| Summary: | 2nd International Ocean Research Conference, One planet one ocean, 17-21 November 2014, Barcelona, Spain Uncultured phylotypes of the algal class Prymnesiophyceae were recently discovered in symbiosis with the unicellular nitrogen-fixing cyanobacterium UCYN-A. Here we studied a particular algal phylotype which harbors the most abundant clade of UCYN-A (UCYN-A1). This partnership was detected at station ALOHA and in the North Atlantic Subtropical Gyre close to the Azores. We designed a new probe specific for the UCYN-A1 host (UPRYM69) and applied it by double Catalyzed Reporter Deposition-Fluorescent in situ Hybridization (CARD-FISH) to identify both partners and to assess their cell abundances and distribution. Our sampling includes worldwide marine surface waters taken during the Malaspina expedition. Additionally, several profiles were analyzed in detail to study its vertical distribution pattern. UCYN-A1 host was detected in all ocean basins in stations embracing a wide range of environmental gradients, expanding the previous niche proposed. However, it presented a patchy distribution, being detected in 45% of the stations analyzed. When present, its abundance reached up to ~500 cells ml-1. The vertical trend observed was that the UCYN-A1 host occupied the upper water column and decreased in abundance until disappearing towards the Deep Chlorophyll Maximum (DCM) peak. At surface waters it represented an important fraction (up to 85%) of the prymnesiophyte assemblage, which generally peaked at the DCM. We seldom detected non-associated hosts or free symbionts and, when detected, their abundances were highly correlated suggesting that they derived from disruption and that the association is obligated for both, host and symbiont Peer Reviewed
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