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Fine scale sampling unveils diazotroph patchiness in the South Pacific Ocean

Résumé : Diazotrophs are important contributors to reactive nitrogen availability in the ocean. Oceanographic cruise data accumulated along decades has revealed a heterogeneous distribution of diazotroph species at regional to global scales. However, the role of dynamic fine scale structures in distributing diazotrophs is not well understood. This is due to typical insufficient spatiotemporal resolution sampling and the lack of detailed physical studies in parallel. Here we show the distribution of five groups of diazotrophs in the South Pacific at an unprecedented resolution of 7-16 km. We find a patchy distribution of diazotrophs, with each group being differently affected by parameters describing fine scale structures. The observed variability could not have been revealed with a lower resolution sampling, highlighting the need to consider fine scale physics to resolve the distribution of diazotrophs in the ocean.
Abstract : Diazotrophs are important contributors to reactive nitrogen availability in the ocean. Oceanographic cruise data accumulated along decades has revealed a heterogeneous distribution of diazotroph species at regional to global scales. However, the role of dynamic fine scale structures in distributing diazotrophs is not well understood. This is due to typical insufficient spatiotemporal resolution sampling and the lack of detailed physical studies in parallel. Here we show the distribution of five groups of diazotrophs in the South Pacific at an unprecedented resolution of 7-16 km. We find a patchy distribution of diazotrophs, with each group being differently affected by parameters describing fine scale structures. The observed variability could not have been revealed with a lower resolution sampling, highlighting the need to consider fine scale physics to resolve the distribution of diazotrophs in the ocean. Main body of text The surface ocean is constantly stirred by currents that swirl and mix different seawater masses creating a dynamic mosaic of biogeochemical properties [1]. Numerical modeling and satellite data have shown that 'fine scale' structures such as filaments and eddies (with typical spatiotemporal scales of 1 to 100 km and days to weeks) impact the distribution of phytoplankton, primary productivity and carbon export in the ocean [2-4]. While these remote approaches provide valuable synoptic views at large scales, in situ sampling remains imperative to resolve the diversity, metabolism and trophic interactions of marine microbes at fine scales. However, the spatiotemporal resolution of typical at-sea sampling efforts is too coarse to resolve fine scale processes [5], which hampers our understanding of physicalbiological interactions and their impact on marine microbes [1]. Understanding the effect of fine scales on biogeochemically relevant microbial groups is of particular importance. Dinitrogen (N2) fixers or 'diazotrophs' provide a significant source of bioavailable nitrogen in the ocean [6]. Some studies have shown a preferential accumulation of diazotrophs in anticyclonic eddies [7, 8], where eddy pumping deepens isopycnals impoverishing surface waters in inorganic nitrogen presumably favoring diazotroph growth [reviewed in 9]. However, other studies have reported accumulations of diazotrophs in cyclonic eddies instead, attributed to wind-driven Ekman pumping [10]. Diazotrophs encompass a wide diversity of prokaryotic microorganisms with different tolerance to environmental and nutrient availability conditions. For example, cyanobacteria such as Trichodesmium and UCYN-B abound in oligotrophic (sub)tropical waters, while UCYN-A displays a wide geographical distribution spanning from the tropics to the polar seas [6]. Non-cyanobacterial diazotrophs cannot photosynthesize and obtain carbon and energy from organic matter compounds, which presents fundamentally different bottom-up controls as
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https://hal.archives-ouvertes.fr/hal-03007380
Contributor : Mar Benavides Connect in order to contact the contributor
Submitted on : Wednesday, November 25, 2020 - 2:34:51 PM
Last modification on : Tuesday, May 31, 2022 - 1:28:06 PM

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Mar Benavides, Louis Conradt, Sophie Bonnet, Ilana Berman-Frank, Stéphanie Barrillon, et al.. Fine scale sampling unveils diazotroph patchiness in the South Pacific Ocean. 2020. ⟨hal-03007380⟩

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