Background/Question/Methods Nitrogen (N) is often considered as a macronutrient that limits phytoplankton productivity in the ocean. Stratified waters in (sub)tropical regions typically are deplete of inorganic N and carry phytoplankton communities that are dominated by the unicellular cyanobacteria Synechococcus and Prochlorococcus. These cyanobacteria display plasticity in their N-assimilation potential allowing them to satisfy their requirement with different N complements in different ocean niches. They remain N-replete in environments with nanomolar concentrations of ammonium, nitrite and nitrate. We addressed the question of how N-distributions shape cyanobacterial communities and their N-physiologies. Cyanobacterial N-stress responses are controlled by a single transcriptional activator, encoded by ntcA. Results/Conclusions Studies of ntcA provide windows on characterization of cyanobacterial community structure (phylotype) as well as on composition of the N-acquisition regulon (ecotypes or “nitrotypes”). Based on reverse ecology approaches and molecular field studies we will define nitrotypes and discuss global distributions of marine cyanobacteria in relation to N availability. We further inquired how microbial community structure correlates with N-distributions in marine environments. Spatial and temporal changes in distribution of archaeal/bacterial taxa were analyzed vis-ŕ-vis those of inorganic N species, while distributions of protist grazers were correlated to those of organic N (urea, cyanate) in the Northern Red Sea, a marine ecosystem with dramatic seasonal change in N-status. Aspects of the terms of coexistence and interspecies competition in marine microbial communities will be discussed.