Aquatic life is strongly structured by the distribution of light, which, besides attenuation in intensity, exhibits a continuous change in the spectrum with depth¹. The extent to which these light changes are perceived by phytoplankton through photoreceptors is still inadequately known. We addressed this issue by integrating functional studies of diatom phytochrome (DPH) photoreceptors in model species² with environmental surveys of their distribution and activity. Here, by developing an in vivo dose–response assay to light spectral variations mediated by DPH, we show that DPH can trigger photoreversible responses across the entire light spectrum, resulting in a change in DPH photoequilibrium with depth. By generating dph mutants in the diatom Thalassiosira pseudonana, we also demonstrate that under simulated low-blue-light conditions of ocean depth, DPH regulates photosynthesis acclimation, thus linking optical depth detection with a functional response. The latitudinal distribution of DPH-containing diatoms from permanently stratified regions to seasonally mixed regions suggests an adaptive value of DPH functions in coping with vertical displacements in the water column. By establishing DPH as a detector of optical depth, this study provides a new view of how information embedded in the underwater light field can be exploited by diatoms to modulate their physiology throughout the photic zone.

水生生物在很大程度上由光的分布构成，光的分布除了强度衰减外，光谱还表现出随深度¹ 的连续变化。浮游植物通过光感受器感知这些光变化的程度仍然不充分。我们通过将模式物种² 中硅藻植物色素 （DPH） 光感受器的功能研究与其分布和活性的环境调查相结合来解决这个问题。在这里，通过开发一种针对 DPH 介导的光谱变化的体内剂量 - 反应测定法，我们表明 DPH 可以触发整个光谱的光可逆反应，导致 DPH 光平衡随深度变化。通过在硅藻 Thalassiosira pseudonana 中产生 dph 突变体，我们还证明在模拟的海洋深度低蓝光条件下，DPH 调节光合作用驯化，从而将光学深度检测与功能响应联系起来。含 DPH 硅藻的纬度分布从永久分层区域到季节性混合区域，表明 DPH 函数在应对水柱中垂直位移方面具有适应性价值。通过将 DPH 建立为光学深度检测器，本研究为硅藻如何利用嵌入水下光场的信息来调节它们在整个光区的生理机能提供了新的观点。