Airborne microbes contribute to N2 fixation in surface water of the Northern Red Sea

Desert dust storms are frequent in the Northern Red Sea (NRS) region, providing nutrients (i.e., PO₄) and trace-metals (i.e., Fe) that may stimulate dinitrogen (N₂) fixation. Dust also carries a high diversity of airborne microbes (bacteria, archaea), including diazotrophs, that may remain viable during transport and upon deposition. Here we evaluate the impact of atmospheric deposition and its associated airborne diazotrophs on N₂ fixation in the surface water of the low-nutrients NRS, using mesocosm bioassay experiments. We compared the chemical (nutritional) and sole airborne microbial impact of aerosol additions on N₂ fixation using ‘live-dust’ (release nutrients/trace-metals and viable airborne microorganisms) and ‘UV-killed dust’ (release only chemicals). Airborne diazotrophy accounted for about one-third of the measured N₂ fixation (0.35±0.06 nmol N L^-1 d^-1 and 0.29±0.06 nmol N L^-1 d^-1, for ‘February 2017’ and ‘May 2017’, ’live-dust’ additions, respectively). Two nifH sequences related to cluster III diazotrophs were amplified from the dust samples, consistent with the N₂ fixation measurement results. We postulate that the deposition of viable airborne diazotrophs may enhance N₂ fixation, especially in marine provinces subjected to high aerosol loads. We speculate that the relative contribution of airborne N₂ fixation may increase in the future with the predicted increase in dust deposition.