Adaptation du phytoplancton – PhytAdapt

Plankton is composed of organisms drifting in the water column that represent an amazing 98% of ocean biomass. Phytoplankton is the autotrophic component of the plankton community and is responsible for much of the oxygen present in the Earth's atmosphere and half of the total amount produced on the planet. Their cumulative energy fixation into carbon compounds is the basis of oceanic food webs. In addition to being key components of the carbon cycle, they are also involved in several other major biogeochemical cycles, such as silicification. They have therefore shaped the current biogeochemistry of our planet, they determine major climate patterns, and perform essential ecosystem services. They are also likely to be affected by global climate change but we do not know clearly how. In order to improve our understanding of these poorly studied microorganisms, a systematic characterization of the physiology of the key organism groups is required, especially for eukaryotes which are less known. In the current project we propose to study Prasinophytes and diatoms, exploiting the expertise uniquely available in France for a genome-enabled exploration of photosynthetic activities in these organisms. Both organism classes are widely distributed in marine environments, and are derived from highly divergent evolutionary processes, the former being algae emerging at the base of the green lineage, the latter being the result of a secondary endosymbionsis within the Chromalveolates. The project will focus on Ostreococcus tauri as Prasinophyte green micro-alga, and Phaeodactylum tricornutum as pennate diatom, and will exploit powerful resources as their recently published genome sequence, numerous ESTs, availability of reverse genetics tools, a range of characterized ecotypes from different locations and non invasive spectroscopic studies. Using these resources we will perform detailed analyses of growth and photosynthetic parameters (photosynthetic quantum yield, non photochemical dissipation of excess absorbed energy) in a range of ecologically-relevant conditions such as different physical and nutrient sources, using laboratory cultures corresponding to a range of ecotypes of each strain/species. These studies will aim to understand the mechanisms of photosynthesis in both groups and will also highlight differences. These studies are likely to reveal major differences, for example in how excess electrons are shuttled away from carbon fixation and reactive oxidations into alternative pathways in the plastids or other components. This will show how these reactions are utilized in different growth conditions. In a second time, we will isolate Ostreococcus and Phaeodactylum strains from the natural environment (the former in the Gulf of Lion, Mediterranean, the latter off Plymouth, UK) and will characterize them with respect to the ecotypes examined extensively beforehand. In parallel we will examine their genetic relationship with respect to the previously characterized ecotypes and will relate their photosynthetic activities to the physico-chemical conditions of the marine environment from which they were isolated. Finally, a modelling component will examine whether it is possible to improve our estimates of carbon fixation rates in different oceanic regions, currently based on available satellite and oceanographic data, as well as species distributions. Through this multidisciplinary approach we therefore propose to explore major questions about growth and photosynthetic activity in two key groups of marine phytoplankters for which genomics tools are available. This will provide a more solid foundation for inferring true carbon fixation rates in different oceanic regions