extended abstractThe classical paradigm about m rine N2-fixation establishes that this
process is
mainly constrained to
oligotrophic tropical
and subtropical regions and attributed
mostly to the cyanobacterium Trichodesmium.
However, the development of molecular techniques led to the discovery of a larger variety
of marine
diazotrophs, which extends the range of environments where N2-fixation may be relevant. Between February 2014 and December 2015 we carried out 16 cruises in the upwelling ecosystem off NW Iberia with the following goals: 1) to quantify the magnitude of N2-fixation, 2) to investigate its biogeochemical role as mechanism of new
nitrogen
supply, and
3) to identify and quantify the main diazotrophs in the region under contrasting
hydrographic regimes. Our results indicate that the magnitude of N2-fixation in this region is comparable to the lower-end of rates described for subtropical regions. All the N2-fixation activity was detected in the smaller- sized (<10µm) fraction. The comparison with nitrate diffusive fluxes reveals the minor role of this process (<1%) as a mechanism of new nitrogen supply into the productive euphotic layer. Results obtained through phylogenetic analyses by Illumina® (NGS) show that the composition of the diazotrophic community presents a seasonal variability depending on hydrographic conditions. Additional expe iments carried out in the field and in the lab demonstrate that 15N-labeled contaminants included in some commercial 15N2 stocks are assimilable by non-diazotrophs organisms. This could result in an up to 15-fold overestimation of N2-fixation rates. Overall, our findings support the emerging view that mesotrophic regions should be considered in global budgets of marine N2-fixation.RADIALES-20 (IEO), NICANOR (Xunta de Galicia, EM2013/021
