LE 15 DÉCEMBRE 2015 à 09h00 en salle Jean Maetz

Composition du Jury : Andreas OSCHLIES (rapporteur), Richard B. RIVKIN (rapporteur), Laurent MÉMERY (rapporteur), Paul NIVAL (Examinateur), Ingrid OBERNOSTERER (Examinateur), Maurizio RIBERA D'ALCALA (Examinateur)

 ABSTRACT : Phytoplankton is a major actor of biogeochemical cycles in the ocean with effects on the marine environment at the global scale.

This manuscript gathers studies on the responses of planktonic, phototrophic organisms to their physico-chemical environment, using both experimental and modeling developments. I focus on processes, at small scale, that drive the daily dynamics of phytoplankton such as photosynthesis, nutrient acquisition, storage of reserve compounds... I then develop models to study how the growth properties of a given species will determine its adaptation and competitiveness in the natural environment. The dual, experimental--‐theoretical approach allows for an analysis of the relations that take place between the different components of a biological system. Combined theory and experiments allow picturing how environmental conditions exert control on the metabolism, and hence on populations, at different spatial and/or temporal scales. When carefully calibrated, models are fantastic investigation tools... They can for instance be used to grasp the combination of events leading to disequilibria, such as the appearance and maintenance of phytoplankton blooms. How physical processes affect the growth of organisms and how in turn, growth processes may affect the environment in which these organisms evolve is the underlying theme to the different research lines I have been pursuing since my PhD. The concept of cascading time scales is therefore present as a wallpaper this manuscript. Particular interest is paid to biological nitrogen (N2) fixation in marine planktonic cyanobacteria, which has been my main research line since my earliest years as a researcher.