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Heterotrophic organisms dominate nitrogen fixation in the South Pacific Gyre

Heterotrophic organisms dominate nitrogen fixation in the South Pacific Gyre
Heterotrophic organisms dominate nitrogen fixation in the South Pacific Gyre
Oceanic subtropical gyres are considered biological deserts because of the extremely low availability of nutrients and thus minimum productivities. The major source of nutrient nitrogen in these ecosystems is N(2)-fixation. The South Pacific Gyre (SPG) is the largest ocean gyre in the world, but measurements of N(2)-fixation therein, or identification of microorganisms involved, are scarce. In the 2006/2007 austral summer, we investigated nitrogen and carbon assimilation at 11 stations throughout the SPG. In the ultra-oligotrophic waters of the SPG, the chlorophyll maxima reached as deep as 200 m. Surface primary production seemed limited by nitrogen, as dissolved inorganic carbon uptake was stimulated upon additions of (15)N-labeled ammonium and leucine in our incubation experiments. N(2)-fixation was detectable throughout the upper 200 m at most stations, with rates ranging from 0.001 to 0.19 nM N h(-1). N(2)-fixation in the SPG may account for the production of 8-20% of global oceanic new nitrogen. Interestingly, comparable (15)N(2)-fixation rates were measured under light and dark conditions. Meanwhile, phylogenetic analyses for the functional gene biomarker nifH and its transcripts could not detect any common photoautotrophic diazotrophs, such as, Trichodesmium, but a prevalence of ?-proteobacteria and the unicellular photoheterotrophic Group A cyanobacteria. The dominance of these likely heterotrophic diazotrophs was further verified by quantitative PCR. Hence, our combined results show that the ultra-oligotrophic SPG harbors a hitherto unknown heterotrophic diazotrophic community, clearly distinct from other oceanic gyres previously visited.
1751-7362
1238-1249
Halm, Hannah
7c769c24-0976-4b18-87b0-813cd8d3bd79
Lam, Phyllis
996aef80-a15d-4827-aed8-1b97b378f6ad
Ferdelman, Timothy G.
608e7e72-65a9-4df1-9333-0dd5cfbe0cbe
Lavik, Gaute
29014780-d97c-41c0-8b59-a230bdfcdb37
Dittmar, Thorsten
38cc8f62-b49f-402c-a3d1-1b95cae4ec56
LaRoche, Julie
21d1685a-3dbd-4ccd-9ae4-e968c8792297
D'Hondt, Steven
06530267-4061-4f9b-b8ec-3e6009d4ac6e
Kuypers, Marcel M.M.
b6288cfb-42bc-469c-93fe-8fbb40d97bec
Halm, Hannah
7c769c24-0976-4b18-87b0-813cd8d3bd79
Lam, Phyllis
996aef80-a15d-4827-aed8-1b97b378f6ad
Ferdelman, Timothy G.
608e7e72-65a9-4df1-9333-0dd5cfbe0cbe
Lavik, Gaute
29014780-d97c-41c0-8b59-a230bdfcdb37
Dittmar, Thorsten
38cc8f62-b49f-402c-a3d1-1b95cae4ec56
LaRoche, Julie
21d1685a-3dbd-4ccd-9ae4-e968c8792297
D'Hondt, Steven
06530267-4061-4f9b-b8ec-3e6009d4ac6e
Kuypers, Marcel M.M.
b6288cfb-42bc-469c-93fe-8fbb40d97bec

Halm, Hannah, Lam, Phyllis, Ferdelman, Timothy G., Lavik, Gaute, Dittmar, Thorsten, LaRoche, Julie, D'Hondt, Steven and Kuypers, Marcel M.M. (2012) Heterotrophic organisms dominate nitrogen fixation in the South Pacific Gyre. The ISME Journal, 6 (6), 1238-1249. (doi:10.1038/ismej.2011.182). (PMID:22170429)

Record type: Article

Abstract

Oceanic subtropical gyres are considered biological deserts because of the extremely low availability of nutrients and thus minimum productivities. The major source of nutrient nitrogen in these ecosystems is N(2)-fixation. The South Pacific Gyre (SPG) is the largest ocean gyre in the world, but measurements of N(2)-fixation therein, or identification of microorganisms involved, are scarce. In the 2006/2007 austral summer, we investigated nitrogen and carbon assimilation at 11 stations throughout the SPG. In the ultra-oligotrophic waters of the SPG, the chlorophyll maxima reached as deep as 200 m. Surface primary production seemed limited by nitrogen, as dissolved inorganic carbon uptake was stimulated upon additions of (15)N-labeled ammonium and leucine in our incubation experiments. N(2)-fixation was detectable throughout the upper 200 m at most stations, with rates ranging from 0.001 to 0.19 nM N h(-1). N(2)-fixation in the SPG may account for the production of 8-20% of global oceanic new nitrogen. Interestingly, comparable (15)N(2)-fixation rates were measured under light and dark conditions. Meanwhile, phylogenetic analyses for the functional gene biomarker nifH and its transcripts could not detect any common photoautotrophic diazotrophs, such as, Trichodesmium, but a prevalence of ?-proteobacteria and the unicellular photoheterotrophic Group A cyanobacteria. The dominance of these likely heterotrophic diazotrophs was further verified by quantitative PCR. Hence, our combined results show that the ultra-oligotrophic SPG harbors a hitherto unknown heterotrophic diazotrophic community, clearly distinct from other oceanic gyres previously visited.

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Published date: June 2012
Organisations: Ocean Biochemistry & Ecosystems

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Local EPrints ID: 349910
URI: http://eprints.soton.ac.uk/id/eprint/349910
ISSN: 1751-7362
PURE UUID: 08b1454a-b7b2-49fb-a97e-5d8bec4a5209
ORCID for Phyllis Lam: ORCID iD orcid.org/0000-0003-2067-171X

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Date deposited: 13 Mar 2013 13:25
Last modified: 15 Mar 2024 03:47

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Contributors

Author: Hannah Halm
Author: Phyllis Lam ORCID iD
Author: Timothy G. Ferdelman
Author: Gaute Lavik
Author: Thorsten Dittmar
Author: Julie LaRoche
Author: Steven D'Hondt
Author: Marcel M.M. Kuypers

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