Microorganisms, including both single-celled prokaryotes and eukaryotes, play a key role in mediating the biogeochemical cycles in marine environments; and for this reason quantifying and understanding their abundance and diversity has been the subject of an enormous research effort in the last few decades. Coevolution of prokaryotes and eukaryotes has promoted the immense diversity of parasitic, symbiotic and commensal relationships present in the environment and also stimulated the development of an inter-kingdom signalling communication system. Some of the most interesting symbiotic interactions are the ones established between diazotrophs (organism capable of biological N2 fixation, reduction of di-nitrogen N2 to ammonia, NH3) and microbial eukaryotes. These diazotrophic partnerships are widespread in the open ocean and co-occur with free-living and unicellular N2 fixing cyanobacteria; combined they primarily support the new and primary production of the open ocean gyres. In the less studied Baltic Sea, between 20 and 40% of the nitrogen loading depend on the N2 fixation of diazotrophic bacteria. Considering the latter premises, this project aims at exploring the interaction in the microbial world at inter-kingdom level. The steep gradient in salinity and nutrient concentrations together with the eutrophication of anthropogenic origin make the Baltic Sea a dynamically ideal habitat for our proposed project.