Séminaire

In oceanic ecosystems, planktonic organisms play a pivotal role in oceanic food webs and global biogeochemical cycles. Although most of our knowledge of these processes stems from organisms reliably sampled, there is an emerging recognition of unicellular organisms as key players of ocean biogeochemical processes. Among them, the Rhizaria is a diverse a group of single-celled eukaryotes (protists), encompassing organisms that build complex silicified skeletons (e.g., Radiolaria and Phaeodaria) and thus, an extensive fossil record extremely valuable for micropaleontology. However, as it is currently impossible to culture them, we lack the most fundamental information about their basic biology, ecology and ultimately the true role of rhizarians in oceanic ecosystems and global biogeochemical cycles. In recent years, I applied innovative approaches (e.g., single-cell methods, in situ imaging) to refine their emerging role as key components of planktonic community structure. For instance, using single-cell barcoding followed by environmental sequencing, I explored the diversity patterns of Collodaria, one group of radiolarians, recently highlighted as major components of planktonic communities by the results of the Tara Ocean Expedition. More recently, using a combination of standard sediment trap deployments coupled with simultaneous cutting-edge in situ imagery, I have shown that one family of giant phaeodarian represents a significant part of biogenic silica fluxes in the twilight zone. This same family may intercept up to 20% of sinking particles produced above in the euphotic zone. These different examples suggest that rhizarians could play a major role in oceanic ecosystems, either in the silica cycle but also the biological pump, by attenuating carbon fluxes in the twilight zone. However, beyond the ecological significance of specific groups, rhizarians also encompass a large diversity of species, spanning a wide size range, from tens of micrometers to several centimeters. Also, while many rhizarians inhabit euphotic regions, some typically thrive below in the twilight zone. Therefore, additional studies are required to better appreciate the full extent of rhizarians contribution to oceanic ecosystems and biogeochemical processes, and ultimately include this emerging component into biogeochemical models.

francesco.dovidio@locean-ipsl.upmc.fr