Diazotrophs Distribution and N2 fixation Drivers in the Northern North Atlantic Ocean

The biological conversion of dissolved dinitrogen (N₂) gas by bacterioplankton referred to as diazotrophs, constitutes the major source of nitrogen (N) to the global ocean. Marine primary production (i.e., biological fixation of atmospheric carbon dioxide, CO₂ dissolved in the ocean) is mostly limited by the availability of dissolved inorganic nitrogen (e.g., nitrate, ammonium) in the global ocean. Surveys of the nitrogenase reductase-encoding gene (nifH), used as a diazotroph phylogenetic marker, complemented by metagenomic assembly and binning approaches have revealed that the ability of marine microbial communities to fix gaseous N₂ is widely distributed in the surface ocean. While there is some molecular marker evidence for the presence of diazotrophs in high latitude regions mostly shelf areas, concurrent N₂ fixation observations are still too scarce and scattered to determine their biogeochemical importance. During the cross-Atlantic GEOVIDE expedition from Lisbon, Portugal to St. John’s, Newfoundland, Canada (GEOTRACES GA01, May-June 2014), we carried out rates measurements of N₂ fixation and primary production, surveyed the nifH gene distribution and abundance of predominant diazotroph groups, to be gauged against key environmental variables monitored during or at the time of our sampling such as seawater trace metals concentrations, climatological nutrient data and remotely sensed atmospheric depositions. We aim at gaining a better understanding of the factors regulating diazotrophic activity in this temperate to sub-polar basin, while highlighting the important role played in those regions by non-photosynthetic diazotrophs, i.e. unicellular diazotrophic cyanobacterium of group A (UCYN-A) and non-cyanobacterial diazotrophs (i.e. Proteobacteria, Firmicutes, and Archaea).