Active role of the mucilage in the toxicity mechanism of the harmful benthic dinoflagellate Ostreopsis cf. ovata

Ostreopsis cf. ovata is a harmful benthic dinoflagellate, widespread along most of the Mediterranean coasts. It produces a wide range of palytoxin-like compounds and variable amounts of mucus that may totally cover substrates, especially during the stationary phase of blooms. Studies on different aspects of the biology and ecology of Ostreopsis spp. are increasing, yet knowledge on toxicity mechanism is still limited. In particular, the potential active role of the mucilaginous matrix has not yet been proven, although when mass mortalities have occurred, organisms have been reported to be covered by the typical brownish mucilage. In order to better elucidate toxicity dependence on direct/indirect contact, the role of the mucilaginous matrix and the potential differences in toxicity along the growth curve of O. cf. ovata, we carried out a toxic bioassay during exponential, stationary and late stationary phases. Simultaneously, a molecular assay was performed to quantify intact cells or to exclude cells presence. A liquid chromatography - high resolution mass spectrometry (LC-HRMS) analysis was also carried out to evaluate toxin profile and content in the different treatments. Our results report higher mortality of model organism, especially during the late stationary phase, when a direct contact between model organism and intact microalgal cells occurs (LC50-48h < 4 cells/ml on A. salina), although also growth medium devoid of microalgal cells but containing O. cf. ovata mucilage cause significant toxic effects. This finding is also supported by chemical analysis which shows the highest toxin content in pellet extract (95%) and around to 5% of toxins in the growth medium holding mucus, while the treatment deprived of both cells and mucilage didn’t contain any detectable toxins. 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 cellular features.