Oceanic environments are sensitive to climate change. The most emblematic example is the bleaching of coral reefs due to the breakdown of the symbiosis between the coral and its dinoflagellate microalgal symbionts in response to temperature rise. Such mutualistic photosymbioses are not only a highly significant process for evolution, but also a key ecological interaction supporting the functioning of whole ecosystems. While corals are highly symbolic and attract most of the research attention, they represent only a small fraction of photosymbioses in the global ocean. In the plankton, arguably one of the least explored compartments of the biosphere, photosymbiotic relationships with dinoflagellates are frequently observed and hold a key position in pelagic ecosystems. Considering the obvious significance of plankton on the one hand and photosymbiosis on the other, coral bleaching could just be the tip of the iceberg of a more global unnoticed phenomenon occurring at the surface of all oceans. “Plankton bleaching”, which has been reported from fossil records to have already occurred in the middle Eocene (40 Million years ago), could have a considerable impact on oceanic ecosystem structure and function. We hypothesize that fundamental biological processes underlying benthic and planktonic photosymbioses are based on common molecular pathways and have been selected to respond to similar environmental settings. In this context, in the IMPEKAB project we will seek i- to evaluate the sensibility of planktonic photosymbiosis to environmental changes, ii- to unveil fundamental biological processes involved in the response of photosymbiosis to thermal stress across eukaryotic lineages by comparing outcomes of similar experiments performed on marine benthic and planktonic host models. Finally, based on our comprehensive understanding of photosymbiosis stress processes and considering that plankton bleaching could impact the whole pelagic ecosystem, we will iii- apply an original eco-systems biology approach to evaluate the thermal stress response of planktonic photosymbiosis in the environment. The research strategy we propose in order to attain our objectives is based on promising preliminary results obtained on the sea anemone and the Radiolaria, our ecologically relevant benthic and planktonic biological models, respectively. Taking advantage of the strong expertise of the partners on their respective biological models, we have developed a carefully planned experimental strategy. Briefly, once the physiological characteristics of the models with respect to temperature variation have been defined, we will use this framework to conduct experiments to decipher the genetic and metabolic responses associated to thermal stress. Data acquired and outcomes of comparative analysis between experimental conditions and biological models will be used to implement innovative modelling approaches to integrate all results into an environmental context. Ultimately we aim to achieve a detailed physio-genomic understanding of how marine photosymbiosis responds to environmental stressors in order to develop tools that will facilitate in situ monitoring of the potentially critical "plankton bleaching" phenomenon. This joint initiative, ambitious and highly original in the national and international research landscape, will involve recognized scientists from the fields of biology, ecology, chemistry, bioinformatics, and computational modelling, promoting strong exchanges of concepts and technical expertise and contributing to the development of the novel “eco-systems biology” field. As a consortium, we will implement a carefully organized strategy for scientific dissemination, outreach, and valorization of the results obtained in IMPEKAB.