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 N2gas into biologically available ammonia, is the main source of new nitrogen (N) in the ocean. For more than 50 years, oceanic N2fixation has mainly been attributed to the activity of the colonial cyanobacterium Trichodesmium1,2. Other smaller N2-fixing microorganisms (diazotrophs)-in particular the unicellular cyanobacteria group A (UCYN-A)-are, however, abundant enough to potentially contribute significantly to N2fixation in the surface waters of the oceans3-6. Despite their abundance, the contribution of UCYN-A to oceanic N2fixation has so far not been directly quantified. Here, we show that in one of the main areas of oceanic N2fixation, the tropical North Atlantic7, the symbiotic cyanobacterium UCYN-A contributed to N2fixation 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 N2fixation 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 N2fixation. 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 N2fixation in a much larger oceanic area than previously thought. Based on their high rates of N2fixation and cosmopolitan distribution, we hypothesize that UCYN-A plays a major, but currently overlooked role in the oceanic N cycle.
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Clara
http://orcid.org/0000-0002-9600-3331
C
Martínez-Pérez
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Wiebke
http://orcid.org/0000-0002-1126-1455
W
Mohr
Department of General Microbiology, Christian-Albrechts-University Kiel, Am Botanischen Garten 1-9, 24118 Kiel, Germany.
Carolin R
CR
Löscher
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Julien
J
Dekaezemacker
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Sten
S
Littmann
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Pelin
P
Yilmaz
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Nadine
N
Lehnen
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Bernhard M
BM
Fuchs
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Gaute
G
Lavik
Department of General Microbiology, Christian-Albrechts-University Kiel, Am Botanischen Garten 1-9, 24118 Kiel, Germany.
Ruth A
RA
Schmitz
Department of Biogeochemistry, Helmholtz Centre for Ocean Research (GEOMAR), Düsternbrooker Weg 20, 24105 Kiel, Germany.
Julie
J
LaRoche
Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359 Bremen, Germany.
Marcel M M
MM
Kuypers
D000864:Antarctic Regions; D001267:Atlantic Ocean; D000458:Cyanobacteria / Q000378:metabolism*; D058458:Nitrogen Cycle; D009586:Nitrogen Fixation; D009591:Nitrogenase / Q000378:metabolism; D012336:RNA, Ribosomal, 16S; D012623:Seawater / Q000382:microbiology*; D013559:Symbiosis
D012336:RNA, Ribosomal, 16S; D009591:Nitrogenase