Pole-to-pole LOV-domain receptor diversity points to an early stramenopile origin and global dominance of aureochromes

Light regimes are a fundamental environmental cue for marine organisms. Yet molecular adaptations of photoreceptors to the extreme seasonality of polar regions remain poorly understood. We demonstrate that the diversity of candidate blue-light-sensing LOV-domain proteins increases at higher latitudes. We identify aureochromes, blue-light-sensitive transcription factors unique to stramenopiles, as the most frequent LOV-domain receptors across ocean latitudes. Phylogenetic reconstructions reveal early aureochrome diversification in the stramenopile lineage, originating in a heterotrophic ancestor before photosynthesis was acquired. This challenges the assumption that aureochromes are restricted to phototrophs, suggesting an ancestral role in spectral light signaling. We find that, despite changes in light quality and photoperiod by latitude, diatom aureochrome domain architecture remains conserved. This implies adaptation through regulatory tuning, such as altered expression dynamics or post-translational control. Our results emphasise the importance of spectral-light sensing in polar oceans and link the widespread dominance of aureochromes to their ancient origin and conservation.