The harmful dinoflagellate Ostreopsis cf. ovata has been causing toxic events along the Mediterranean coasts and other temperate and tropical areas, with increasing frequency during the last decade. Despite many studies, important biological features of this species are still poorly known. An integrated study, using different microscopy and molecular techniques, Raman microspectroscopy and high resolution liquid chromatography-mass spectrometry (HR LC-MS), was undertaken to elucidate cytological aspects, and identify main metabolites including toxins. The species was genetically identified as O. cf. ovata, Atlantic-Mediterranean clade. The ultrastructural results show unique features of the mucilage network abundantly produced by this species to colonize benthic substrates, with a new role of trichocysts, never described before. The amorphous polysaccharidic component of mucilage appears to derive from pusule fibrous material and mucocysts. In all stages of growth, the cells show an abundant production of lipids. Different developmental stages of chloroplasts are found in the peripheral cytoplasm and in the centre of cell. In vivo Raman microspectroscopy confirms the presence of the carotenoid peridinin in O. cf. ovata, and detects in several specimen the abundant presence of unsaturated lipids structurally related to docosahexaenoic acid. The HR LC-MS analysis reveals that ovatoxin-a is the predominant toxin, together with decreasing amounts of ovatoxin-b, -d/e, -c and putative palytoxin. Toxins concentration on a per cell basis increases from exponential to senescent phase. The results suggest that benthic blooms of this species are probably related to features such as the ability to create a unique mucilaginous sheath covering the sea bottom, associated with the production of potent toxins as palytoxin-like compounds. In this way, O. cf. ovata may be able to rapidly colonize benthic substrates outcompeting other species.

Ostreopsis cf. ovata, a benthic dinoflagellate often blooming along the Mediterranean coasts, has been associated with toxic events ranging from dyspnea to mild dermatitis. In late September 2009, an Ostreopsis cf. ovata bloom occurred in the Gulf of Trieste (Northern Adriatic Sea; Italy), causing pruritus and mild dermatitis in beachgoers. An integrated study was initiated to characterize Ostreopsis cells by light and confocal microscopy, PCR techniques, immunocytochemistry, and high resolution liquid chromatography-mass spectrometry (HR LC-MS). The presence of Ostreopsis cf. ovata of the Atlantic/Mediterranean clade was unambiguously established by morphological and genetic analyses in field samples. Several palytoxin-like compounds (ovatoxin-a,-b,-c,-d,-e) were identified by HR LC-MS, ovatoxin-a being the most abundant (45-64 pg/cell). Surprisingly, no palytoxin was detected. For the first time, monoclonal and polyclonal antipalytoxin antibodies revealed the intracellular cytoplasmic localization of ovatoxins, suggesting their cross-reactivity with these antibodies. Since harmful dinoflagellates do not always produce toxins, the immunocytochemical localization of ovatoxins, although qualitative, can provide an early warning for toxic Ostreopsis cells before their massive diffusion and/or concentration in seafood.

To better understand the dynamics of diarrhetic shellfish poisoning (DSP) contamination a field study was carried out on the feeding behavior of Mytilus galloprovincialis (Lmk) during an important DSP outbreak. The study was focused on the relationships between phytoplankton in seawater and algal cells, or their remnants, in mussel stomachs. During the period studied, M. galloprovincialis seemed to feed selectively on dinoflagellates rather than diatoms. Further selection was observed among different dinoflagellate genera, a preference for the genus Dinophysis being particularly evident. In addition, mussels seemed to open the thecae of Dinophysis cells and digest them more easily than other dinoflagellates. Due to the high variability of the results of phytoplankton analysis in the mussel stomachs, no correlation was found between the abundance of Dinophysis species in the mussels' stomachs and the content of okadaic acid plus dinophysistoxin-1 in their digestive glands, evaluated with an ELISA assay. Conversely, the presence of Dinophysis fortii (the main DSP-causative agent in the area studied) in integrated net samples of the whole water column and the toxin content of the digestive glands presented similar temporal trends

In September 2015, a massive occurrence of the Ostreopsis species was recorded in central Adriatic Kaštela Bay. In order to taxonomically identify the Ostreopsis species responsible for this event and determine their toxin profile, cells collected in seawater and from benthic macroalgae were analyzed. Conservative taxonomic methods (light microscopy and SEM) and molecular methods (PCR-based assay) allowed the identification of the species Ostreopsis cf. ovata associated with Coolia monotis. The abundance of O. cf. ovata reached 2.9 × 104 cells L−1 in seawater, while on macroalgae, it was estimated to be up to 2.67 × 106 cells g−1 of macroalgae fresh weight and 14.4 × 106 cells g−1 of macroalgae dry weight. An indirect sandwich immunoenzymatic assay (ELISA) and liquid chromatography–high-resolution mass spectrometry (LC-HRMS) were used to determine the toxin profile. The ELISA assay revealed the presence of 5.6 pg palytoxin (PLTX) equivalents per O. cf. ovata cell. LC-HRMS was used for further characterization of the toxin profile, which showed that there were 6.3 pg of the sum of ovatoxins (OVTXs) and isobaric PLTX per O. cf. ovata cell, with a prevalence of OVTXs (6.2 pg cell−1), while the isobaric PLTX concentration was very low (0.1 pg cell−1). Among OVTXs, the highest concentration was recorded for OVTX-a (3.6 pg cell−1), followed by OVTX-b (1.3 pg cell−1), OVTX-d (1.1 pg cell−1), and OVTX-c (0.2 pg cell−1).

The theca and the flagella of the marine dinoflagellate Prorocentrum minimum were studied by means of scanning and transmission electron microscopy. The relative positions of emergence of the flagella, the spatial conformation of the "transverse" flagellum and its characteristic arrangement in an anti-clockwise circle suggest new considerations on the construction of the theca of this species. A new interpretation of the theca is proposed and discussed in relation to the previous ones, considering evolutionary implications.