Brichta, M. & E.-M. NöthigTitle : Proboscia inermis: A key diatom species in Antarctic autumnThe large diatoms of the genus Proboscia frequently dominate summer-autumn communities in warm-temperate to sub-polar regions. During an antarctic autumn bloom and its decay phase, in the Bellingshausen Sea, different morphotypes of Proboscia inermis accounted for 21% of phytoplankton carbon, being the most important species at that time. Their empty valves increased with depth (empty:full ratio from 1:1 at surface to 23:1 at depth) and exceeded those of other empty diatom frustules in numbers (26%-96% of all empty frustules found) being the major component of the biogenic silica pool. Auxospores, spring, winter and intermediate forms were encountered. Transitional stages from spring forms to winter forms were as well found; many frustules showed newly semiendogenously formed winter valves within the spring valves. When the formation of a winter valve is completed the remnant empty valves contribute significantly to the biogenic silica in the water column, eventually being important for the opal flux. Possible life strategies and adaptation to ice environments will also be discussed.