Our findings strongly suggest that microbial life arrives into the hyperarid core of the Atacama from the Pacific Ocean and the Coastal Range of the Atacama mainly in the afternoon hours, to be further dispersed around in the morning hours. Importantly, these findings also reveal not only the time frames in which microbial life may move across the hyperarid core of the Atacama in greater numbers, but also the time of the day at which a higher percentage of viable microorganisms may arrive, as RH increases6,7,12 and UV radiation decreases12 in the late afternoon hours at the hyperarid core, giving microbial life a higher chance to survive the transportation process. Similar to other experiments performed at the hyperarid core6, we detected a low percentage of colonized plates independent of the growth media used, particularly in the case of the Iquique transect. In most cases, none or only one out of ten plates exposed per site/sampling date showed signs of microbial growth after two weeks of observation after inoculation (Table S1). Despite the limitations of the culture-dependent approach, this allowed us to detect a number of new microbial isolates in both transects.

The species detected at site 1 of the Iquique transect suggested the potential source of origin and subsequent transport route of microbial life through the hyperarid core; Oceanobacillus oncorhynchi is a halotolerant obligate alkaliphile first described in watery environments13, which in this case may had arose from either the Pacific Ocean and/or from the coast of the Atacama (the first area of this desert encountered by the winds coming from the ocean and then moving into the hyperarid core) explaining why it is detected at the sites closer to the coast in both transects. Oceanobacillus species were first isolated from deep-sea sediments lending support to this hypothesis14, an observation which also applies to Bacillus oceanisediminis, first isolated from marine sediments from the South Sea in China15. In turn, bacterial species like Bacillus simplex have been isolated from the plant rhizosphere16, suggesting a potential origin from the soils of the top of the Coastal Range of the Atacama, which contain a few plant species that use fog as their main source of water17, plants which are completely absent in the coast of the Atacama and also further inland in the inspected region. A similar case applies to Ophiosphaerella herpotricha, a fungal species known to affect the roots of grass species18.

An analogous analysis applies to the species detected further inland (sites 2 and 3); Terribacillus saccharophilus is a moderately halophilic bacterial species isolated from soils in Japan and is closely related to species of the Oceanobacillus genus19, and may also have originated at the coast of the Atacama and then moved further inland. This could also be the case of Bacillus litoralis, an halophilic bacterial species isolated from a tidal flat of the Yellow Sea in Korea20, while Cladosporium bruhnei belongs to a genus with several species isolated from hypersaline environments21. Other species again suggest an origin at the Coastal Range hills; Aspergillus versicolor is a highly ubiquitous fungus commonly isolated from soil, plant debris and marine environments22, and Chaetomium globosum is a mesophilic saprophytic fungus that primarily resides in habitats like mountain soils across various biomes23.

Similar to the Iquique transect, a number of bacterial species in the sites closer to the coast of the Tocopilla transect also suggest a marine origin; Oceanobacillus oncorhynchi, Bhargavaea cecembensis (isolated from deep-sea sediments samples collected at the Chagos-Laccadive ridge system in the Indian Ocean24), and Brachybacterium paraconglomeratum, which belongs to a small genus containing species isolated from coastal sands25. In turn, Salinicoccus roseus is a moderately halophilic bacterium isolated from solar salterns in Spain26, and Microbacterium barkeri is a moderately halophilic actinobacteria first isolated from sea-water samples taken from Amursky Bay of the Gulf of Peter the Great (Russia)27.

In turn, other species may have also arose from the sparse plant-covered areas of fog oases on top of the hills of the Coastal Range; Solibacillus silvestris is a moderately halophilic bacterium first found in forest soils28, members of the genus Paenibacillus are facultatively anaerobic bacteria and have been isolated from decomposing plant material29, Bacillus amyloliquefaciens is a Gram-positive found as part of the plant rhizosphere30,31, Microbacterium paraoxydans has been isolated from arsenic polluted vegetated soils32, Bacillus firmus is an alkaliphilic bacterium that has been isolated from oil reservoirs and vegetation covered soils33, while Bacillus paralicheniformis has been isolated from soybean and other plants34,35 (the case of Bacillus firmus is of particular interest, because although it was found at site 2, is one of the few microbial species able to survive in María Elena6, the driest place on the Atacama and on Earth, located only eleven kilometers further east from site 2). A higher number of plant-associated microorganisms along the Tocopilla transect may be explained by the fact that the amount and diversity of plants at the Coastal Range hills steadily increases south.

Interestingly, two of the species detected by us have been reported as airborne bacteria; Kocuria flava36, an actinobacteria isolated in Xinjiang, China, and Bacillus altitudinis, collected in the atmosphere at altitudes between 24 and 41 km during a balloon flight from Hyderabad, India37. Whether these two species may have arrived from even more distant places is been now investigated.

