Communication between conspecific individuals is an essential part of life both in terrestrial and marine realms. Until recently, social behavior in marine phytoplankton was assumed to rely mainly on the secretion of a variety of infochemicals that allowed population-scale collective responses. Here, we demonstrate that pelagic diatoms also use Sun-stimulated fluorescence signals for synchronizing their behavior. These unicellular microorganisms, playing a key biogeochemical role in the ocean, use photoreceptor proteins and red–far-red fluorescent radiation to communicate. A characteristic beaconing signal is generated by rhythmic organelle displacement within the cell cytoplasm, triggering coordinated population behavior. These light-based communication networks could critically determine major facets of diatom ecology and fitness and regulate the dynamics of larger-scale ocean processes.