Abstract The phytochrome superfamily, a group of proteins that enable some organisms to detect changes in light intensity and quality, is widespread in terrestrial and marine microbes, fungi, algae, and plants. In terrestrial plants, these photosensory receptors monitor variations in the light environment by sensing red ( R ) and far-red ( FR ) regions of the spectrum and trigger important developmental, metabolic, and physiological responses. However, the role of these photosensors in marine microbes, living in environments where, due to absorption of water molecules R and FR radiation does not penetrate beyond the upper few meters, remains controversial. Here, we investigate the role of phytochromes in light perception of the marine diatom Phaeodactylum tricornutum and their involvement in light-driven collective behavior. We perform experiments comparing the social conduct of wild-type and phytochrome knock-out strains to different light wavelengths. Our results show that cell movements become synchronized in a coordinated wobbling dance upon activation of their phytochromes by blue or far-red light, therefore, demonstrating the key role of phytochrome in light-mediated diatom collective behaviour. Furthermore, our experiments suggest that the observed phytochrome-mediated concerted dance implies a form of intercell communication, proposedly mediated by variable R/FR autofluorescence emission in the frequency range of diatom wobbling movements. Our findings provide new insights into communication pathways in aquatic microorganisms and emphasize the importance of social conduct in the sea at all ecological levels.