Molecular analysis of the light-dependent rhythmic regulation in a marine diatom – DiaRhythm

Diatoms belong to the world’s most diverse group of algae, being responsible for a large part of the primary production in aquatic environments. In addition, they represent peculiar evolutionary chimeric cells, as they have evolved by the uptake of a eukaryotic alga into another eukaryotic cell, thus having a very different genetic background than green algae and plants. As most organisms, diatoms experience daily light/dark changes and show rhythms of basic biological processes to adjust their life cycle to it. Although recent studies revealed that these rhythms persist in continuous light conditions, indicating the existence of an endogenous circadian oscillator in diatoms, the mechanisms regulating cellular rhythmicity in these algae are still obscure. Interestingly, no genes for homologs of the circadian clock components known from bacteria, plants or animals have been found in the diatom genomes, indicating a diversification of the circadian clock machinery in these algae. Therefore, the aim of this project is to elucidate the light-dependent rhythmic mechanisms in the model diatom Phaeodactylum tricornutum and to assess their physiological relevance by integrating complementary interests and expertise of two independent research groups in Germany and France.
This project is based on important recent discoveries by the two teams: i) diatoms integrate light signals from the environment as well as from an endogenous circadian clock to regulate diel cellular activities; ii) the bHLH-PAS transcription factor RITMO1 has been recently identified as the first regulator of the diatom circadian rhythms; iii) diverse diatom photoreceptors involved in transcription regulation, such as the Aureochromes and the animal-like Cryptochrome (PtCPF1) appear as key players for light input to the clock; iv) the two labs have independently generated RITMO1 and photoreceptor KO mutants and established a system for studying diatom circadian rhythms. The combined resources offer an unprecedented opportunity to characterize the diatom circadian clock system and its entrainment by light. Therefore, by characterizing circadian rhythms in wild type (WT) and a mutant collection, the project will allow an extended characterization of the diatom circadian clock features, and will identify the photoreceptors of the input pathways. It will further investigate the RITMO1 partners by transcriptomic analyses in WT and RITMO1 mutants and by studying RITMO-binding sites in genome-wide protein/DNA interaction assays. Testing interactions of candidate photoreceptor/transcription factors to potential binding sites by Yeast one hybrid and protein/protein interactions by Yeast two-hybrid and immunoprecipitation will contribute to identify the first regulatory loop(s) generating cellular rhythmicity in marine diatoms. The proposed molecular analysis will provide new insights into the evolution of biological rhythms and their significance for life in the marine environment.