Role of Rhizaria in biogeochemical Cycles in the epi- and mesopelagic ocean

• Main Author: Laget, Manon
• Other Authors: Laboratoire d’Océanologie et de Géosciences (LOG) - UMR 8187 (LOG), Institut national des sciences de l'Univers (INSU - CNRS)-Université du Littoral Côte d'Opale (ULCO)-Université de Lille-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD France-Nord ), Université du Littoral Côte d'Opale (ULCO), Université du Littoral Côte d'Opale, Urania Christaki, Tristan Biard
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: HAL CCSD 2023
• Subjects: Rhizaria; Carbon cycle; Silicon cycle; In situ imaging; Biomass; Sinking speed; Cycle du carbone; Cycle du silicium; Imagerie in situ; Biomasse; Vitesse de sédimentation; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDV.EE]Life Sciences [q-bio]/Ecology; environment; Southern Ocean
• Online Access: https://theses.hal.science/tel-04552165; https://theses.hal.science/tel-04552165/document; https://theses.hal.science/tel-04552165/file/these_LAGET_Manon.pdf

The epipelagic and mesopelagic oceans play a key role in the production, recycling, and transfer of both organic and mineral matter to the deep ocean. Rhizaria (including Radiolaria and Phaeodaria) are planktonic protists thriving in these layers throughout the world ocean. They can display either mixotrophic or heterotrophic feeding behavior, and some of them form silica skeletons. Furthermore, these organisms can aggregate detrital material around them, forming fast-sinking particles. Sampling these fragile organisms is challenging, but advances in in situ imaging techniques have improved estimates of their abundance and roles in element fluxes. Still, due to a lack of measurements at the cellular level, our understanding of their global carbon biomass and their roles in biogeochemical processes remains limited. To fill this gap, the carbon content of diverse rhizarian taxa was measured, covering a broad size spectrum, and an allometric relationship was established, revealing an overall low carbon density compared to smaller protists. Using boosted regression trees and a global Underwater Vision Profiler (UVP) 5 dataset, including >167,000 rhizarian images recorded all over the world ocean, global carbon biomass of >600-μm Rhizaria was reestimated to be 1.7% of the total mesozooplankton biomass within the upper 500 m of the water column. This biomass was found to be 10-fold higher in the mesopelagic than in the epipelagic layer. Subsequently, mesopelagic flux-feeder Phaeodaria were estimated to intercept 3.8-9.2% of the gravitational POC flux exported out of the euphotic zone. In the Southern Ocean, where their abundance was previously shown to be low, this interception rate can reach as high as 11.2-23.4%. In addition, biogenic silica (bSi) production rates of Phaeodaria were estimated, being the first quantification of bSi production in the mesopelagic layer. As the sole bSi producers in this layer, they play a significant role in its recycling, co-dominating the silicon cycle along with diatoms and ...