Marine phytoplankton are the basis of marine food webs and are responsible for nearly 50% of the global annual carbon-based primary production. Since phytoplankton exert such a global-scale influence on climate, we are interested in understanding what controls their cell fate during bloom “boom and bust” dynamics. Despite their importance, the molecular basis for their ecological success is still in its infancy. In recent years, the wealth of genomic information from marine microbes, coupled with molecular resources and analytical tools, provide an unprecedented opportunity to address fundamental questions about their unique evolutionary history and ecological role. Nevertheless, there is a critical need to “decode” the genomic resources and translate them into cellular mechanisms, community structure and, eventually, to their role in ecosystem function. This proposed research aims to provide novel insights into the role of a chemical-based “arms race” that mediates and structures the microbial interactions in the marine environment. We will dissect unexplored signaling pathways employed by phytoplankton during sensing and acclimation to changes in their environment. Our overarching objective is to unravel the role of infochemicals and related gene products in regulating phytoplankton surveillance systems in response to environmental stress conditions. We will focus on the three major biotic interactions of key dominant algal groups in the oceans; intercellular signaling in diatoms, host-virus interactions in coccolithophores and predator-prey interactions. We will provide a suite of cellular probes and metabolic biomarkers that will allow in situ detection of chemical signaling and biotic interactions in the oceans and will highlight their role in shaping microbial food webs. Our vision is to provide novel cellular concepts and molecular tools to the link between the intricate mechanisms of cell signaling and stress response, and large biogeochemical cycles