Distribution of a consortium between unicellular algae and the N2 fixing cyanobacterium UCYN‐A in the North Atlantic Ocean. (17th April 2014) - Krupke, Andreas
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 North Atlantic Ocean. The N2 fixation activity coincided with the detection of UCYN‐A cells and was only observed in oligotrophic (< 0.067 NO3− μM and < 0.04 PO43− μM) and warm (> 18°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.