Picoeukaryotic diversity in the Gulf of Gabès: Variability patterns and relationships to nutrients and water masses

Marine picoeukaryotes show high phylogenetic diversity worldwide, notably in oligotrophic waters. In the Gulf of Gabès (south-eastern Mediterranean), characterized by oligotrophic conditions and a complex water mass circulation, information on picoeukaryotic diversity is still lacking. In this study, we investigated the diversity and spatial variability of picoeukaryotic assemblages in relation to nutrient availability, physical parameters and water masses in 3 cruises carried out in the Gulf of Gabès in June of 2008, April of 2009 and November of 2009. High-throughput sequencing revealed a dominance of sequences from non photosynthetic picoeukaryotes, mostly represented by the presumably parasitic marine alveolate MALV-II (33.20%) and the bacterivorous Bicosoecida (13.56%). Differences in picoeukaryotic assemblages were higher between coastal and open-sea stations, and depth in the water column also affected community differences, with surface (5 m), intermediate (25−100 m) and mesopelagic (>200 m) samples forming separate groups. A clear temporal variability was also evident, particularly for communities collected from the surface layer and open-sea stations. Co-inertia analysis revealed that picoeukaryotic groups were more affected by salinity in deep waters, whereas at the surface, they were dependent on nutrients and temperature. During the November cruise, samples that shared similar water mass properties generally clustered together. The Levantine water mass, observed for the first time in this area, was characterized by the presence of Acantharia and Polycystinea. Our study highlights the role of physical and chemical features, such as water mass origin, the wide continental shelf and trophic status, in determining the diversity of marine picoeukaryotes.