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Mediterranean Institute of Oceanography

Oceanomed building
Aix-Marseille University - Luminy Campus - Case 901
13288 MARSEILLE Cedex 09, FRANCE

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sophie.bonnet@mio.osupytheas.fr

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SOPHIE BONNET IRD/MIO

©2020 par Sophie Bonnet IRD/MIO. Créé avec Wix.com

Our science recently presented at the Aquatic Sciences Meeting 2021

 

 

QUANTIFYING DI-NITROGEN FIXATION AND ITS CONTRIBUTION TO EXPORT PRODUCTION USING D15N BUDGETS NEAR THE TONGA ARC IN THE WESTERN SUB-TROPICAL SOUTH PACIFIC

 

Heather Forrer, Sophie Bonnet, Cécile Guieu, Angela Knapp

 

Aquatic Sciences Meeting ASLO, Virtual, 2021

  

Identifying the spatial distribution of the largest di-nitrogen (N2) fixation fluxes to the ocean remains a critical goal of chemical oceanography. The location of these fluxes informs our understanding of the environmental sensitivities of N2 fixation and the capacity for the dominant marine nitrogen (N) source and sink processes to respond to each other, influencing the global carbon cycle and climate. Here we quantify rates of N2 fixation as well as its importance for supporting export production using d15N budgets at stations sampled near the Tonga subduction zone. Recent observations indicate that shallow hydrothermal plumes may provide significant dissolved iron to the euphotic zone in this region, thereby stimulating N2 fixation. We present measurements of water column nitrate+nitrite d15N that are compared with the d15N of sinking particulate N collected by drifting sediment traps at stations both proximal and distal to subsurface hydrothermal activity. Preliminary d15N budget results suggest very high rates of N2 fixation at stations proximal to hydrothermal activity, supporting the majority (>50%) of export production. These findings are consistent with prior results from the region, however are in contrast to observations from d15N budgets in most other oligotrophic regions, where N2 fixation typically supports <10% of export production. Consequently, this region is expected to contribute significant low-d15N N to the thermocline, balancing the elevated nitrate+nitrite d15N generated in the oxygen deficient zones in the eastern tropical Pacific.

 

A GROUP-SPECIFIC APPROACH TO QUANTIFY IRON UPTAKE BY DIAZOTROPHS AND ASSOCIATED MICROBIAL COMMUNITIES

 

Caroline Lory, France Van Wambeke, Marion Fourquez, Aude Barani, Chloé Tiliette, Dominique Marie, Sandra Nunige, Cécile Guieu, Sophie Bonnet

 

Aquatic Sciences Meeting ASLO, Virtual, 2021

 

In oligotrophic oceans, biological N2 fixation is often limited by iron (Fe) as both photosynthesis and N2 fixation confer high Fe requirements to diazotrophs. In the Western Tropical South Pacific (WTSP), shallow hydrothermal sources provide new Fe to the euphotic layer, which is hypothesized to sustain the high N2 fixation rates reported in the region. Yet, the Fe demand of diazotrophs and their competition for this new resource with the rest of the microbial community remain unknown. By coupling 55Fe uptake experiments on three size fractions (0.2-2 µm, 2-10 µm and >10 µm) with cell-sorting by flow cytometry, we assess for the first time, the specific Fe needs of diazotrophs in their natural environment and across dissolved Fe gradients (above and away from a submarine volcano). We discuss bulk and size fraction Fe uptake rates along the studied gradients and compare the specific Fe uptake rates of filamentous and unicellular diazotrophs with other sorted organisms. This group-specific approach reveals that Trichodesmium and non-diazotrophic pico-plankton are the major contributors to the biological Fe demand in this remote ecosystem.

 

P-ANHYDRIDES AS A POTENTIAL SOURCE OF DOP FOR DIAZOTROPHS IN THE SOUTH PACIFIC

 

Alba Filella, France van Wambeke, Elvira Pulido-Villena, Sandra Nunige, Olivier Grosso, Sophie Bonnet, Lasse Riemann, Solange Duhamel, Mar Benavides

 

Aquatic Sciences Meeting ASLO, Virtual, 2021

 

In phosphate limited ocean regions, diazotrophs may rely on dissolved organic P (DOP). Oceanic DOP contains P-monoesters, phosphonates and P-anhydrides. While the two first are known to promote diazotrophy, the lability of the latter to diazotrophs is unknown. Here we explore the role of inorganic and organic P-anhydrides on diazotrophs in low and moderate phosphate availability regions of the South Pacific (TONGA cruise https://doi.org/10.17600/18000884). Surface communities were incubated with AMP (P-monoester), ATP (P-ester and P-anhydride bonds) or 3polyP (inorganic P-anhydride). After 48h, we measured N2 fixation rates, diazotroph and microbial community abundance and composition, bulk elemental composition, bacterial production rates and ectoenzymatic activities. Crocosphaera abounded in both regions, while Trichodesmium occurred mainly in mesotrophic waters.  Overall, N2 fixation was stimulated by AMP additions compared to the P-anhydrides tested, and although N2 fixation rates were ≥100-fold greater at the mesotrophic station, the addition of AMP prompted a greater response at the oligotrophic station. Conversely, enhanced N2 fixation rates measured in 3polyP treatments were comparable between sites. Interestingly, ATP additions mainly boosted growth of heterotrophic bacteria to a similar extent at both sites, but not N2 fixation. Overall, our results suggest a differential repartition of the P pool among diazotrophic vs non-diazotrophic communities and a potential role of P-anhydrides as a source of P for marine diazotrophs in tropical waters.

  

POTENTIAL ROLE OF MARINE PICOCYANOBACTERIA IN THE DISTRIBUTION OF DISSOLVED METHANE IN THE WESTERN TROPICAL SOUTH PACIFIC OCEAN

 

Cédric Boulart, Pierre Le Moal, Jean-Philippe Gac, Estelle Bigeard, Mathilde Ferrieux, Laurence Garczarek, Sophie Bonnet, Cécile Guieu

 

Aquatic Sciences Meeting ASLO, Virtual, 2021

 

Oceans are often considered as a minor source of methane (CH4) to the atmosphere but recent observations highlighted their oversaturation at the global scale, making them a significant source to the atmosphere. Recently marine picocyanobacteria emerged as potential important players, producing CH4 as a byproduct of methylphosphonate decomposition in phosphate-depleted, oxic surface waters. As part of the TONGA Cruise (NO L’Atalante, Nov. 2019, https://doi.org/10.17600/18000884) in the Western Tropical South Pacific Ocean (WTSP), we sampled the 0-400 m water column along a 1,500 nm W-E transect from Noumea (New Caledonia) to determine the CH4 concentrations and genetic diversity of marine picocyanobacteria. Results indicate a CH4 oversaturation of the oxic mixed layer over the whole transect, strongly correlated to phosphate concentrations below detection limits, the abundance of Prochlorococcus and Synechococcus cells as well as the relative abundance of specific Synechococcusclades. These results are in agreement with the recent findings from lab-based experiments showing the ability of cyanobacteria to produce CH4 under both light and dark conditions. Furthermore, analysis of the Tara Oceans metagenomes showed that several genes potentially involved in the transport and assimilation of phosphonates and/or phosphites, are specifically present in phosphate-limited regions of the world ocean. Further studies are required to identify the genes involved in the CH4 production in the surface layer of the WTSP as well as to evaluate the fate of CH4 in the water column.

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