AUTHOR=Ababou Fatima-Ezzahra , Le Moigne Frédéric A. C. , Cornet-Barthaux Véronique , Taillandier Vincent , Bonnet Sophie 

TITLE=Composition of the sinking particle flux in a hot spot of dinitrogen fixation revealed through polyacrylamide gel traps

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 10 - 2023

YEAR=2024

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1290625

DOI=10.3389/fmars.2023.1290625

ISSN=2296-7745

ABSTRACT=Diazotrophs regulate marine productivity in the oligotrophic ocean by alleviating nitrogen limitation, contributing to particulate organic carbon (POC) export to the deep ocean. Yet, the characterization of particles composing the sinking POC flux has never been explored in such ecosystems. Moreover, the contribution of the direct gravitational export of diazotrophs to the overall flux is seldom assessed. Here we explore the composition of the sinking POC flux in a hot spot of N2 fixation (the western sub-tropical South Pacific) using polyacrylamide gel-filled traps deployed at two stations (S05M and S10M) and three depths (170 m, 270 m, 1000 m) during the TONGA expedition (November-December 2019). Image analyses of particles collected in the gels was used to classify them into 5 categories (fecal aggregates, phytodetrital aggregates, mixed aggregates, cylindrical fecal pellets, and zooplankton carcasses). Fecal aggregates were the most abundant at both stations and all depths and dominated the flux (average of 56 ± 28% of the POC flux), followed by zooplankton carcasses (24 ± 19%), cylindrical fecal pellets (15 ± 14%) and mixed aggregates (5 ± 4%), whereas phytodetrital aggregates contributed less (<1%). In parallel, random confocal microscopy examination performed on sinking particles revealed that diazotrophs were present in several categories of exported particles, with a potential contribution to overall POC fluxes increasing with depth.Our results provide the first characterization of particle categories composing the sinking flux and their contribution to the overall flux in a hot spot of N2 fixation.Samples were collected during TONGA expedition (GEOTRACES GPpr14, https://doi.org/10.17600/18000884) onboard the R/V L'Atalante from November 1 st to December 5 th of 2019 (beginning of austral summer) in the WTSP (Figure 1). A surfacetethered mooring line (~1000 m) equipped with sediment traps was deployed at 2 stations: S05M (21.157°S; 175.153°W) and S10M (19.423°S; 175.133°W) (Figure 1) for 5 and 3.8 days respectively and at 3 depths: 170 m (below the base of the euphotic layer), 270 m and 1000 m.Each trap was made up of four particle interceptor tubes (PITs) of a collecting area of 0.0085