scholarly journals Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N<sub>2</sub> fixation activity and diversity in the Western Tropical South Pacific

2018 ◽  
Author(s):  
Mathieu Caffin ◽  
Hugo Berthelot ◽  
Véronique Cornet-Barthaux ◽  
Sophie Bonnet
Keyword(s):  
Food Web ◽  
N2 Fixation ◽  
Secondary Ion Mass Spectrometry ◽  
Heterotrophic Bacteria ◽  
South Pacific ◽  
Open Ocean ◽  
Planktonic Food ◽  
Fixation Activity ◽  
Zooplankton Communities ◽  
Secondary Ion

Abstract. Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood due to technical limitations. The main goals of this study were to (i) quantify how much of DDN is released to the dissolved pool during N2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, (ii) to compare the DDN release and transfer efficiencies under contrasting N2 fixation activity and diversity the oligotrophic waters of the Western Tropical South Pacific (WTSP) Ocean. We used nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling and flow cytometry cell sorting to track the DDN transfer to plankton, in regions were the diazotroph community was either dominated by Trichodesmium or by UCYN-B. After 48 h, ~ 20–40 % of the N2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYN-B dominated. ~ 7–15 % of the total fixed N (net N2 fixation + release) was transferred to non-diazotrophic plankton within 48 h, with higher transfer efficiencies (15 ± 3 %) when UCYN-B dominated as compared to when Trichodesmium dominated (9 ± 3 %). Most of the DDN (> 90 %) was transferred to picoplankton (Synechococcus, Prochlorococcus and bacteria) in all experiments. The cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries (~ 65–70 % of the DDN transfer), followed by heterotrophic bacteria (~ 23–34 % of the DDN transfer). The DDN transfer in bacteria was the highest (34 ± 7 %) when UCYN-B were dominating the diazotroph community. Regarding higher trophic level, the DDN transfer to the dominant zooplankton species was more efficient when the diazotroph community was dominated by Trichodesmium (~ 5–9 % of the DDN transfer) than when it is dominated by UCYN-B (~ 28 ± 13 % of the DDN transfer). To our knowledge, this study provides the first quantification of DDN release and transfer to phytoplankton, bacteria and zooplankton communities in open ocean waters. It reveals that despite UCYN-B fix N2 at lower rates compared to Trichodesmium in the WTSP, the DDN from UCYN-B is much more available and efficiently transferred to the planktonic food web than the DDN coming from Trichodesmium.

scholarly journals Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N<sub>2</sub> fixation activity and diversity in the western tropical South Pacific Ocean

2018 ◽  
Vol 15 (12) ◽  
pp. 3795-3810 ◽  
Author(s):  
Mathieu Caffin ◽  
Hugo Berthelot ◽  
Véronique Cornet-Barthaux ◽  
Aude Barani ◽  
Sophie Bonnet
Keyword(s):  
Food Web ◽  
N2 Fixation ◽  
Secondary Ion Mass Spectrometry ◽  
Heterotrophic Bacteria ◽  
South Pacific ◽  
Open Ocean ◽  
Trophic Levels ◽  
Planktonic Food ◽  
South Pacific Ocean ◽  
Fixation Activity

Abstract. Biological dinitrogen (N2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric deposition and riverine inputs to the N supply. Yet the fate of the diazotroph-derived N (DDN) in the planktonic food web is poorly understood. The main goals of this study were (i) to quantify how much of DDN is released to the dissolved pool during N2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, and (ii) to compare the DDN release and transfer efficiencies under contrasting N2 fixation activity and diversity in the oligotrophic waters of the western tropical South Pacific (WTSP) Ocean. We used nanometre-scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling and flow cytometry cell sorting to track the DDN transfer to plankton, in regions where the diazotroph community was dominated by either Trichodesmium or by UCYN-B. After 48 h, ∼ 20–40 % of the N2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYN-B dominated; ∼ 7–15 % of the total fixed N (net N2 fixation + release) was transferred to non-diazotrophic plankton within 48 h, with higher transfer efficiencies (15 ± 3 %) when UCYN-B dominated as compared to when Trichodesmium dominated (9 ± 3 %). The pico-cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries of the DDN transferred (∼ 65–70 %), followed by heterotrophic bacteria (∼ 23–34 %). The DDN transfer in bacteria was higher (34 ± 7 %) in the UCYN-B-dominating experiment compared to the Trichodesmium-dominating experiments (24 ± 5 %). Regarding higher trophic levels, the DDN transfer to the dominant zooplankton species was less efficient when the diazotroph community was dominated by Trichodesmium (∼ 5–9 % of the DDN transfer) than when it was dominated by UCYN-B (∼ 28 ± 13 % of the DDN transfer). To our knowledge, this study provides the first quantification of DDN release and transfer to phytoplankton, bacteria and zooplankton communities in open ocean waters. It reveals that despite UCYN-B fix N2 at lower rates compared to Trichodesmium in the WTSP, the DDN from UCYN-B is much more available and efficiently transferred to the planktonic food web than the DDN originating from Trichodesmium.

