## 4 Discussion
### Microscopy Analysis of Iberulites and Dust Samples  
CLSM revealed the presence of organic matter in the iberulites' surfaces (Figure 6). This finding would be supported by surface chemical composition analysis (XPS), indicating a high proportion of organic C in the dust particles' surfaces (Table 7). New evidence of biological activity in iberulites was recognised through SEM (Figure 7). The iberulite rind was partially covered by some filaments (Figure 8). On the biofilm, some polymicrobial aggregates co-associated with entrapped mineral particles were observed. Biological spore-like forms in the external rind were previously visualised in iberulites, suggesting that active biologic agents might be transported attached to cavities, crevasses, cracks and any other surface irregularities in the external rind [16]. Thus, iberulites would provide protection against UV and a nutritive medium for microorganisms, acting as a potential source of microorganisms beneficial for plant growth. Mineralogical composition was also illustrated by the SEM images. Figure 9 illustrates a quartz grain, a major constituent in dust samples. The general roundness would evidence the aeolian transport. Ellipsoidal nano-sized particles over the surface might be bacterial cells or spores, providing new evidence of the biological activity in atmospheric dust. [PERSON] et al. (2013) [57] observed similar morphologies in sand from Saharan soils. The mineral-bacteria association was clearly suggested by SEM. Figures 10-12 show different examples of microorganisms attached on mineral surfaces or as part of mineral aggregates. During atmosprogenesis processes, mineral particles can also form unions or even cover bacterial cells, with these being either isolated or in colonies. Figure 10 depicts a bacterial cell over a mineral grain of dolomite. Figure 11 illustrates a bunch of bacteria, a small colony, merged in an aggregate of mineral particles, in which the presence of a kaolinite particle is hinted. In Figure 12, some bacteria appear to be partially covered by tiny platelets, which might be kaolinite particles due to their crystal habit (with some clear hexagonal edges). These particles seem to be neoforming and regrowing amid the space they share with microorganisms. The relationship between bacteria and mineral particles in the atmospheric dust studied here is demonstrated with SEM. This relationship can be formulated using at least four different approaches:  
1. Bacteria might use the mineral surfaces to achieve major stability and stay in the air for a longer time period, creating polymicrobial interactions in close spatial association during the transport. Thus, both dust particles and iberulites contribute primarily to the transport and dispersion of microorganisms in the atmosphere in a continuous and essential process [58]. 2. Bacteria seem to grow and even form colonies over particles, provided humidity is high enough, in microsites within mineral particles (Figure 11), aggregating around them. Dust suspended in the air would account for a unique culture medium, as a kind of soil solution, though it has never been described as such. 3. The third piece of evidence, and the most noteworthy, is the relationship between microorganisms and larger airborne particles--the iberulites--which resembles a \"symbiosis\", in this case, between biotic and abiotic material. Both subjects receive a mutual benefit and share a particular bond, creating polymicrobial aggregates. On the one hand, microbial cells find shelter and protection against UV radiation and extreme temperatures. Microsites within mineral particles might retain some water and scarce nutrients, being subsequently suitable for bacterial growth. On the other hand, mineral particles improve their consistence owing to the bacterial exopolysaccharides, which may act as cement, co-adjuvant or, in other words, as an aggregating substance for agglomeration; a biofilm (Figure 8). 4.