Chemical, molecular, and eco-toxicological investigation of Ostreopsis sp. from Cyprus Island: structural insights into four new ovatoxins by LC-HRMS/MS

Blooms of benthic dinoflagellates of the genus Ostreopsis (mainly O. cf. ovata and occasionally O. cf. siamensis) represent a serious concern for humans in the Mediterranean area, due to production of palytoxin-like compounds listed among the most potent marine toxins known. In this work six strains of Ostreopsis spp. from Cyprus Island were analyzed through an integrated approach based on molecular, chemical and eco-toxicological methods. Cypriot Ostreopsis spp. was found to be a species distinct from O. cf. ovata and O. cf. siamensis, belonging to the Atlantic/Mediterranean Ostreopsis spp. clade. Some variability in toxin profiles emerged: three strains produced ovatoxin-a (OVTX-a), OVTX-d, OVTX-e and isobaric palytoxin, so far found only in O. cf. ovata, the other three strains produced only new palytoxin-like compounds, that we named ovatoxin-i, -j1, -j2, and -k. The new ovatoxins present the same carbon skeleton as ovatoxin-a differing primarly for an additional C2H2O2 moiety and an unsaturation in the region C49-C52. Other minor structural differences were found, including the presence of a hydroxyl group at C44 (in OVTX-j1 and -k) and the lack of a hydroxyl group in the region C53-C78 (in OVTX-i and -j1). Toxin content of the analyzed Ostreopsis spp. strains was in the range 0.06-2.8 pg/cell, definitely lower than that of a Ligurian O. cf. ovata strain cultured under the same conditions. Accordingly, eco-toxicological test on A. salina nauplii demonstrated that Ostreopsis spp. presents a very low toxicity compared to O. cf. ovata. The whole of these data suggest that Ostreopsis spp. from Cyprus Island pose a relatively low risk to humans