Programme REID 2012x - Inra

DR JEKYLL AND MR HYDE: HARMFUL AND BLOOM-FORMINGPHYTOPLANKTON IN SYMBIOSIS IN WORLDWIDE OCEANSJ Decelle, I Probert, Y Desdevises, L Bittner, S Colin,R Simo, M Gali & F Not.UMR Adaptation et Diversité en Milieu Marin, Station Biologique, RoscoffDiversity and function of symbiotic relationships between microalgae and a heterotroph(photosymbiosis) remain poorly known, although it is a common strategy in the plankton in the vastnutrient-poor oceans. Here, we found an original symbiosis where different species of the harmfuland bloom-forming Phaeocystis, identified by various genes (the nuclear 18S and 28S rDNA, andplastidial rbcL and psbA), can live in endosymbiosis worldwide within the single-celled Acantharia(Radiolaria). Bayesian molecular clocks suggest that the symbiotic interaction would have emerged inthe Jurassic (about 175 Mya), probably forced by the scarcity of nutrients at this period, and wouldcorrespond to the first diversification of modern Phaeocystis. Cophylogenetic analyses do not reveala strong cospeciation pattern between Phaeocystis and Acantharia, probably due to the absence ofhost-specificity in the interaction. Biogeography seems rather to be the main driver of this flexiblesymbiosis. Finally, we found that Phaeocystis in symbiosis (30 cells in average per Acantharia)produces a great quantity of DMSP, DMS and DMSO, which are likely transferred and accumulated inthe host, suggesting an important physiological role. The dual lifestyle of Phaeocystis, which holds akey role in marine ecology both in symbiosis and as free-living, brings new insights into the biologyand evolution of this ecologically-important microalga, and more generally into photosymbiosis inthe plankton.Mots-clés : photosymbiosis-protists-Acantharia-Phaeocystis-oligotrophic waters27