The characterisation and summer distribution of ultraphytoplankton in the Gulf of Gabes (Eastern Mediterranean Sea, Tunisia) by using flow cytometry
Résumé
TheGulfofGabèsintheEasternMediterraneanBasinisoneofthelargestcontinentalshelvesandisahighly
productivearea.ThisisincontrasttotheEasternMediterraneanBasinasawholewhichisoligotrophicto
ultraoligotrophic.Overrecentyears,duetoincreasedpressurefromanthropologicalactivities,therehas
beenadeclineinthebiologicalresourcesintheGulfofGabès.Thishasledscientiststofocustheirresearch
on thedifferenttrophiclevelsofthefoodchainandinparticularonphytoplankton.Studiesdedicatedto
phytoplanktonintheGulfofGabèshavepreviouslyreliedonconventionalglobalapproaches.Thisworkon
theotherhand,examinesultraphytoplankton(o10 mm) distributionatthesinglecelllevelindifferent
watermasses,duringasummercruise(2008).Threedistinctwatermasseswereidentified:modified
Mediterraneanwaters(MMW),modified Atlanticwaters(MAW)andIonianwaters(IW).Despitethe
summerperiod,nutrientswereabundantandnon-limiting.Flowcytometryresolved five cellgroups,
Prochlorococcus, Synechococcus, picoeukaryotes,nanoeukaryotesandagroupofunidentified cells.These
cellswerecharacterisedbylow fluorescencesignalsandscattersignalslargerthanthatofcyanobacteriaand
picoeukaryotesandthuswerelabelledLFNanoforlow fluorescencenanoeukaryotes.Theywerefoundin
veryhighabundances,whereastheotherusualgroupswererelativelylowinabundancedespitethe
favourablegrowthconditions.LFNanofoundduringa2012summercruiseinthesamearea,werealso
sorted by flowcytometryandobservedbyscanningelectronmicroscopy.Choanoflagellateswereidentified,
andthesenanoflagellatesappeartoaccountforthelowabundanceintheusualultraphytoplanktongroup
due totheirefficientfeeding/grazingmechanism.However,themostabundantLFNanocouldnotbe
identified.Resultsstronglysuggestatop-downcontroloftheecosystem,consistentwiththehigh
productivityoftheGulfthatimpliesarapidtransferofmatterandenergytohighertrophiclevels.