scholarly journals In depth characterization of diazotroph activity across the Western Tropical South Pacific hot spot of N<sub>2</sub> fixation

2018 ◽  
Author(s):  
Sophie Bonnet ◽  
Mathieu Caffin ◽  
Hugo Berthelot ◽  
Olivier Grosso ◽  
Mar Benavides ◽  
...  
Keyword(s):  
N2 Fixation ◽  
Secondary Ion Mass Spectrometry ◽  
Hot Spot ◽  
Average Rate ◽  
South Pacific ◽  
Isotopic Analyses ◽  
Nitrogenase Genes ◽  
South Pacific Gyre ◽  
Secondary Ion

Abstract. Here we report quantification of N2 fixation rates over a ~ 4000 km transect in the western and central tropical South Pacific. Water samples were collected along a west to east transect from 160° E to 160° W, covering contrasting trophic regimes, from oligotrophy in the Melanesian archipelagoes (MA) waters to ultra-oligotrophy in the South Pacific Gyre (GY) waters. N2 fixation was detected at all 17 sampled stations with an average rate of 631 ± 286 µmol N m−2 d−1 (range 196–1153 µmol N m−2 d−1) in MA waters and of 85 ± 79 µmol N m−2 d−1 (range 18–172 µmol N m−2 d−1) in GY waters. Exceptionally high rates of N2 fixation in MA waters were favored by availability of both iron and phosphate and the observed warm sea surface temperatures (> 28 °C). Trichodesmium and UCYN-B cyanobacteria dominated the diazotroph community (> 80 %) and gene expression of nitrogenase genes (cDNA > 105 nifH copies L−1) in MA waters, and single-cell isotopic analyses performed by nanoscale secondary ion mass spectrometry at selected stations reveal that Trichodesmium was always the major contributor to N2 fixation in MA waters, accounting for 47.1 to 83.8 % of bulk N2 fixation.

scholarly journals Biogeochemical and biological impacts of diazotroph blooms in a Low Nutrient Low Chlorophyll ecosystem: synthesis from the VAHINE mesocosm experiment (New Caledonia)

2016 ◽  
Author(s):  
Sophie Bonnet ◽  
Melika Baklouti ◽  
Audrey Gimenez ◽  
Hugo Berthelot ◽  
Ilana Berman-Frank
Keyword(s):  
Food Web ◽  
N2 Fixation ◽  
New Caledonia ◽  
External Source ◽  
Marine Ecosystems ◽  
Inorganic Phosphorus ◽  
Food Web Structure ◽  
West Pacific ◽  
Planktonic Food ◽  
South West

Abstract. In marine ecosystems, N2 fixation provides the predominant external source of nitrogen (N) (140 ± 50 Tg N yr−1), contributing more than atmospheric and riverine inputs to the N supply. Yet the fate and magnitude of the newly-fixed N, or diazotroph-derived N (hereafter named DDN) in marine ecosystems is poorly understood. Moreover, it remains unclear whether the DDN is preferentially directly exported out of the photic zone, recycled by the microbial loop, and/or transferred into larger organisms, subsequently enhancing indirect particle export. These questions were investigated in the framework of the VAHINE (VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific) project. Triplicate large volume (~50 m3) mesocosms were deployed in the tropical South West Pacific coastal ocean (New Caledonia) to maintain a stable water-mass without disturbing ambient light and temperature conditions. The mesocosms were intentionally fertilized with ~0.8 μM dissolved inorganic phosphorus (DIP) at the start of the experiment to stimulate diazotrophy. A total of 47 stocks, fluxes, enzymatic activities and diversity parameters were measured daily inside and outside the mesocosms by the 40 scientists involved in the project. The experiment lasted for 23 days and was characterized by two distinct and successive diazotroph blooms: a dominance of diatom-diazotroph associations (DDAs) during the first half of the experiment (days 2–14) followed by a bloom of UCYN-C during the second half of the experiment (days 15–23). These conditions provided a unique opportunity to compare the DDN transfer and export efficiency associated with different diazotrophs. Here we summarize the major experimental and modelling results obtained during the project and described in the VAHINE Special issue, in particular those regarding the evolution of the main standing stocks, fluxes and biological characteristics over the 23-days experiment, the contribution of N2 fixation to export fluxes, the DDN released to dissolved pool and its transfer to the planktonic food web (bacteria, phytoplankton, zooplankton). We then apply our Eco3M modelling platform further to infer the fate of DDN in the ecosystem and role of N2 fixation on productivity, food web structure and carbon export. Recommendations for future work are finally provided in the conclusion section.

scholarly journals N<sub>2</sub> fixation in eddies of the eastern tropical South Pacific Ocean

2016 ◽  
Vol 13 (10) ◽  
pp. 2889-2899 ◽  
Author(s):  
Carolin R. Löscher ◽  
Annie Bourbonnais ◽  
Julien Dekaezemacker ◽  
Chawalit N. Charoenpong ◽  
Mark A. Altabet ◽  
...  
Keyword(s):  
Pacific Ocean ◽  
N2 Fixation ◽  
South Pacific ◽  
Open Ocean ◽  
Cyclonic Eddy ◽  
Geochemical Data ◽  
Intermediate Water ◽  
N Loss ◽  
Mode Water ◽  
South Pacific Ocean

Abstract. Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen-deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations into their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce.We examined an open-ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open-ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies.In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November 2012 and December 2012 also revealed a reduction in N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore.

