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Sponge skeletons as an important sink of silicon in the global oceans

Silicon (Si) is a pivotal element in the biogeochemical and ecological functioning of the ocean. The marine Si cycle is thought to be in internal equilibrium, but the recent discovery of Si entries through groundwater and glacial melting have increased the known Si inputs relative to the outputs in the global oceans. Known outputs are due to the burying of diatom skeletons or their conversion into authigenic clay by reverse weathering. Here we show that non-phototrophic organisms, such as sponges and radiolarians, also facilitate significant Si burial through their siliceous skeletons. Microscopic examination and digestion of sediments revealed that most burial occurs through sponge skeletons, which, being unusually resistant to dissolution, had passed unnoticed in the biogeochemical inventories of sediments. The preservation of sponge spicules in sediments was 45.2 ± 27.4%, but only 6.8 ± 10.1% for radiolarian testa and 8% for diatom frustules. Sponges lead to a global burial flux of 1.71 ± 1.61 TmolSi yr−1 and only 0.09 ± 0.05 TmolSi yr−1 occurs through radiolarians. Collectively, these two non-phototrophically produced silicas increase the Si output of the ocean to 12.8 TmolSi yr−1, which accounts for a previously ignored sink that is necessary to adequately assess the global balance of the marine Si cycle.

References

Maldonado, M., López-Acosta, M., Sitjà, C., García-Puig, M., Galobart, C., Ercilla, G., & Leynaert, A. (2019). Sponge skeletons as an important sink of silicon in the global oceans. Nature Geoscience. https://doi.org/10.1038/s41561-019-0430-7

 

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Seminar by Julie Laroche, Professor at Dalhousie University (Canada) on Tuesday, May 28

Julie LaRoche, is a visiting professor at LEMAR as part of the OFI and EUR ISBlue, will hold a seminar on Tuesday 28 May at IUEM in amphitheatre D from 11:00 am.

Julie LaRoche, is professor and Canada Research Chair in Marine Microbial Genomics and Biogeochemistry, Department of Biology, Dalhousie University, Nova Scotia, Canada.

The title of her presentation will be: Dynamics of microbial community structure and marine dinitrogen fixation at a microbial observatory in the Northwest Atlantic Ocean.

Abstract:
Primary productivity is limited by the availability of fixed nitrogen in large regions of the oceans. Dinitrogen fixation, the only biological input pathway into the marine N cycle, is an energetically expensive biochemical process that reduces N2 gas into NH3, a form of fixed nitrogen that is readily incorporated into biomolecules. The nitrogen fixers, or diazotrophs, are a selected group of prokaryotic microorganisms that can carry out this biochemical process. Historically, marine nitrogen fixation was thought to be a process carried out primarily by cyanobacteria and important mainly in the tropical and subtropical oligotrophic waters. Recent realization concerning the wide diversity of marine microbes harboring the nitrogenase enzyme indicates that we do not fully understand the roles of the diverse diazotrophs that populate the ocean. In the context of the Ocean Frontier Institute located at Dalhousie University, the microbial community structure and function in Northwest Atlantic (NWA) have been assessed through next-generation sequencing of hypervariable regions of 16S and 18S rRNA genes, nifH gene and metagenomics at existing time-series stations since 2014. The nifH gene, a marker gene for diazotrophy, has shown that both cyanobacterial and non-cyanobacterial diazotrophs are members of the microbial communities in our NWA microbial observatories. The lecture will focus on the microbial community structure in the NWA, with a specific attention to the diazotrophs. In particular, the potential metabolic pathways identified from the genome annotation of a novel bacterial isolate, belonging to a clade of gamma-proteobacteria widely distributed in the Tara expedition database, will be discussed in a global context.

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LMI DISCOH

The main objective of LMI DISCOH is to study ocean-atmosphere, biogeochemical and ecological dynamics in the CHS in order to understand and anticipate the effect of intra-seasonal, interannual, decadal and climate change variability on coastal ecosystem dynamics. LMI contributes to the effective implementation of the ecosystem approach to fisheries.

From a scientific point of view, the LMI “DISCOH” complements projects under development (ANRs PEPS and TOPINEME, etc.) and aims to improve coordination between projects. In particular, LMI aims to direct the efforts of its participants towards major cross-cutting scientific issues.

In addition, LMI “DISCOH” supports the development of high-level Masters in Marine Sciences in Peru by promoting the participation of IRD researchers and other foreign scientists (courses and supervision of theses) and by supporting student and faculty exchanges, particularly in the region.

More information on the LMI DISCOH website