Cheminant-Navarro, S. | bioRxiv Science

bioRxiv2024

Circadian regulation of key physiological processes by the RITMO1 clock protein in the marine diatom Phaeodactylum tricornutum

O_LIPhasing biological and physiological processes to periodic light-dark cycles is crucial for the life of most organisms. Marine diatoms, as many phytoplanktonic species, exhibit biological rhythms, yet their molecular timekeepers remain largely uncharacterized. Recently, the bHLH-PAS protein RITMO1 has been proposed to act as a regulator of circadian rhythms. C_LIO_LIIn this study, we first determined the physiological conditions to monitor circadian clock activity and its perturbation in the diatom model species Phaeodactylum tricornutum by using cell fluorescence as a circadian output. Employing ectopic overexpression, targeted gene mutagenesis, and functional complementation, we then investigated the role of RITMO1 in various circadian processes. C_LIO_LIOur findings reveal that RITMO1 significantly influences the P. tricornutum circadian rhythms not only of cellular fluorescence, but also of photosynthesis and of the expression of clock-controlled genes, including transcription factors and putative clock input/output components. RITMO1 effects on rhythmicity are unambiguously detectable under free running conditions. C_LIO_LIBy uncovering the complex regulation of biological rhythms in P. tricornutum, these results provide a key step in understanding the endogenous regulators of phytoplankton physiological responses to environmental changes. Furthermore, these studies position diatoms as instrumental and novel model systems for elucidating key mechanistic principles of oscillator functions in marine ecosystems. C_LI

molecular biology↗

bioRxiv2018

A bHLH-PAS protein regulates light-dependent rhythmic processes in the marine diatom Phaeodactylum tricornutum

Periodic 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 a Phaeodactylum tricornutum bHLH-PAS protein, PtbHLH1a, in the regulation of light-dependent diurnal rhythms. Peak expression of PtbHLH1a mRNA occurs toward the end of the light period and it adjusts to photoperiod changes. Ectopic over-expression of PtbHLH1a 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 by PtbHLH1a 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.\n\nSIGNIFICANCE 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 diatom Phaedoactylum tricornutum. The identification of PtbHLH1a-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.

molecular biology↗