| Summary: | Mamiellales are an order of unicellular cosmopolitan green algae with ecologically important species such as Bathycoccus, Micromonas or Ostreococcus, major contributors to the primary production. This thesis uses this phytoplankton group with known reference genomes as a study model in order to better analyze the impact of the environment on plankton using samples from the Tara Oceans expedition.To do this, different analyses were carried out to define their biogeography and ecological preferences, first in temperate waters then in the cold, nutrient-rich waters of the Arctic Ocean. In both cases, temperature was shown to be the main factor distinguishing the environment in which the different genomes were found. We then carried out a more detailed study in particular on Bathycoccus prasinos, a species abundant in these two distinct environments, in order to establish its population structure, which proved to be clearly separated into three groups: southern , arctic and temperate samples, again showing an impact of temperature but not only in view of the genomic distance between the first two basins.Finally, our study was extended with various collaborations, allowing us to observe a group of heterotrophic protists, the stramenopiles, and to perform analyses at the much larger scale of communities. All of these results conclude once again, among other things, on the strong impact of temperature, leading us to contribute to the question about the current context of climate change and its impact on plankton. Les Mamiellales sont un ordre d'algues vertes unicellulaires cosmopolites comprenant des espèces d'importance écologique telles que Bathycoccus, Micromonas ou encore Ostreococcus, des contributeurs majeurs à la production primaire. Cette thèse prend pour modèle d'étude ce groupe phytoplanctonique aux génomes de référence connus afin d'analyser au mieux l'impact de l'environnement sur le plancton grâce aux échantillons provenant de l'expédition Tara Oceans.Pour cela, différentes analyses ont été menées afin ...
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