Distribution of a consortium between unicellular algae and the N 2 fixing cyanobacterium UCYN ‐ A in the North Atlantic Ocean

Summary The globally abundant, uncultured unicellular cyanobacterium UCYN ‐ A was recently discovered living in association with a eukaryotic cell closely related to a prymnesiophyte. Here, we established a double CAtalysed Reporter Deposition‐Fluorescence In Situ Hybridization ( CARD ‐ FISH ) approach to identify both partners and provided quantitative information on their distribution and abundance across distinct water masses along a transect in the N orth A tlantic O cean. The N 2 fixation activity coincided with the detection of UCYN ‐ A cells and was only observed in oligotrophic (< 0.067 N O 3 −  μ M and < 0.04 P O 4 3 − μ M ) and warm (> 18° C ) surface waters. Parallel 16 S ribosomal RNA gene analyses among unicellular diazotrophs indicated that only UCYN ‐ A cells were present. UCYN ‐ A cells were associated with an algal partner or non‐associated using the double CARD ‐ FISH approach. We demonstrated that UCYN ‐A cells living in association with H aptophyta were the dominant form (87.0 ± 6.1%), whereas non‐associated UCYN ‐ A cells represented only a minor fraction (5.2 ± 3.9%). Interestingly, UCYN ‐ A cells were also detected living in association with unknown single‐celled eukaryotes in small amounts (7.8 ± 5.2%), presumably A lveolata. The proposed ecological niche of UCYN ‐ A as an oligotrophic, mesophilic and obligate symbiotic nitrogen‐fixing microorganism is evident for the N orth A tlantic O cean.