The Relationship between Air-Mass Trajectories and the Abundance of Dust-Borne Prokaryotes at the SE Mediterranean Sea

Abstract

Airborne prokaryotes are transported along with dust/aerosols, yet very little attention is given to their temporal variability above the oceans and the factors that govern their abundance. We analyzed the abundance of autotrophic (cyanobacteria) and heterotopic airborne microbes in 34 sampling events between 2015–2018 at a coastal site in the SE Mediterranean Sea. We show that airborne autotrophic (0.2–7.6 cells × 103 m−3) and heterotrophic (0.2–30.6 cells × 103 m−3) abundances were affected by the origin and air mass trajectory, and the concentration of dust/aerosols in the air, while seasonality was not coherent. The averaged ratio between heterotrophic and autotrophic prokaryotes in marine-dominated trajectories was ~1.7 ± 0.6, significantly lower than for terrestrial routes (6.8 ± 6.1). Airborne prokaryotic abundances were linearly and positively correlated to the concentrations of total aerosol, while negatively correlated with the aerosol’s anthropogenic fraction (using Pb/Al or Cu/Al ratios as proxies). While aerosols may play a major role in dispersing terrestrial and marine airborne microbes in the SE Mediterranean Sea, the mechanisms involved in the dispersal and diversity of airborne microorganisms remain to be studied and should include standardization in collection and analysis protocols.

Aerosol Microbiome over the Mediterranean Sea Diversity and Abundance

“…Bacterial abundances in aerosols collected over the MS were in agreement with previous studies from the eastern Mediterranean coast [10] and the Atlantic, Pacific, and Indian ocean basins [1], yet were lower than those reported in the East China Sea [54] and the Red Sea [55]. Rahav et al [10] measured the abundance of airborne prokaryotes at a coastal site located at the easternmost MS during 34 sampling events (between 2015 and 2018) and found that abundances were positively correlated to the concentration of aerosols in the air (mg m −3 air). Here, however, we did not find such a correlation, likely because the range of concentrations during non-dust-storm conditions, represented by our samples, was relatively small in comparison to previous studies.…”

“…To our knowledge, there are only two other studies of airborne microbes in the MS during non-storm conditions [10,32]. The study site of Gat et al [32] was done at a coastal city in Israel (~10 Km away from the shoreline) and the study site of Rahav et al [10] was at the rooftop of a building directly next to the ocean. Thus, these studies represent different ecological systems than the open ocean.…”

“…We used filters to collect aerosols, but different studies have used other techniques, such as liquid impingement [69][70][71] or electrostatic precipitation [72][73][74]. Similarly, we measured bacterial abundance directly from filters after sonication to promote detachment from the filter, while others used different methods, such as qPCR [54], culture-based methods [3] and flow cytometry [10]. As a result, it is difficult to reliably compare results between studies, even if the sampling site and environmental conditions are similar.…”

“…These airborne microbes can originate from both land [2,3] and the ocean [4,5]. Upon aerosolization, wind can transport microbes over great distances, including across large ocean basins and seas [6][7][8][9][10]. The residence time of microbes in the air can reach up to seven days [1], which enables them to cross thousands of kilometers.…”

A cross-biomes bacterial diversity shed light on ocean-atmosphere microbial transmission

“…Actinobacteria abundance was also significantly higher in the Atlantic AMBL compared to the water (t-test, p-value < 0.0001). Airborne Actinobacteria and Firmicutes have been previously connected to desert dust samples in the Eastern Mediterranean (23)(24)(25), and detected in different studies of airborne bacteria (26)(27)(28). In addition, Firmicutes are usually more abundant in soils than in marine surface water (2,21).…”