LAUSR

Desert dust storms are frequent in the Northern Red Sea 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 and 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-nutrient Northern Red Sea, 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 · day 1 and 0.29 ± 0.06 nmol N · L 1 · day , 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. Plain Language Summary Aerosols and dust are regularly transported across the oceans supplying nutrients and trace metals to the surface water. In addition, aerosols may also contain a wide array of different airborne microorganisms (heterotrophic bacteria, virus, cyanobacteria, and fungi) that can be easily transported for thousands of kilometers away from their place of origin within a few days. Here we examined the role of airborne N2 fixers (diazotrophs) in the surface water of the low-nutrient Northern Red Sea during the summer. To this end, 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). Our results demonstrate that airborne N2 fixationmay be an important source of new bioavailable N in the Northern Red Sea, fueling primary production. In accordance with these measurements, two nifH sequences related to cluster III diazotrophs were amplified from the dust samples. Our results suggest that airborne N2 fixation may play an important role in marine environments subjected to high aerosol loads. We further suggest that the role of airborne diazotrophs may likely increase in the future due to global desertification processes and thus an increase in dust deposition.