Direct DNA extraction from dust particles unveiled a number of bacterial species already detected by cultivation (Oceanobacillus oncorhynchi, Paenibacillus sp, Terribacillus saccharophilus, Arthrobacter sp., Staphylococcus equorum, and Bacillus firmus), (Fig. 8), but also a greater number of bacterial species not detected by the culture dependent method used. Although archaea specific primers were used, no archaeal sequences were found. This is puzzling, and may be explained by the absence of sources of origin for these type of species in the transects analyzed. If one of our transects had crossed the Salar Grande located at the Coastal Range of the Atacama (where archaea has been reported38), we may have detected them, provided that they are as resilient to wind transport as shown for bacteria and fungi.

Figure 8 Main OTUs identified from DNA extracted from dust particles and its distribution at the sites of the Tocopilla transect. Species names in bold highlight bacterial species also detected by cultivation. Values inside site distribution bars indicate percentages of total OTUs. Full size image

Similar to the analysis shown above, the microbial species detected by direct DNA extraction from dust particles again suggest the source of origin for the species found inland; i.e., Oceanobacillus oncorhynchi and Halobacillus (Pacific Ocean/Coastal Range) or the Coastal Range of the Atacama (Lactobacillus curvatus, Bacillus firmus).

Several of the species found by both culture dependent and independent methods used have been reported in the inspected region; i.e., Bacillus simplex39,40, Bacillus litoralis41, Bacillus subtilis42, Aspergillus nidulans43, Penicillium chrysogenum44, Bacillus amyloliquefaciens45, and Geodermatophilus obscurus6.

It is also interesting that 71% of the microbial species detected either by cultivation or direct DNA extraction from the dust samples collected can also use spores to disperse in the environment (e.g., Bacillus and fungal species), suggesting a fraction of the species that may also be arriving as spores, with the remaining 29% of the species (e.g., Microbacter, Brachybacterium) that should be using only wind-transported dust to reach the hyperarid core of the Atacama.

Altogether, the analysis of dust particles collected across the hyperarid core of the Atacama shows that microbial life is able to efficiently move across the driest and most UV irradiated desert on Earth unharmed, particularly in the late afternoon hours. Considering the wind speeds measured in the studied transects (~10–20 km/h), and the distances covered (~100 km), the total transport time required by microbial life to reach deep into the hyperarid core of the Atacama from their source origin is only 5–10 hours, transport times that will vary depending on the season of the year. Thus, the result presented here are consistent with the hypothesis that the microbes detected in the soils of the hyper-arid core of the Atacama - delineated by the presence of nitrate deposits - are carried there by the wind with transport times of days or less. Given the aforementioned explanations, and that the transects studied are separated by more than 200 kms (a distance selected with the purpose of understanding whether these are processes that extend to all of the Atacama), we may assume with confidence that our conclusions apply to the other regions of this desert, that is, that the main driver for the transport of microbial life in this desert is the presence and speed of winds.

Our data unveiled the potential point of origin for the species found at the hyperarid core of the Atacama in the coasts of this desert and its Coastal Range. What happens to these species after arriving to the hyperarid core is now been investigated, as well as whether the atmospheric transport of microbial life and subsequent arrival may be affected by the recent unusual rain events in this region45.

Implications for mars

It is well known that Mars is constantly affected by winds46 and dust storms that in many cases can cover its entire surface47,48,49. Our results suggest that any potential viable microbial life on Mars may similarly spread all over significant distances using dust-mediated transport, either now or in the past, and that extreme aridity does not fully prevent this from taking place.

The Martian atmosphere is full of dust, with loads that greatly fluctuate with the season of the year50. Airborne dust interacts with solar visible and thermal infrared radiation, perturbing atmospheric heating, modifying the circulation and thermal structure of the atmosphere, and ultimately leading to sudden atmospheric perturbations which cause the uplift of dust (thus potentially collecting new microbial cells from different regions of the planet) and the subsequent development of planet-encircling sandstorms48,49 (which then may redistribute these microbes). Therefore, it would be interesting to collect and analyze fresh samples of airborne Martian dust in search of potential biosignatures.

The potential transportation of microbes by Martian dust would have been (or may be) of particular relevance in a planet where habitable niches may have been separated both in time and space51, particularly after the Noachian period52. Favorable conditions for the appearance and continuity of life on Mars may have been met at different times and discrete places, spanning a minimum time range of about 2 million years53. As it has been argued that the lack of continuously hospitable conditions would have deterred a continuous biological evolution on Mars53, our results offer a way to sort out this problem, as the lack of inter-connectivity between dispersed habitable environments by ancient water flow or tectonics may have been balanced by wind dispersal of microorganisms during extended periods of time. As a consequence, the aeolian distribution of life may have allowed some degree of evolution of microbial Martian life forms.

Finally, our results are of critical application for planetary protection, as terrestrial microorganisms hitchhiking in rovers and landers (and their discarded landing material) may have been widely dispersed all over the Martian surface by planetary-level dust storms54.