The classical paradigm about marine N2 fixation establishes that this process is mainly constrained to oligotrophic tropical and subtropical regions, and attributed mostly to the filamentous 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 10 cruises in the upwelling ecosystem off NW Iberia in order to: 1) quantify the magnitude of N2 fixation, 2) investigate its biogeochemical role as a mechanism of new nitrogen supply, and 3) 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. The comparison with nitrate diffusive fluxes reveals the minor role of N2 fixation (<5%), as a mechanism of new nitrogen supply into the productive euphotic layer. All the N2 fixation activity was detected in the smaller-sized (<10μm) fraction. Results obtained through phylogenetic analyses by Illumina® (NGS technology) show that the diazotrophic community was dominated by UCYN-A (the most abundant sublineage UCYN-A2), bacteria from Class γ-Proteobacteria and from Phylum Firmicutes. However, the diazotrophic community composition exhibited an important seasonal variability, being the highest diversity associated with autumn and winter mixing and downwelling conditions. Overall, our findings support the emerging view that mesotrophic regions should be considered in global budgets of marine N2 fixation.

Xunta de Galicia (EM2013/021)

Conference communication