Séminaire du Dr Michel Denis (Directeur de Recherche Emerite)
Aix-Marseille Université, Université de Toulon, CNRS/INSU, IRD, MIO, 163 avenue de Luminy, Case 901, 13288 Marseille cedex 09, France. firstname.lastname@example.org
Titre : Evidence for top-down control of ultraphytoplankton in the Gulf of Gabès (Eastern Mediterranean Basin, Tunisia)
The Gulf of Gabès, one of the largest continental shelves in the oligotrophic to ultra-oligotrophic Eastern Mediterranean Basin, is a very highly productive area under strong anthropogenic pressure. The ultraphytoplankton (< 10 µm) summer (June 2008) distribution was determined in the Gulf by flow cytometry that resolved 5 cell groups, nanoeukaryotes (photosynthetic eukaryotic cells belonging to the size class 3-20 µm), picoeukaryotes (<3 µm), Prochlorococcus, Synechococcus and one cluster of unknown cells that was the most abundant (0.1-2.9 106 cells.cm-3) and apparently larger than 2 µm. These cells were characterised by low orange and red fluorescence whereas from their apparent size, they should emit a red fluorescence signal at least as large as that of nanoeukaryotes and were thus labelled LFNano. In contrast, the abundance of the usual photosynthetic cell groups was found abnormally low considering the absence of nutrient limitation, which would suggest a top-down control of ultraphytoplankton by predation activity. Flow cytometry analysis of samples collected in the same area in July 2012 yielded similar results. Cell sorting followed by scanning electron microscopy revealed the presence of a choanoflagellate species, however the unknown cell group could not be identified. The ultraphytoplankton abundance was similar to that observed in winter in the eastern Mediterranean Basin, well below that reported in the western Mediterranean Basin in spring. Because nutrients in the Gulf of Gabès are never fully exhausted, the abnormally low abundance level was explained by predation activity applying a top down control on the ultraphytoplankton. The 2012 samples enabled sorting cells of the LFNano group and their observation by scanning electron microscopy. Though a choanoflagellate species could be detected, the dominant cells in the LFNano group could not be identified. DNA sequencing is considered to achieve their identification. Despite this unresolved question, results support the existence of a top-down control of photosynthetic pico- and nanoplankton, insuring a rapid transfer of matter and energy to higher trophic levels, consistently with the well-known high productivity of the Gulf of Gabès. Nanoflagellates are likely candidates to run this top down control.