Insights on the toxic effect of Ostreopsis cf. ovata: the role of direct/indirect contact with microalgal cells

Description

Ostreopsis cf. ovata is a harmful benthic dinoflagellate, widespread along most of the Mediterranean coasts. It produces several toxins and variable amounts of mucus that may totally cover substrates, especially during the stationary phase of blooms, when cells are aggregated into a mucilaginous matrix. Studies on different aspects of the biology and ecology of Ostreopsis spp. are increasing, yet knowledge on toxicity mechanism is still scarce. In particular, the potential active role of the mucilaginous matrix has not yet been proved, although when mass mortalities have occurred, organisms were reported to be covered by the typical brownish mucilage. In order to better elucidate toxicity dependence of direct/indirect contact with O. cf. ovata cells, we carried out toxicity bioassays exposing Artemia salina nauplii and ephyrae of Aurelia aurita, as innovative model organism in ecotoxicology, to several treatments of O. cf. ovata culture. Simultaneously, molecular and chemical analyses were performed to validate our methods and characterize the toxin profile. Our results have reported higher effects when a direct contact between model organisms and intact microalgal cells occurs (LC50-48h < 4 cells/ml on A. salina, EC 50-48h = 9 cells/ml on A. aurita), although also growth medium devoid of microalgal cells but containing O. cf. ovata mucilage and filaments, caused significant toxic effects. Additionally, the connection between mucilaginous matrix and thecal plates, pores and trychocysts was explored by way of atomic force microscopy (AFM) to investigate cell surface at a sub-nanometer resolution, providing a pioneering description of the inner side of O. cf. ovata thecal plates

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