Bonnet Sophie auteur aut IRD Benavides Mar auteur aut IRD Le Moigne F. A. C. auteur aut IRD Camps M. auteur aut IRD Torremocha A. auteur aut IRD Grosso O. auteur aut IRD Dimier C. auteur aut IRD Spungin D. auteur aut IRD Berman-Frank I. auteur aut IRD Garczarek L. auteur aut IRD Cornejo-Castillo F. M. auteur aut IRD text journalArticle eng text/pdf reformatted digital access Diazotrophs are widespread microorganisms that alleviate nitrogen limitation in 60% of our oceans, thereby regulating marine productivity. Yet, the group-specific contribution of diazotrophs to organic matter export has not been quantified, which so far has impeded an accurate assessment of their impact on the biological carbon pump. Here, we examine the fate of five groups of globally-distributed diazotrophs by using an original combination of mesopelagic particle sampling devices across the subtropical South Pacific Ocean. We demonstrate that cyanobacterial and non-cyanobacterial diazotrophs are exported down to 1000 m depth. Surprisingly, group-specific export turnover rates point to a more efficient export of small unicellular cyanobacterial diazotrophs (UCYN) relative to the larger and filamentous Trichodesmium. Phycoerythrin-containing UCYN-B and UCYN-C-like cells were recurrently found embedded in large (>50 mu m) organic aggregates or organized into clusters of tens to hundreds of cells linked by an extracellular matrix, presumably facilitating their export. Beyond the South Pacific, our data are supported by analysis of the Tara Oceans metagenomes collected in other ocean basins, extending the scope of our results globally. We show that, when diazotrophs are found in the euphotic zone, they are also systematically present in mesopelagic waters, suggesting their transport to the deep ocean. We thus conclude that diazotrophs are a significant part of the carbon sequestered in the deep ocean and, therefore, they need to be accounted in regional and global estimates of export. specialized PACIFIQUE SUD 034 084 [Early access] [12 p.] 2022 1751-7362 https://www.documentation.ird.fr/hor/fdi:010086131 10.1038/s41396-022-01319-3 1751-7362 [F B010086131] https://www.documentation.ird.fr/hor/fdi:010086131 https://horizon.documentation.ird.fr/exl-doc/pleins_textes/2022-11/010086131.pdf IRD - Base Horizon / Pleins textes 2022-11-09 2025-02-24 fdi:010086131 fre