Beyond red/far‐red sensing: phytochrome perception of the marine light field by microalgae

Phytochromes (PHYs) are a major group of photoreceptors, described as red and far-red light sensors in land plants. Recent genomic and metagenomic explorations have revealed the presence of PHYs also in various eukaryotic microalgae originating from distinct endosymbiotic events. Growing evidence indicates that these PHYs are spectrally and functionally tuned to shorter wavelengths, which are prevalent in the aquatic environments as depth increases. Investigations using emerging phytoplankton model species, along with environmental surveys, are uncovering new PHY-mediated responses that likely influence their growth and distribution in marine environments. This Tansley Insight explores the implications of these discoveries for understanding the evolution and functional significance of this major photoreceptor class in the upper ocean, where light drives both energy and information flow.