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

International audienceThe Mediterranean Thau Lagoon is an important European oyster farming area. Oyster growth levels are among the highest in France, although chlorophyll a concentration is low. Previous studies have demonstrated that picophytoplankton, nano-microphytoplankton, dinoflagellates and loricate ciliates such as tintinnids are abundant in the Thau Lagoon. Moreover, heterotrophic flagellates and aloricate ciliates have not been investigated. The aim of this study was to assess picophytoplankton, protist and zooplankton abundances in water columns of the Thau Lagoon and to understand the particular structure of the Thau microbial food web, which may explain such a paradoxical oyster growth. In oligotrophic waters in the Thau Lagoon, the picoeukaryote Ostreococcus tauri is the dominant autotrophic picoplankter with a maximum Summer abundance. On 17 August 1998, the picophytoplankton and nanophytoplankton abundances were not as high as expected and we observed the development of large diatoms. At this time, available carbon resources arose from microphytoplankton (84.5 %) and picoplanktonic cells represented only 1.27 % in terms of carbon. The heterotrophic cells were few in abundance and constitued only < 14 % of carbon resources. In order to evaluate the importance of the "protozoan trophic link" for energy transfer from "microbial food web" to large benthic suspension feeders, the oyster Crassostrea gigas was offered a planktonic community as potential prey. In the grazing experiment, all > 5 μm flagellates, microphytoplankton, dinoflagellates, ciliates and large zooplankton were retained by the oyster gills. Only < 5 μm flagellates and picoeukaryotic cells, Ostreococcus tauri, were not very well retained (45 % and 2 %). The high clearance rates of Crassostrea gigas found in this experiment can be explained by a low suspended particulate matter (0.65 mg l-1). Oysters adapted their retention mechanism when they lived in oligotrophic waters. These results indicate that, under the given experimental conditions, picophytoplankton did not represent a valuable trophic resource for farmed oysters because (1) Crassostrea can not retain picoparticles and (2) the picoplankton represented a poor available carbon resource to be transferred via a weak heterotrophic protist community. In the oyster pends of the Thau Lagoon during this study, which followed a rainfall event, microphytoplanktonic primary producers, in particular diatoms, could be considered as the main food sources for bivalve suspension feeders

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.