A NEW MARINE PICOEUCARYOTE - OSTREOCOCCUS TAURI GEN ET SP-NOV (CHLOROPHYTA, PRASINOPHYCEAE)

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 field studies, the cells were discovered by their flow 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 Chl c-like pigment, Mg 3,8 DVP (=Mg 2,4 DVP) argues for placing it in the Prasinophyceae. It differs from 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.