The Harmful Benthic Dinoflagellate Ostreopsis cf. Ovata: Biotic Factors Affecting its Growth and Toxicity

Over the last two decades, Ostreopsis cf. ovata blooms have been reported with increased frequency, intensity and areal distribution in the Mediterranean Sea. The epiphytic/benthic dinoflagellate proliferates under low hydrodinamism, and produces toxins (palytoxin-like compounds) causing human health problems and massive mortalities of benthic invertebrate communities. While several studies have assessed some effects of abiotic factors like hydrodynamics, water temperature and inorganic nutrients on O. cf. ovata bloom dynamics, biotic factors have been barely addressed. In an effort to provide insights about competition with other microalgal species and microalgal-bacterial interactions in affecting O. cf. ovata growth, physiology and ultimately bloom dynamics, this thesis investigated: (i) the inorganic nutrients uptake and organic phosphorus utilization by O. cf. ovata; (ii) the phylogenetic structure of bacterial assemblages co-occurring with O. cf. ovata bloom in situ; (iii) the microbial (bacteria and viruses) dynamics and toxin production along the O. cf. ovata growth in batch cultures. Data collected show the high efficiency of O. cf. ovata in both inorganic and organic phosphate acquisition, an aspect that could confer advantages towards competing species co-occurring in the microphytobenthos. Recurrent functional associations between O. cf. ovata and Alphaproteobacteria have been evidenced both in situ and in batch cultures, and indicate the latter as a reliable tool for future investigations on O. cf. ovata-bacteria interactions. The ability of some O. cf. ovata-associated Roseobacters (i.e. Ruegeria, Jannaschia, Dinoroseobacter, Roseovarius) to switch from mutualistic to antagonistic relationships depending on algal physiological status suggests an important role of dinoflagellate-bacterial interactions in bloom dynamics. Furthermore, while the vast majority of studies on harmful algal blooms to date have focused on macronutrients’ dynamics, data in this thesis pave the way for further investigations on the role of allochthonous vitamins in O. cf. ovata bloom development and maintenance.