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Unveiling the role of Rhizaria in the silicon cycle

Abstract : Rhizaria are ubiquitous planktonic protists, thriving from surface waters to bathypelagicecosystems. A number of rhizarians taxa (i.e., polycystines, phaeodarians) build complex and robust siliceous skeletons. Yet, basic knowledge such as abundances, diversity, cellular content (biogenic silica-bSi, carbon and nitrogen) and silicon (Si) uptake rates of rhizarians are poorly known. This lack of knowledge prevents an accurate estimate of their contribution to the Si biogeochemical cycle. To address this issue, Si uptake rates —using the 32Siradioisotope— and cellular content were measured during three research cruises (Mediterranean Sea, Ross Sea and Atlantic Ocean). High individual production rates (up to 9 nmol-Si cell-1d-1) and Si content (up to 11.9 nmol-Si cell-1) were reported, as compared to diatoms. Combining these first results with previously published abundance data, the contribution of these organisms to the biogeochemical cycling of Si was assessed,revealing that Rhizaria contribute up to 22% of the bSi production in the global ocean. Additionally, the abundance, biomass and diversity of Rhizaria were quantified by pairing imaging tools (FlowCAM, Zooscan and UVP) and molecular barcode techniques to cover the wide size range represented by Rhizaria in the NW Mediterranean Sea. This integrative approach revealed that Rhizaria represent up to 6% of the bSi in the first 500 metres of the water column. Overall, this work provides unique quantitative data that highlight the impact of rhizarians in the cycling of Si in the ocean. This thesis work also emphasizes the need toexplore further the deep ocean to refine our first estimates of Rhizarian contribution to the Sibiogeochemical cycle at local and global scales
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Submitted on : Monday, June 14, 2021 - 1:09:08 PM
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Natalia Llopis Monferrer. Unveiling the role of Rhizaria in the silicon cycle. Other. Université de Bretagne occidentale - Brest, 2020. English. ⟨NNT : 2020BRES0041⟩. ⟨tel-03259625⟩

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