Photoperiodism in ubiquist model microalgae of marine phytoplankton: molecular mechanisms and sensitivity to ocean temperature increase – CLIMA-CLOCK

At the basis of marine food webs, phytoplankton is a major player of carbon sequestration and transfer to higher trophic levels. Marine phytoplankton encompasses a large phylogenetic diversity of microalgae with a worldwide distribution from equator to pole. Accordingly, in polar and temperate regions, phytoplankton growth follows specific patterns, with seasonal bursts (algal blooms). Adaptation to varying day length along seasons and across latitudes, known as photoperiodism, has been well described in terrestrial animals and plants. In these organisms, the circadian clock (CC) is at the heart of photoperiodism. This endogenous molecular timekeeper is essential for perceiving and coordinating physiology not only to periodic light cues through specific photoreceptors (PRs) but also to temperature cues. Uncoupling these cues can have a dramatic effect on fitness or reproductive success of species adapted to specific latitudinal/seasonal niches. Though seasonal rhythms have long been known in marine phytoplankton, the genetic basis regulating physiological adaptation to varying latitudinal and seasonal day length (photoperiodism) remain largely unknown in these organisms.
The overall objective of the CLIMA-CLOCK project is to fill this important gap in our knowledge by uncovering the molecular mechanisms regulating photoperiodism, i.e the genetic adaptation to latitude and seasonal day length, as well as to assess the impact of ocean temperature increase on photoperiodism in phytoplankton. Our integrative gene to ecosystem approach is innovative. It relies on both the analysis of large metagenomic datasets collected during oceanographic cruises such as TARA Oceans and locally in Banyuls bay, France (10 years time series) to identify natural genetic variations in CC/PR genes and on functional genomics in model organisms to test the importance of these variations in photoperiodism. The project will focus on two algal groups of major ecological relevance presenting a worldwide distribution, diatoms and mamiellophyceae, for which available genetic tools have been used to identify PRs and CC components in partners labs. Comparison of CC-driven photoperiodism between mamiellophyceae and diatoms, offers a unique opportunity to identify the shared and specific molecular mechanisms regulating physiological adaptation to day length.
Estimating the importance of natural variations in adaptation of phytoplankton to specific latitudinal and temporal niches and testing experimentally the impact of genetic variations on photoperiodic responses in the lab will go beyond most recent studies which have focus primarily on the presence/absence or expression of genes involved in adaptation to specific ecological niches. By integrating global scale approaches in marine genomics and functional studies of regulators of photoperiodism, CLIMA-CLOCK will help to build a new paradigm for understanding how spatial and temporal dimensions of photoperiodism are connected. An expected outcome of CLIMA-CLOCK is that the genetic variants identified in the project can be used as genetic markers for genomic studies of environmental changes of phytoplankton distribution and seasonality. This first step is needed to be able to forecast the global consequences of rising sea temperatures on the productivity of marine phytoplankton which is an essential component of marine microbial ecosystems accounting for 50 % of the oxygen production of the planet.
The results of CLIMA-CLOCK will be valorized by joint publications in major international peer reviewed journals and communications in national and international congresses and workshops. We also aim at publishing news or articles in traditional media. CLIMA-CLOCK will also communicate to a broad public audience in part through an exhibition at the “Biodiversarium” facility (Banyuls), which is a public aquarium dedicated to scientific mediation, and which is receiving more than 80 000 visitors each year.