Hot spot of N
 2
 fixation in the western tropical South Pacific pleads for a spatial decoupling between N
 2
 fixation and denitrification

Bonnet, Sophie; Caffin, Mathieu; Berthelot, Hugo; Moutin, Thierry

doi:10.1073/pnas.1619514114

Cited by 111 publications

(129 citation statements)

References 11 publications

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“…The BNF activity detected in this study is a further evidence that high nitrate concentrations (and high N:P ratios) do not inhibit BNF activity, such as it was also observed in other upwelling regions (e.g., Voss et al, 2004;Sohm et al, 2011;Subramaniam et al, 2013;Benavides et al, 2014). Indeed, recent studies demonstrated that BNF can occur upon exposure to elevated DIN (nitrate and/or ammonium), as long as phosphate is available (Knapp, 2012), and point toward iron as the primary control factor of BNF (Knapp et al, 2016;Bonnet et al, 2017). Thus, the sensitivity of marine BNF to high DIN is not clear.…”

Section: Magnitude and Biogeochemical Relevance Of Bnfsupporting

confidence: 76%

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

et al. 2017

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The classical paradigm about marine N 2 fixation establishes that this process is mainly constrained to nitrogen-poor tropical and subtropical regions, and sustained by the colonial cyanobacterium Trichodesmium spp. and diatom-diazotroph symbiosis. However, the application of molecular techniques allowed determining a high phylogenic diversity and wide distribution of marine diazotrophs, which extends the range of ocean environments where biological N 2 fixation may be relevant. Between February 2014 and December 2015, we carried out 10 one-day samplings in the upwelling system off NW Iberia in order to: (1) investigate the seasonal variability in the magnitude of N 2 fixation, (2) determine its biogeochemical role as a mechanism of new nitrogen supply, and (3) quantify the main diazotrophs in the region under contrasting hydrographic regimes. Our results indicate that the magnitude of N 2 fixation in this region was relatively low (0.001 ± 0.002 -0.095 ± 0.024 µmol N m −3 d −1 ), comparable to the lower-end of rates described for the subtropical NE Atlantic. Maximum rates were observed at the surface during both upwelling and relaxation conditions. The comparison with nitrate diffusive fluxes revealed the minor role of N 2 fixation (<2%) as a mechanism of new nitrogen supply into the euphotic layer. Small diazotrophs (<10 µm) were responsible for all N 2 fixation activity detected in the region. Quantitative PCR targeting the nifH gene revealed the highest abundances of two sublineages of Candidatus Atelocyanobacterium thalassa or UCYN-A (UCYN-A1 and UCYN-A2), mainly at surface waters during upwelling and relaxation conditions, and of Gammaproteobacteria γ-24774A11 at deep waters during downwelling. Maximum abundance for the three groups were up to 6.7 × 10 2 , 1.5 × 10 3 , and 2.4 × 10 4 nifH copies L −1 , respectively. Our findings demonstrate measurable N 2 fixation activity and presence of diazotrophs throughout the year in a nitrogen-rich temperate region.

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Section: Magnitude and Biogeochemical Relevance Of Bnfsupporting

confidence: 76%

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

et al. 2017

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“…Bonnet et al (2017) combined the N2 fixation measurements from 6 cruises in the WTSP (including OUTPACE cuise) and concluded for a hot spot of N2 fixation in this region, consistent with the sharp increase of N* in the thermocline waters of the Coral Sea in the WTSP (Fig. 12a, 12b, Fig.…”

supporting

confidence: 72%

“…S2) suggest that the spatial coupling would occur at the basin scale through the westwards and eastward transport of the SEC, the EAC and finally of the SPC between a High Iron/High Nitrogen/High Phosphate (HI-HN-HP) zone in the 530 eastern part and a High Iron/Low Nitrogen/Low Phosphate (HI-LN-LP) zone in the western part of the subtropical Pacific. As hypothesized by Bonnet et al (2017) and Moutin et al (this issue), the spatial coupling could thus be spatially more distant than predicted by Deutsh et al (2007) and Gruber (2016).…”

mentioning

confidence: 53%

Excess nitrogen as a marker of intense dinitrogen fixation in the Western Tropical South Pacific Ocean: impact on the thermocline waters of the South Pacific

Fumenia

Moutin

Bonnet

et al. 2018

Preprint

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Abstract. As part of the Oligotrophy to UlTra-oligotrophy PACific Experiment cruise, which took place in the Western 15Tropical South Pacific during the austral summer (March-April 2015), we present data on nitrate, phosphate and on particulate and dissolved organic matter. The stoichiometric nitrogen-to-phosphorus ratios of the inorganic and organic material and the tracer N* are described. N* allows to trace changes in the proportion of fixed nitrogen due to diazotrophy and/or denitrification.Our results showed that the Melanesian archipelago waters between 160° E and 170° W are characterized by a deficit of nitrate and phosphate in the productive layer, significant dinitrogen fixation rates and an excess of particulate organic nitrogen 20 compared to the canonical ratio of Redfield. A positive N* anomaly was observed in the productive layer reflecting the combined effect of phosphate uptake by diazotrophic organisms and remineralization of excess particulate organic nitrogen.The South Pacific Gyre waters between 170° W and 160° W were depleted in nitrate but rich in phosphate. Surface waters exhibited very low dinitrogen fixation rates, an absence of excess particulate organic nitrogen and a N* signal close to zero.The higher iron availability coupled with an absence of nitrate in the suface water of the Melanesian archipelago could 25 stimulate the diazotrophic activity, which in turn will introduce excess nitrogen, deplete the surface waters in phosphate and be the explanation for the positive N* anomaly in the Melanesian archipelago waters. In the thermocline waters, the N* tracer revealed its full complexity, with notably the cumulative effect of the remineralization of particulate organic nitrogen and the

