters, displacing other Alexandrium species. This mixotrophic alga produces goniodomins (GDs) and bioactive extracellular substances (BECs) that may pose a threat to coastal ecosystems and other marine resources. This study demonstrated the adverse effects of A. pseudogonyaulax on four marine trophic levels, including microalgae (Rhodomonas salina), microzooplankton (Polykrikos kofoidii) and mesozooplankton (Acartia tonsa), as well as fish gill cells (RTgill-W1, Oncorhynchus mykiss), ultimately leading to enhanced mortality and cell lysis. Furthermore, cell-free supernatants collected from A. pseudogonyaulax cultures caused complete loss of metabolic activity in the RTgill-W1 cell line, indicating ichthyotoxic properties, while all tested GDs were much less toxic. In addition, cell-free supernatants of A. pseudogonyaulax led to cell lysis of R. salina, while all tested GDs were non-lytic. Finally, reduced egg hatching rates of A. tonsa eggs exposed to cell-free supernatants of A. pseudogonyaulax and impaired mobility of P. kofoidii and A. tonsa exposed to A. pseudogonyaulax were also observed. Altogether, bioassay results suggest that the toxicity of A. pseudogonyaulax is mainly driven by BECs and not by GDs, although further research into factors modulating the lytic activity of Alexandrium spp. are needed.