## 4 Discussion
### Description of Culturable Microbial Communities  
Microorganisms are ubiquitous in the environment, and the atmosphere is no exception. However, airborne bacterial communities remain relatively unexplored. Enhancing our understanding of these communities and the mineral composition of dust particles is crucial for comprehending the impact on biodiversity and health. In total, 23 culturable microorganisms were isolated, being mostly present in samples M2 (ncm = 14) and M3 (ncm = 6) (Figure 13). In contrast, sample M1 showed the lowest number (ncm = 3). As previously stated, sample M1 not only had a finer granulometry, but also the longest backward trajectory compared to sample M2 (Tables 2 and 4 and Figure 2). The major distance travelled might imply a major dust elevation from the soil on the ground, causing a bacterial deprivation. Furthermore, the backward trajectory's length, corroborated by the wind speed, would justify the residence time in the atmosphere, compatible with the time needed for bacterial growth (Table 2). Prior studies revealed that the bacterial abundance depended on dust granulometry [63; 64; 65]. The major abundance corresponded to sample M2, with its coarser granulometry and minor percentage of clay minerals (Tables 4 and 6, Figures 3 and 4). [PERSON] et al. (2021) [65] observed a higher bacterial abundance in coarse particles (particles > 10 \(\upmu\)m), likely being derived from local sources. In our case, the highest bacterial abundance bound to the coarser granulometry might be attributed to the atmospheric dust genesis, considering the minor distance from soil. Thus, bacteria would travel attached to mineral particles or inside polynineral aggregates with strategic microsites suitable for bacterial growth. The visualisation of bacteria over the surface of mineral particles, as well as the geomicrobial interactions observed by SEM-EDX, would support these statements (Figures 7-12). Particle size had also an influence on the airborne bacterial communities. Some studies found that spore-forming microorganisms were more abundant in large particles, whereas viable microorganisms were mostly present in particles of reduced size [63; 64]. Additionally, [PERSON] et al. (2021) [65] reported a major abundance of pathogens in fine particles (particles < 2.5 \(\upmu\)m), which can be suspended in air for a longer period of time. Therefore, fine particles pose a threat to human health due to their biological content and respirable size. In total, 19 out of 23 isolated culturable microorganisms were identified by 16S RNAr sequencing (Table 8). The identified microorganisms belonged to _Pseudomonas_, _Bacilota_ and _Actinomycetota_, being consistent with the dominant phyla in other dust studies [9; 65; 67]. _Pseudomonas_ (nmc = 6) was the predominant phylum in sample M2, followed by _Bacilotta_ (nmc = 4) and _Actinomycetota_ (nmc = 2). On the other hand, sample M3 showed the same proportions in each phylum. Sample M1 was represented by only one microorganism belonging to _Bacilota_, corroborating its low bacterial abundance. Most of the identified microorganisms had previously been isolated from soil samples, with some of them being described as extremophiles. Bacteria with beneficial effects on plants, such as Plant Growth-Promotion Rhizobacteria (PGPR), were detected in our samples. _Peribacillus frigoritolerans_, isolated for the first time in arid soils from Morocco, is an extremophilic microorganism, involved in soil bioremediation, with a promising PGPR activity [68; 69; 70]. _Bacillus safensis_ subsp. _safensis_ and _Paenarthrobacter nitroguajacolicus_ possess PGPR properties that may be potentially viable even in extreme environments due to their tolerance to harsh conditions [71; 72]. On the other hand, _Pseudomonas alloptida_ and _Pseudomonas hunanensis_ have PGPR properties and are also efficient in soil bioremediation [73; 74].