An Inverse Model Analysis of Planktonic Food Webs in Experimental Lakes

1994 ◽  
Vol 51 (9) ◽  
pp. 2034-2044 ◽  
Author(s):  
Alain F. Vézina ◽  
Michael L. Pace
Keyword(s):  
Food Web ◽  
Heterotrophic Bacteria ◽  
Grazing Pressure ◽  
Inverse Methods ◽  
Trophic Levels ◽  
Microbial Food Web ◽  
Food Web Structure ◽  
Planktonic Food ◽  
Size Groups ◽  
The Impact

We used inverse methods to reconstruct carbon flows in experimental lakes where the fish community had been purposely altered. These analyses were applied to three years of data from a reference lake and two experimental lakes located in Gogebic County, Michigan. We reconstructed seasonally averaged flows among two size groups of phytoplankton, heterotrophic bacteria, microzooplankton, cladocerans, and copepods. The inverse analysis produced significantly different flow networks for the different lakes that agreed qualitatively with known chemical and biological differences between lakes and with other analyses of the impact of fish manipulations on food web structure and dynamics. The results pointed to alterations in grazing pressure on the phytoplankton that parallel changes in the size and abundance of cladocerans and copepods among lakes. Estimated flows through the microbial food web indicated low bacterial production efficiencies and small carbon transfers from the microbial food web to the larger zooplankton. This study demonstrates the use of inverse methods to identify and compare flow patterns across ecosystems and suggests that microbial flows are relatively insensitive to changes at the upper trophic levels.

scholarly journals Evidence of high N<sub>2</sub> fixation rates in productive waters of the temperate Northeast Atlantic

2018 ◽  
Author(s):  
Debany Fonseca-Batista ◽  
Xuefeng Li ◽  
Virginie Riou ◽  
Valérie Michotey ◽  
Forian Deman ◽  
...  
Keyword(s):  
North Atlantic ◽  
N2 Fixation ◽  
Spring Bloom ◽  
Open Ocean ◽  
Qualitative Assessment ◽  
Shelf Area ◽  
Bay Of Biscay ◽  
Fixation Activity ◽  
Basin Scale ◽  
Iberian Margin

Abstract. Diazotrophic activity and primary production (PP) were investigated along two transects (Belgica BG2014/14 and GEOVIDE cruises) off the western Iberian Margin and the Bay of Biscay (38.8–46.5° N; 8.0–19.7° W) in May 2014 close to the end of the spring bloom. We report substantial N2 fixation activities, reaching up to 65 nmol N L−1 d−1 and 1533 µmol N m−2 d−1 close to the Iberian Margin between 38.8° N and 40.7° N. Similar figures in the basin have only been reported in the temperate and tropical western North Atlantic waters with coastal, shelf or mesohaline characteristics, as opposed to the mostly open ocean conditions studied here. In agreement with previous studies, the qualitative assessment of nifH gene diversity (encoding the nitrogenase enzyme that fixes N2) suggested a predominance of heterotrophic diazotrophs, and the absence of filamentous cyanobacteria. At the sites where N2 fixation activity was highest sequences affiliated to UCYN-A1, obligate symbiont of eukaryotic prymnesiophyte algae, were recovered. The remaining phylotypes were non-cyanobacterial diazotrophs, known to live in association with suspended particles and zooplankton (i.e., Bacteroidetes, Firmicutes and Proteobacteria). Outside the area of exceptional activity, N2 fixation in the open ocean and at shelf-influenced sites was also relatively high, ranging from 81 to 384 µmol N m−2 d−1, but was undetectable in the central Bay of Biscay. We propose that the unexpectedly high heterotrophic N2 fixation activity recorded at the time of our study was sustained by the availability of phytoplankton derived organic matter (dissolved and/or particulate) resulting from the ongoing to post spring bloom. We pose that this organic material not only sustained bacterial production, but also provided sufficient nutrients essential for the nitrogenase activity (e.g., phosphorus). Dissolved Fe was supplied through atmospheric dust deposition during the month preceding our study and through advection of surface waters from the subtropical region and the shelf area. Our findings stress the need for a more detailed monitoring of the spatial and temporal distribution of oceanic N2 fixation in productive waters of the temperate North Atlantic to better constrain the basin-scale nitrogen input to the ocean inventory.

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