Interactions between marine microorganisms such as symbiosis, competition, and allelopathy determine the structure and function of microbial communities, yet are relatively unstudied. We must understand these interactions at multiple levels in order to predict how marine microbial communities will evolve in a changing world. Here, I propose to study interactions between Prochlorococcus, the most abundant photosynthetic organism in the oceans, and heterotrophic marine bacteria, utilizing a cross-scale approach combining physiology, chemical ecology, functional genomics and ecological modeling. We will: 1) Determine the general mechanism of interaction and the physiological response of two Prochlorococcus and five heterotrophic strains to liquid batch co-culture; 2) Elucidate genes and pathways responding to and mediating these interactions through comparative transcriptomic experiments; 3) Formulate, constrain and test mathematical models of these co-cultures, utilizing the physiological and genomic observations to explore the complexity needed to mathematically represent microbial interactions. The proposed interdisciplinary study will, for the first time, link multiple mechanisms of interaction between marine microorganisms to the genes and pathways involved. The experimentally-informed and tested mathematical models will provide a critical stepping stone towards explicitly representing microbial interactions in global biogeochemical models. Our results will be of special interest given the dominance of Prochlorococcus as a primary producer in the open ocean, and may shed light on the paradox of how microbial interactions can occur in such a diffuse environment. This interdisciplinary study will promote my integration into the EU research community and facilitate ongoing and future collaborations with EU and American scientists studying microbial communities and their dynamics in the changing oceans.