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

Microbial dinitrogen (N) fixation, the nitrogenase enzyme-catalysed reduction of N gas into biologically available ammonia, is the main source of new nitrogen (N) in the ocean. For more than 50 years, oceanic N fixation has mainly been attributed to the activity of the colonial cyanobacterium Trichodesmium. Other smaller N-fixing microorganisms (diazotrophs)-in particular the unicellular cyanobacteria group A (UCYN-A)-are, however, abundant enough to potentially contribute significantly to N fixation in the surface waters of the oceans. Despite their abundance, the contribution of UCYN-A to oceanic N fixation has so far not been directly quantified. Here, we show that in one of the main areas of oceanic N fixation, the tropical North Atlantic, the symbiotic cyanobacterium UCYN-A contributed to N 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 N, contributing ∼20% to N 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 N 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 N fixation in a much larger oceanic area than previously thought. Based on their high rates of N fixation and cosmopolitan distribution, we hypothesize that UCYN-A plays a major, but currently overlooked role in the oceanic N cycle.