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 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 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 Aphotic N<sub>2</sub> fixation along an oligotrophic to ultraoligotrophic transect in the Western Tropical South Pacific Ocean

2018 ◽  
Author(s):  
Mar Benavides ◽  
Katyanne M. Shoemaker ◽  
Pia H. Moisander ◽  
Jutta Niggemann ◽  
Thorsten Dittmar ◽  
...  
Keyword(s):  
N2 Fixation ◽  
New Caledonia ◽  
Nifh Gene ◽  
South Pacific ◽  
French Polynesia ◽  
Mode Water ◽  
South Pacific Ocean ◽  
Fixation Activity ◽  
Nitrogen Inputs ◽  
Cyclotron Mass

Abstract. The western tropical South Pacific (WTSP) Ocean has been recognized as a global hotspot of dinitrogen (N2) fixation. Here, as in other marine environments across the oceans, N2 fixation studies have focused in the sunlit layer. However, studies have confirmed the importance of aphotic N2 fixation activity, although until now only one had been performed in the WTSP. In order to increase our knowledge of aphotic N2 fixation in the WTSP, here we measure N2 fixation rates and identify diazotrophic phylotypes in the mesopelagic layer along a transect spanning from New Caledonia to French Polynesia. Because non-cyanobacterial diazotrophs presumably need external dissolved organic matter (DOM) sources for their nutrition, we also identified DOM compounds using Fourier Transform Ion Cyclotron Mass Spectrometry (FTICRMS). N2 fixation rates were low (average 0.63 ± 0.07 nmol N L−1 d−1), but consistently detected across all depths and stations, representing ~ 6–88 % of photic N2 fixation. N2 fixation rates were not significantly correlated to DOM compounds. The analysis of nifH gene amplicons revealed a wide diversity of non-cyanobacterial diazotrophs, majorly matching clusters 1 and 3. Interestingly, a distinct phylotype from the major nifH subcluster 1G dominated at 650 dbar, coinciding with the oxygenated Sub-Antarctic Mode Water (SAMW). This consistent pattern suggests that the distribution of aphotic diazotroph communities is to some extent controlled by water mass structure. While the data available is still too scarce to elucidate the distribution and controls of mesopelagic non-cyanobacterial diazotrophs in the WTSP, their prevalence in the mesopelagic layer and the consistent detection of active N2 fixation activity at all depths sampled during our study suggest that aphotic N2 fixation may contribute significantly to fixed nitrogen inputs in this area.

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

2015 ◽  
Vol 12 (22) ◽  
pp. 18945-18972 ◽  
Author(s):  
C. R. Löscher ◽  
A. Bourbonnais ◽  
J. Dekaezemacker ◽  
C. N. Charoenpong ◽  
M. 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 on 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 and December 2012 revealed also a reduction of 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.

scholarly journals Evidence for efficient regenerated production and dinitrogen fixation in nitrogen-deficient waters of the South Pacific Ocean: impact on new and export production estimates

2008 ◽  
Vol 5 (2) ◽  
pp. 323-338 ◽  
Author(s):  
P. Raimbault ◽  
N. Garcia
Keyword(s):  
Pacific Ocean ◽  
Nitrogen Uptake ◽  
N2 Fixation ◽  
Inorganic Nitrogen ◽  
South Pacific ◽  
Dinitrogen Fixation ◽  
The South ◽  
New Production ◽  
Marquesas Islands ◽  
South Pacific Ocean

Abstract. One of the major objectives of the BIOSOPE cruise, carried out on the R/V Atalante from October-November 2004 in the South Pacific Ocean, was to establish productivity rates along a zonal section traversing the oligotrophic South Pacific Gyre (SPG). These results were then compared to measurements obtained from the nutrient – replete waters in the Chilean upwelling and around the Marquesas Islands. A dual 13C/15N isotope technique was used to estimate the carbon fixation rates, inorganic nitrogen uptake (including dinitrogen fixation), ammonium (NH4) and nitrate (NO3) regeneration and release of dissolved organic nitrogen (DON). The SPG exhibited the lowest primary production rates (0.15 g C m−2 d−1), while rates were 7 to 20 times higher around the Marquesas Islands and in the Chilean upwelling, respectively. In the very low productive area of the SPG, most of the primary production was sustained by active regeneration processes that fuelled up to 95% of the biological nitrogen demand. Nitrification was active in the surface layer and often balanced the biological demand for nitrate, especially in the SPG. The percentage of nitrogen released as DON represented a large proportion of the inorganic nitrogen uptake (13–15% in average), reaching 26–41% in the SPG, where DON production played a major role in nitrogen cycling. Dinitrogen fixation was detectable over the whole study area; even in the Chilean upwelling, where rates as high as 3 nmoles l−1 d−1 were measured. In these nutrient-replete waters new production was very high (0.69±0.49 g C m−2 d−1) and essentially sustained by nitrate levels. In the SPG, dinitrogen fixation, although occurring at much lower daily rates (≈1–2 nmoles l−1 d−1), sustained up to 100% of the new production (0.008±0.007 g C m−2 d−1) which was two orders of magnitude lower than that measured in the upwelling. The annual N2-fixation of the South Pacific is estimated to 21×1012g, of which 1.34×1012g is for the SPG only. Even if our "snapshot" estimates of N2-fixation rates were lower than that expected from a recent ocean circulation model, these data confirm that the N-deficiency South Pacific Ocean would provide an ideal ecological niche for the proliferation of N2-fixers which are not yet identified.

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