Final Activity Report Summary - CRENARC (Functional diversity of Crenarchaeota in aquatic ecosystems) Microbes are abundant and ubiquitous component of aquatic ecosystems, and through their role in biogeochemical cycling, they can influence the earth's oceanic and atmospheric composition. Little is known, however, about the factors that influence the activity and dynamics of microbial communities. The main goal of our research project was to determine the diversity and function of Crenarchaeota in aquatic ecosystems. Crenarchaeota is a phylum of Archaea (the third domain of life) widely represented in the world's oceans but which role in the ecosystem remains poorly understood. Recent findings indicate that Crenarchaeota are involved in ammonium oxidation and CO2 fixation, suggesting a major role for the group in the biogeochemical cycles of nitrogen and carbon. The project chose to work simultaneously on different approaches to better cover the different aspects of the functional diversity of Crenarchaeota in aquatic ecosystems. The main approaches involved the study of the temporal dynamic of crenarchaeotal communities through long term monitoring of microbial abundance, and the detailed description of crenarchaeotal assemblages via high throughput sequencing. The work carried out showed that in the coastal Mediterranean Sea Crenarchaeota had a marked seasonal dynamics that reoccurred year after year. Their abundance was highest during winter and significantly correlated with nitrite and archaeal ammonia monooxygenase (amoA) gene involved in ammonia oxidation. The close association between Crenarchaeota and a major ecosystem processes indicates that Crenarchaeota may play an important ecological role in the coastal Mediterranean Sea. Further, by using the new pyrocequencing technology the project could report a very detailed description of Archaea communities in the deep Arctic Ocean. It showed that Crenarchaeota were always an important component of the deep Arctic but that the communities differed from one water mass to the other. It concluded that water masses probably act as physical barriers limiting the dispersal and controlling the diversity of Archaea in the ocean. This suggests that it is essential to consider the coupling between microbial and physical oceanography to fully understand the diversity and function of marine microbes.