Light is one of the most informative environmental signals. Its perception by photoreceptor proteins enable organisms to adjust their physiology to the surroundings. Light sensing in marine environments remains largely unexplored. Marine microalgae, such as diatoms, which are a prominent group of phytoplankton, possess numerous photoreceptors sensitive to blue light, in line with the deepest penetration of this band in the water column, while violet and red ones are rapidly absorbed. They also have phytochrome photoreceptors, known in land plants, bacteria and fungi to primarily absorb red and far-red light and photoconvert between an active and an inactive form based on the ratio of these two bands. In some prasinophyte and glaucophyte microalgae, phytochromes exhibit a blue-shifted absorption spectrum, interpretated as an adaptation to the predominant wavelengths in the ocean.
By combining different approaches, we investigated the light sensing properties and the possible roles phytochromes play in diatoms. We found that diatom phytochromes (DPH) of different taxonomic and geographical origins, exhibit a conserved red/far-red absorption spectra. In the molecular model species Phaeodactylum tricornutum, we established a DPH-response reporter system and found that DPH triggers photoreversible responses not only depending on the red/far-red wavebands but across the entire visible light spectrum, with a strong influence of blue, green, and red bands. Quantification and modeling of these responses in oceanic light fields predicted a DPH-dependent regulation with depth. Functional investigations of diatom photophysiology in cells acclimated to « depth »- or « surface »-like conditions revealed that DPH can regulate photosynthesis acclimation at depth. Combined with analysis of the biogeography distribution pattern of DPH-containing diatoms, mostly located in temperate and polar regions, we hypothesized that DPH could have an adaptive function in coping with light variations associated to vertical displacements in waters characterized by strong seasonal variations of the mixed layer depth.