Diatom phytochromes integrate the entire visible light spectra for photosensing in marine environments

Abstract Aquatic life is strongly structured by the distribution of light which, besides attenuation in intensity, exhibits a continuous change in the spectrum with depth. The extent to which light changes are perceived by phytoplankton is largely unknown. By focusing on marine diatoms, we here provide the first in vivo quantitative assessment of responses to spectral variations mediated by a marine phytochrome photoreceptor. Our findings reveal that diatom Phytochromes (DPH) display widely conserved R/FR absorption spectra but trigger photoreversible responses across the entire light spectrum, which results in activation of phytochrome responses by the blue/green light found at depth (400-520 nm) and inactivation by longer wavelengths (520-700 nm) at surface. This response to the spectral changes in aquatic systems makes DPH an ideal detector of optical depth, which responds in the opposite way to expectations based on the sole response to red and far-red light. Taken together our results provide a completely novel view on how information embedded in the underwater light field could be successfully exploited throughout the photic zone.