## 4 Discussion
Cyanobacteria, as many other microorganisms, can form biofilms, which have huge weathering potential [55] and can cause both physical and chemical degradation of the stone. The physical actions involve breaking of the surface and penetration of the microorganisms between the grains of the rock with consequent decreases in grain cohesion, while chemical actions include dissolution and chelating processes bringing chemical changes to the stone substrate [56]. In our study, through optical microscopy and the use of SEM, we could visualize areas of the samples with cyanobacterial biofilms that were heavily weathered. In fact, associated with cyanobacteria aggregates, we observed disintegration of the rock, with loose mineral fragments and consequent detachment of the black crust (Supplementary Figure S6). Species of the genus _Gloecapas_, which was relatively abundant in our samples, and of the genus _Leptolyngbya_, also detected in our metagenomic analysis but with lower proportions, were, for example, found responsible for the formation of biofilms in historic stone monuments, with resulting bioweathering of the lithic substrate [57]. Within the Actinobacteria, the genus _Rubrobacter_ was very dominant in all the samples (as clearly visible in the heatmap of Figure 4). _Rubrobacter_ spp. are often associated with desert varnish and in general as a bacterium inhabiting stones in desert environments [16,58]. This genus is known to be highly resistant to many types of environmental stress, such as exposure to gamma and UV radiation, desiccation, high temperature fluctuations, low water availability and high levels of salts [59,60]. In fact, _Rubrobacter_ spp. are often reported to grow on walls and stone monuments rich in salts, causing the so-called rosy discoloration due to the release of carotenoid pigments such as Bacteriowberin [61,62,63]. The genera _Pontibacter_, _Hymenopteraacher_ and _Adhaeribacter_ are also highly radiation resistant bacteria [64] with the ability to synthesize carotenoids and indeed show pink to red shades [65,66]. The eukaryotic communities were affiliated with the fungi kingdom, which was mostly represented by the phylum Ascomycota. High biodiversity at the genus level was observed, but each genus showed low proportion and no dominant taxa, except for a few genera contributing with slightly higher proportions, such as _Letharia_, _Alternaria_, _Lachnellula_, _Metharizium_ and _Talaromyces_. Most of the fungi detected are not stone-dwelling species, but they are found ubiquitously in the environment and are probably deposited on the stone as spores. In fact, we found little evidence that fungi are established on the rock surface. Within the fungi detected in this study, only the order _Chaetothyriales_, with _Exophilata_ spp. and the genus _Contiosporium_, with the _Contiosporium apollinis_ species, might actually be established on the rock. The aforementioned fungi are in fact part of the black fungi, or black meristematic fungi group, which have an extraordinary tolerance to extreme environmental conditions and are known to inhabit stone surfaces in hot deserts and moderate climates [67, 68, 69]. Previous studies mentioned the presence of fungi and lichens, such as _Caloplaca_ sp., as a weathering agent of rock substrates due to the production of oxalic acid and consequent formation of calcium oxalates that can facilitate the disintegration of the calcareous matrix [33]. In this study, however, we could not detect the presence of calcium oxalates. Further culture-dependent and culture-independent microbiological studies are being carried out to have better insight of the fungal communities associated with the rocks of petroglyph sites. The characterization of the black crust through XRF and SEM-EDX analysis confirmed that the varnish is composed of clay minerals and Fe and Mn oxides, as reported by many studies [18, 23, 24]. Specifically, the varnish is highly enriched in Mn compared to the windborne dust deposited in the micro-basins of the surface.