Latitudinal diversity in circadian and light-sensing genes in an ecologically vital group of marine picoeukaryote algae.

Organismal life cycles are influenced by Earth's rotation and orbit, generating daily and seasonal light cycles that vary with latitude, especially in temperate and polar zones. Photoperiodism relies on organisms' ability to measure time via the circadian clock and detect light through specific photoreceptors. Molecular basis of photoperiodism is well-characterized in plants, but photoperiod adaptation in phytoplankton remain largely unexplored. Here, we investigated circadian clock components, photoreceptors, and associated effectors in eukaryote picoalga species from Ostreococcus, Bathycoccus, and Micromonas. We showed that the investigated species shared a conserved set of homologous circadian clock-related genes that appeared in the early evolution of Mamielalles order. Furthermore, gene duplication events account for the specific occurrences and uneven gene copy numbers among these genera. Through metagenomic and metatranscriptomic analyses, we assessed the gene expression profiles of candidate photoperiod-related genes across the global ocean. Our findings reveal an unexpected diversity in photoreceptors, particularly within Micromonas, and highlight the CCT domain family, a key group of transcription factors governing circadian rhythms (TOC1 family) and photoperiodism (CONSTANS family) in plants. TOC1, a central component of the circadian clock in Ostreococcus tauri, is either absent or truncated in tropical species. Functional assays further indicate that the TOC1/CCA1 oscillator is nonfunctional in the tropical strain of Ostreococcus sp. RCC809. These results imply that certain circadian mechanisms may be dispensable at low latitudes, underscoring the diversity of photoperiod adaptations in marine phytoplankton. These results provide valuable insights into the molecular evolution of cosmopolitan plankton groups, particularly their mechanisms of local adaptation.