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“…Even if the link between N 2 fixation rates and export is obvious, the possible temporal decoupling between these processes and the potential role of zooplankton need to be further investigated. Finally, as Bonnet et al (2017) have recently shown that the WTSP is a hotspot of N 2 fixation, and as we have shown the importance of this process with regard to the N input and N and C export, we suggest that this region of the world ocean should be further investigated by means of oceanographic cruises and the establishment of time series. This would give us a "big picture" of the role of N 2 fixation on the export in the oligotrophic ocean.…”

Section: Resultsmentioning

confidence: 62%

“…Finally, the high contribution of N 2 fixation to new N input in the photic layer results from the intrinsically high N 2 fixation rates we measured in the WTSP (especially in MA waters), which are part of the hotspot of N 2 fixation reported by Bonnet et al (2017), with rates being in the upper range of rates reported in the global N 2 fixation Marine Ecosystem Data (MAREDAT) database (Luo et al, 2012). Those high N 2 fixation rates are as high as they are westward in the Solomon Sea Bonnet et al, 2015), extending the hotspot of N 2 fixation to the whole of the WTSP .…”

Section: Contribution Of N 2 Fixation To New N Input In the Wtspmentioning

confidence: 72%

N2 fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

et al. 2018

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Abstract. We performed nitrogen (N) budgets in the photic layer of three contrasting stations representing different trophic conditions in the western tropical South Pacific (WTSP) Ocean during austral summer conditions (FebruaryMarch 2015). Using a Lagrangian strategy, we sampled the same water mass for the entire duration of each long-duration (5 days) station, allowing us to consider only vertical exchanges for the budgets. We quantified all major vertical N fluxes both entering (N 2 fixation, nitrate turbulent diffusion, atmospheric deposition) and leaving the photic layer (particulate N export). The three stations were characterized by a strong nitracline and contrasted deep chlorophyll maximum depths, which were lower in the oligotrophic Melanesian archipelago (MA, stations LD A and LD B) than in the ultra-oligotrophic waters of the South Pacific Gyre (SPG, station LD C). N 2 fixation rates were extremely high at both LD A (593 ± 51 µmol N m −2 d −1 ) and LD B (706 ± 302 µmol N m −2 d −1 ), and the diazotroph community was dominated by Trichodesmium. N 2 fixation rates were lower (59 ± 16 µmol N m −2 d −1 ) at LD C, and the diazotroph community was dominated by unicellular N 2 -fixing cyanobacteria (UCYN). At all stations, N 2 fixation was the major source of new N (> 90 %) before atmospheric deposition and upward nitrate fluxes induced by turbulence. N 2 fixation contributed circa 13-18 % of primary production in the MA region and 3 % in the SPG water and sustained nearly all new primary production at all stations. The e ratio (e ratio = particulate carbon export / primary production) was maximum at LD A (9.7 %) and was higher than the e ratio in most studied oligotrophic regions (< 5 %), indicating a high efficiency of the WTSP to export carbon relative to primary production. The direct export of diazotrophs assessed by qPCR of the nifH gene in sediment traps represented up to 30.6 % of the PC export at LD A, while their contribution was 5 and < 0.1 % at LD B and LD C, respectively. At the three studied stations, the sum of all N input to the photic layer exceeded the N output through organic matter export. This disequilibrium leading to N accumulation in the upper Published by Copernicus Publications on behalf of the European Geosciences Union.

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Hot spot of N ₂ fixation in the western tropical South Pacific pleads for a spatial decoupling between N ₂ fixation and denitrification

Cited by 111 publications

References 11 publications

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

Excess nitrogen as a marker of intense dinitrogen fixation in the Western Tropical South Pacific Ocean: impact on the thermocline waters of the South Pacific

N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

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Hot spot of N 2 fixation in the western tropical South Pacific pleads for a spatial decoupling between N 2 fixation and denitrification

Cited by 111 publications

References 11 publications

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

Biological N2 Fixation in the Upwelling Region off NW Iberia: Magnitude, Relevance, and Players

Excess nitrogen as a marker of intense dinitrogen fixation in the Western Tropical South Pacific Ocean: impact on the thermocline waters of the South Pacific

N&lt;sub&gt;2&lt;/sub&gt; fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)

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Hot spot of N ₂ fixation in the western tropical South Pacific pleads for a spatial decoupling between N ₂ fixation and denitrification

N<sub>2</sub> fixation as a dominant new N source in the western tropical South Pacific Ocean (OUTPACE cruise)