"During the history of the planet Earth, acquisition of symbiont-encoded metabolic pathways has allowed hosts to exploit new ecological niches. A mechanistic understanding of the interactions between host and symbionts as well as knowledge about the role of symbionts in the functional diversification and species formation in hosts is, however, currently lacking. It is well known that aquatic ciliates may harbor phototrophic algae or methanogenic Archaea as symbionts. Moreover, I recently found nitrogen-fixing (diazotrophic) symbionts in ciliates (Protozoa) and recent studies report diazotrophic symbionts in marine dinoflagellates and zooplankton. Accordingly, I will in this project examine symbioses in model systems consisting of diazotrophic prokaryotes and aquatic ciliates to address the importance of symbionts for the diversification of hosts. The study implicates usage of cutting-edge microbiological, molecular and marine ecological methodologies. Various symbioses in marine and freshwater ciliates will be examined when exposed to different levels of nitrogen availability. The project will provide new insights into the formation, maintenance and specificity of symbioses between aquatic ciliates and their prokaryotic symbionts in relation to environmental conditions. These results have implications for our understanding of the importance of symbiosis in the formation of new species. Moreover, the study will provide first estimates of the nitrogen fixation of diazotrophic prokaryotes associated with ciliates. This is of relevance to the global cycling and availability of nitrogen in aquatic environments and thereby intimately linked to marine carbon budgets."