"The ROMEO proposal aims at linking aspects of two scientific fields, traditionally separated, zooplankton ecology and microbial oceanography. ROMEO will investigate the influence of zooplankton activity on the prokaryotic community composition and activity in the open North Atlantic. During a four-week expedition in North Atlantic, the dominant crustacean zooplankton will be collected and the community composition of the resident and the transient gut prokaryotes determined during their nocturnal feeding in the surface waters and at daytime when they are residing at mesopelagic depths. Focus will be put on the two dominant calanoid copepod species in the North Atlantic, Calanus finmarchicus and C. helgolandicus. Also, the prokaryotic community and activity will be assessed in the fecal pellets and the successional changes in the community composition and activity followed during the decomposition of the fecal material. For the determination of the prokaryotic community composition, next generation sequencing will be applied on a metagenomic and –transcriptomic level. The hetero- and the autotrophic activity of the archaea will be differentiated from that of the bacteria using selective inhibitors, erythromycin for bacteria, diphtheria toxin for archaea. Bulk activity measurements as well as single-cell analyses will be performed and compared with the activity patterns in the ambient waters. Overall, the project will involve a substantial amount of bioinformatics work, which is the main training part of the researcher in ROMEO. The project will lead to an advancement of our understanding of the interaction between the main food source for North Atlantic fish, the calanoid zooplankton, and the main drivers of the biogeochemical cycles, the prokaryotes. Therefore, the expected results of this project will have impact on our understanding on the interactions between two different functional groups of organisms, playing major roles in the functioning of pelagic systems."