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Abstract

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 North Atlantic Ocean. The N-2 fixation activity coincided with the detection of UCYN-A cells and was only observed in oligotrophic (<0.067 NO3- mu M and < 0.04 PO43- mu M) and warm (>18 degrees C) surface waters. Parallel 16S 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 Haptophyta 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 Alveolata. The proposed ecological niche of UCYN-A as an oligotrophic, mesophilic and obligate symbiotic nitrogen-fixing microorganism is evident for the North Atlantic Ocean.