Phytochromes facilitate social behaviour in marine diatoms.

The phytochrome superfamily comprises photosensory proteins that enable organisms to perceive changes in light intensity and quality and is widespread across plants, fungi, algae, and microbes. In terrestrial plants, phytochromes sense red and far-red light to regulate key developmental and physiological processes. In marine environments, however, where red and far-red wavelengths penetrate only the upper few meters of water, the function of phytochromes has remained unclear. Recent work shows that diatom phytochromes exhibit photoreversible responses across a broad spectral range, extending beyond red and far-red, suggesting a role in underwater light sensing. Here, we examine the role of phytochromes in light perception and collective behavior in the marine diatom Phaeodactylum tricornutum. Comparing wild-type and phytochrome knockout strains under different light wavelengths reveals that activation of phytochromes by blue or far-red light synchronizes cell movements into a coordinated "wobbling dance." This behavior is absent in phytochrome-deficient mutants, demonstrating the essential role of phytochromes. Our results further suggest that this collective motion involves intercellular communication, potentially mediated by variable red and far-red autofluorescence. Together, these findings uncover a previously unrecognized light-driven social behavior in marine diatoms and highlight the ecological significance of phytochrome-mediated communication in microbial communities.