While biological rhythms are crucial to life, the deep sea has long been considered an arrhythmic exception. However, at hydrothermal vents - devoid of diel cues yet shaped by tides - the mussel Bathymodiolus azoricus shows both tidal and, unexpectedly, circadian rhythms at -1700 m. Whether endogenous clock(s) drive these cycles remained unanswered. Here, we report endogenous circadian rhythms in B. azoricus cell cultures under constant conditions: isolated cells display a circadian oscillation despite tidal-dominant rhythms in the natural environment. Reporter assays using genomic regions upstream of the mussel's per gene and containing E-box motifs suggest that a functional transcription-translation feedback loop (TTFL) contributes to circadian timing even in the deep sea. In mussels, BazPeriod lacks autonomous repressive activity but modulates BazCry2. As BazPeriod itself oscillates tidally, it may explain how a single endogenous clock yields both tidal and diel rhythms. The work also spotlights the highly time-sensitive biology of the vastly unexplored deep-sea environment.
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