AIRBORNE MICROBES CONTRIBUTE TO N₂ FIXATION IN THE SURFACE WATER OF THE NORTHERN RED SEA; A MESOCOSM APPROACH

Desert dust storms are common in the Northern Red Sea (NRS) region, providing nutrients and trace-metals that may stimulate N₂ fixation. Dust also carries a high diversity of airborne microbes (bacteria, archaea), including diazotrophs, that may remain viable during transport. 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 the chemical constitutes). 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 oligotrophic marine provinces subjected to high aerosol loads, and that the relative contribution of airborne N₂ fixation may increase in the future with the expansion of oligotrophic areas of the ocean and increased dust deposition.