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Abstract

Light gradients along the watercolumn shape life in the oceans. Indeed, the light field changes in both intensity and quality with depth, with strong attenuation of long wavelengths such as red (R) and far- red (FR). Surprisingly, predominant marine algae such as diatoms have phytochromes photoreceptors, which are red/far-red receptors in land plants. Diatom phytochromes (DPH) exhibit R/FR absorption spectra and induce expression of a set of genes upon FR light exposure. How a R/FR receptor is activated in the red-poor marine environment remains a puzzle. Here we propose an answer with a new model for phytochrome activation in the ocean. By measuring the responses triggered by DPH photoreceptor in vivo thanks to a reporter system, we showed that its action is not restricted to the long R/FR wavebands, but extends to the entire visible light spectra. Considering these new properties, FR light is ineffective in triggering phytochrome responses in an oceanic context, where phytochrome instead detects the balance between other wavebands (particularly red, green and blue), that reflect depth variations and the presence of other photosynthetic organisms. This paradigm shift in phytochrome sensing abilities opens up new insights into the role of phytochromemediated light sensing in the oceans.