ABSTRACTPeriodic light-dark cycles govern the timing of basic biological processes in organisms inhabiting land as well as the sea, where life evolved. Although prominent marine phytoplanktonic organisms such as diatoms show robust diurnal rhythms in growth, cell cycle and gene expression, the molecular foundations controlling these processes are still obscure. By exploring the regulatory landscape of diatom diurnal rhythms, we unveil the function of aPhaeodactylum tricornutumbHLH-PAS protein,PtbHLH1a, in the regulation of light-dependent diurnal rhythms. Peak expression ofPtbHLH1amRNA occurs toward the end of the light period and it adjusts to photoperiod changes. Ectopic over-expression ofPtbHLH1a results in lines showing a phase shift in diurnal cell fluorescence, compared to the wild-type cells, and with altered cell cycle progression and gene expression. Reduced oscillations in gene expression are also observed in overexpression lines compared to wild-type in continuous darkness, showing that the regulation of rhythmicity byPtbHLH1a is not directly dependent on light inputs and cell division.PtbHLH1a homologs are widespread in diatom genomes which may indicate a common function in many species. This study adds new elements to understand diatom biology and ecology and offers new perspectives to elucidate timekeeping mechanisms in marine organisms belonging to a major, but underinvestigated branch of the tree of life.SIGNIFICANCE STATEMENTMost organisms experience diurnal light-dark changes and show rhythms of basic biological processes such that they occur at optimal times of the day. The ocean harbours a huge diversity of organisms showing light-dependent rhythms, but their molecular foundations are still largely unknown. In this study, we discover a novel protein,PtbHLH1a that regulates cell division, gene expression and the diurnal timing of these events in the marine diatomPhaedoactylum tricornutum. The identification ofPtbHLH1a-like genes in many diatom species suggests a conserved function in diurnal rhythm regulation in the most species-rich group of algae in the ocean. This study unveils critical features of diatom biology and advances the field of marine rhythms and their environmental regulation.
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