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 bases controlling these processes are still obscure. By exploring the regulatory landscape of diatom diurnal rhythms, we here unveil the key function of a Phaeodactylum tricornutum bHLH-PAS protein, named PtbHLH1a, in the regulation of light-dependent rhythms. Peak expression of PtbHLH1a mRNA occurs at the end of the light period and it is adjusted to photoperiod changes. Ectopic over-expression of PtbHLH1a results in lines with altered cell division and gene expression and showing a phase shift in diurnal responses, compared to the wild-type cells. Reduced oscillations in gene expression are also observed in continuous darkness, showing that the regulation of rhythmicity by PtbHLH1a is not directly dependent on light inputs and cell division. PtbHLH1a orthologs are widespread in both pennate and centric diatom genomes, hinting at 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 still underinvestigated branch of the tree of life.