EUROFLEETS Cruise Summary Report Project DIAPICNA: DIAzotrophic PIco-Cyanobacteria in the North Atlantic open ocean: their abundance and importance as a source of new nitrogen at the Azores Front/Current.

In the context of increasing atmospheric CO 2 concentrations, studies understanding and quantifying carbon fixation by oceanic biological production and its rate of removal to the deep ocean are of utmost importance to define the resilience of the Earth’s biogeochemical cycles to anthropogenic forcing. Within the nitrogen biogeochemical cycle, Biological N 2 Fixation (BNF) is the only natural process which can relieve nitrogen limitation of atmospheric CO 2 sequestration. However, dissolved N 2 is available to only a small number of organisms (diazotrophs) producing the necessary enzymes for N 2 fixation and we are still at a rudimentary stage regarding our understanding of unicellular diazotrophs’ distribution and contribution to oceanic BNF. The present research cruise aimed at unfolding the inter-linkages between N 2 and CO 2 fixation mediated by diazotrophs in an area of the North Atlantic Ocean where BNF is suspected to occur, but was never measured. The Azores Current region close to the Mid-Atlantic Ridge, an oligotrophic area where contrasting conditions are found at the interface between temperate and subtropical waters, was investigated for the presence, identity, distribution and activity of diazotrophs from the surface to deeper water layers. A South-North transect including 5 geographical sampling stations (Figs. 1 and 2) was performed to investigate the extent of BNF in subtropical/temperate waters. The focus was in particular on the possible impact of the Azores Front/Current system and of deep sea iron injection by the Rainbow hydrothermal system. The results are expected to contribute substantially to our understanding of the mechanisms leading to the development or limitation of BNF in the open ocean.