Final Activity Report Summary - ESUMAST (Ecological significance of uncultured marine stramenopiles) Marine microbes have a significant ecological impact at the global scale. Indeed, they account for a significant contribution to the main biogeochemical cycles (e.g. carbon, nitrogen). Study of their diversity and interactions are major issues in the fields of biological oceanography and marine microbial ecology. Their eukaryotic component (i.e. protists) is by far the poorest known. Among these eukaryotic marine microbes, heterotrophic flagellates have been for long time recognised as fundamental players of marine food webs. However, the study of their function in marine ecosystems has been hampered by the lack of knowledge on the specific populations forming these assemblages. Also, as commonly highlighted in environmental molecular diversity studies, most dominant micro-organisms are presently uncultured. The ESUMAST project focused on uncultured flagellates named MAST for Marine Stramenopiles, which are well represented in natural environments. The first objective of this project aimed to assess the trophic relationships which exist between different MAST flagellates and the other components of the microbial food web, such as bacteria or photosynthetic picoplankton. We demonstrated the existence of different feeding behaviours, such as size selection and/or prey preferences, among the MAST groups. The second and main objective of the project was to obtain these organisms in culture so that we can carry out physiological and genomic studies, which stands nowadays as one of the primary goals in marine microbial ecology. By designing original isolation experiments we achieved to culture not only a representative of a MAST group but a variety of other heterotrophic flagellates, all belonging to uncultured taxa. The achievement of cultures for a range of these eukaryotic microbes opens up new perspectives and has major implications for scientific fields related to genomics, microbial ecology, biological oceanography, and macro-evolution of eukaryotes.