Spatio-temporal distribution, physiological characterization and toxicity of the marine dinoflagellate Ostreopsis (Schmidt) from a temperate area, the Ebre Delta. Phylogenetic variability in comparison with a tropical area, Reunion Island

Abstract

[eng] The toxic epi-benthic dinoflagellate Ostreopsis includes several species producers of palytoxin-like compounds, one of the most potent marine toxins known so far. The aims of this thesis are, a) to contribute to the improvement of the genus taxonomy by molecular, morphological and toxicological characterization in Reunion Island and, b) to investigate, combining field and laboratory analysis, the role of environmental factors and their interaction, involved in the ecology of O. cf. ovata blooms in the NW Mediterranean Sea. Three genotypes of the genus Ostreopsis were described; 1) Isolates with a morphotype fitting O. marinus original description were clustered close to Ostreopsis sp 5 described by Sato et al., (2011) genetic clade. 2) Ostreopsis cf. ovata was present in the benthic microalgae population, corresponding to the Indo-Pacific clade. 3) An unidentified species, could correspond to O. carribeanus, O. belizeanus or a new species. The strains did not show palytoxin-like compounds production. Multifactorial laboratory experiment with O. cf. ovata from the Mediterranean Sea showed that growth was inhibited at 19oC for all salinities. The highest growth rates were registered at 24oC for both strains (0.48±0.05 div. day-1), and a significant growth rate variability was found among salinities at 24 and 28oC. Two groups were distinguished by cell size in all high temperature conditions and a positive correlation between the amount of small cells and growth rate was found. Palytoxin-like concentration in the cultures increased with time and significant higher amount of toxin were found at 28oC in comparison with 24oC. From the field study of the epi-benthic microalgae community, in the adjacent shallow rocky coast area of the Ebre Delta, NW Mediterranean Sea, O. cf. ovata was identified as the only species present from the genus. Temperature, salinity and wind speed had a strong impact on defining the ecological niche of benthic microalgae. The temporal pattern of O. cf. ovata dynamics from mid-July to early-November is probably due to the fact that this species is observed only above a certain temperature threshold. Epiphytic O. cf. ovata cells were smaller in the northern site, coinciding with higher abundance. Toxicity in planktonic cells was negatively correlated with cell abundance in water column; achieving maximum concentrations of 25 pg PLTX eq·cell-1.