SIR -Autotrophic communities of picoplankton (cell size less than 2 !lm diameter), known to be dominated by prokaryotes 1 , are essential in the carbon cycle of estuaries 2 and oceans \,3. Hall and Vincent report that in other nutrient-rich ecosystems, the eukaryotic forms of the picoplankton "can playa major role in generating new production,,4. That seems to be the case for the marine Mediterranean Thau lagoon (France, 43°24' N-3°36' E) where, using flow cytometry, we have discovered a photosynthetic picoeukaryote which is the main component of the phytoplankton. This picoplankter is

A new marine picoeucaryote: Ostreococcus tauri gen. et sp. nov. (Chlorophyta, Prasinophyceae) M.-J. CHRETIENNOT-DINET\ C. COURTIES2, A. VAQVER2, J. NEVEux\ H. CLAUSTRE3, J. LAUTIER2 AND M.e. MACHAD01* lObservatoire Oceanologique de Banyuls, INSUjCNRS-Universite Paris 6, URA 117, Laboratoire Arago, 66650 Banyuls-sur-mer, . 1995. A new marine picoeucaryote Ostreococcus tauri gen. et sp. nov. (Chlorophyta, Prasinophyceae). Phycologia 34: 285-292. A new picoplanktonic alga, Ostreococcus tauri Courties et Chretiennot-Dinet, gen. et sp. nov. (Chlorophyta, Prasinophyceae) is described from the Thau Lagoon on the Mediterranean coast, France. Almost undetectable by light or fluorescence microscopy in fi eld studies, the cells were discovered by their fl ow cytometric signature and appeared numerically as the main component of the phytoplankton. Their ultrastructure is described, with additional information on cell size, pigment and DNA content. Each cell contains a nucleus, a chloroplast, one mitochondrion, one Golgi body and a very reduced cytoplasmic compartment. A starch granule and Chi a and b demonstrate its affinity to the Chlorophyta and the presence of a Chi c-like pigment, Mg 3,8 DVP (=Mg 2,4 DVP) argues for placing it in the Prasinophyceae. It differs fr om other coccoid taxa in ultrastructural details, mode of division and detailed pigment composition. The size and DNA content make Ostreococcus tauri the smallest eucaryote known. CHRETIENNOT-DINET M.J. 1990. Atlas du Phytoplancton marin.