Planktonic organisms play crucial roles in biogeochemical cycles that regulate Earth's climate. Within planktonic communities, Rhizaria are diverse protists that build complex mineralized skeletons, for some, of biogenic silica. As their skeletons sink, they carry biogenic elements to the deep ocean and seafloor where ultimately they contribute to fossil records. In the last decade, Rhizaria emerged as important organisms modulating biogeochemical processes linked to the silica and carbon cycles. This is particularly true in the Southern Ocean where they reach their highest biomasses.Yet, considering their broad size spectra and extended range of vertical habitats, they still lack a full characterization and quantification of their impacts (i.e., export, flux attenuation, dissolution, etc.) on these biogeochemical cycles. Their fate as they sink toward the seafloor remains elusive for many Rhizaria, hampering extensive paloenvironmental reconstruction. To this end, the ChoRhiSO project will evaluate the role and quantitative contribution of Rhizaria in the biological carbon pump and marine silica cycle in the Southern Ocean, extending from the surface to the seafloor. Built around a multidisciplinary team, an original combination of expertise encompassing biogeochemistry, biological oceanography and micropaleontology will be used. We will use a diverse array of tools and methods: single-cell based experiments, meta-omics, ancient DNA, high frequency imagery, isotope geochemistry, biogeochemistry, and traditional taxonomic-based micropaleontology. Ultimately, since the full characterization and quantification of the impact of rhizarians on these processes has not yet been done in any part of the ocean, and given their new-found importance in marine biota, this project will significantly improve upon our general knowledge of rhizarian ecology as well as expand understanding of key biogeochemical processes affecting the Southern Ocean.