The small unicellular diazotrophic symbiont, UCYN-A, is a key player in the marine nitrogen cycle

Microbial dinitrogen (N2) fixation, the nitrogenase enzyme-catalysed reduction of N2 gas into biologically available ammonia, is the main source of new nitrogen (N) in the ocean. For more than 50 years, oceanic N2 fixation has mainly been attributed to the activity of the colonial cyanobacterium Tri...

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Bibliographic Details
Published in:Nature Microbiology
Main Authors: Martínez-Pérez, Clara, Mohr, Wiebke, Löscher, Carolin R, Dekaezemacker, Julien, Littmann, Sten, Yilmaz, Pelin, Lehnen, Nadine, Fuchs, Bernhard M, Lavik, Gaute, Schmitz, Ruth A, LaRoche, Julie, Kuypers, Marcel M M
Format: Article in Journal/Newspaper
Language:English
Published: 2016
Subjects:
Antarctic Regions
Atlantic Ocean
Cyanobacteria/metabolism
Nitrogen Cycle
Nitrogen Fixation
Nitrogenase/metabolism
RNA
Ribosomal
16S
Seawater/microbiology
Symbiosis
Antarctic
Arctic
The Antarctic
Antarc*
North Atlantic
Online Access:https://portal.findresearcher.sdu.dk/da/publications/0a03e86b-4af2-4c9f-bdd0-932e6cebdb3c
https://doi.org/10.1038/nmicrobiol.2016.163
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Summary:Microbial dinitrogen (N2) fixation, the nitrogenase enzyme-catalysed reduction of N2 gas into biologically available ammonia, is the main source of new nitrogen (N) in the ocean. For more than 50 years, oceanic N2 fixation has mainly been attributed to the activity of the colonial cyanobacterium Trichodesmium(1,2). Other smaller N2-fixing microorganisms (diazotrophs)-in particular the unicellular cyanobacteria group A (UCYN-A)-are, however, abundant enough to potentially contribute significantly to N2 fixation in the surface waters of the oceans(3-6). Despite their abundance, the contribution of UCYN-A to oceanic N2 fixation has so far not been directly quantified. Here, we show that in one of the main areas of oceanic N2 fixation, the tropical North Atlantic(7), the symbiotic cyanobacterium UCYN-A contributed to N2 fixation similarly to Trichodesmium. Two types of UCYN-A, UCYN-A1 and -A2, were observed to live in symbioses with specific eukaryotic algae. Single-cell analyses showed that both algae-UCYN-A symbioses actively fixed N2, contributing ∼20% to N2 fixation in the tropical North Atlantic, revealing their significance in this region. These symbioses had growth rates five to ten times higher than Trichodesmium, implying a rapid transfer of UCYN-A-fixed N into the food web that might significantly raise their actual contribution to N2 fixation. Our analysis of global 16S rRNA gene databases showed that UCYN-A occurs in surface waters from the Arctic to the Antarctic Circle and thus probably contributes to N2 fixation in a much larger oceanic area than previously thought. Based on their high rates of N2 fixation and cosmopolitan distribution, we hypothesize that UCYN-A plays a major, but currently overlooked role in the oceanic N cycle.

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