###### Abstract  
Petroglyph sites exist all over the world. They are one of the earliest forms of mankind's expression and a precursor to art. Despite their outstanding value, comprehensive research on conservation and preservation of rock art is minimal, especially as related to biodeterioration. For this reason, the main objective of this study was to explore the factors involved in the degradation of petroglyph sites in the Negev desert of Israel, with a focus on biodegradation processes. Through the use of culture-independent microbiological methods (metagenomics), we characterized the microbiomes of the samples, finding they were dominated by bacterial communities, in particular taxa of Actinobacteria and Cyanobacteria, with resistance to radiation and desiccation. By means of XRF and Raman spectroscopies, we defined the composition of the stone (calcite and quartz) and the dark crust (clay minerals with Mn and Fe oxides), unveiling the presence of carotenoids, indicative of biological colonization. Optical microscopy and SEM-EDX analyses on thin sections highlighted patterns of weathering, possibly connected to the presence of biodeteriorative microorganisms that leach the calcareous matrix from the bedrock and mobilize metal cations from the black varnish for metabolic processes, slowly weathering it. petroglyphs; Negev desert; biodeterioration; nanopore sequencing technology; metagenomics; analytical techniques +
Footnote †: journal: Accepted: 5 July 2022  
play a significant role in the degradation patterns of the stone [10]. Biodeterioration includes surface alterations by crusts and pigments that can change the color of the surface [11], physico-chemical disintegration of the stone material, and dissolution of the lithic substrate by inorganic and organic acids [12]. For example, hyphal growth and biofilm development can cause physical disruptions and cracks in stone materials, while the release of acids can dissolve carbonates, such as limestone, as well as quartz, causing mineral leaching [13]. All these processes contribute to weakening the stone matrix. In the Negev desert of Israel, the majority of petroglyph sites are spread throughout the Negev central highlands and offer a window into the life and culture of desert people who inhabited the region since prehistoric times [14]. The petroglyphs are considered to be from 3000 BCE [15] and include figurative images (zoomorphic and anthropomorphic), geometric shapes, symbols and inscriptions. The engravings are carved through a dark crust, the so called desert or rock varnish, which coats the local Eocene and late Cretaceous limestone rocks [16]. Desert varnish is defined as a thin (10-200 \(\upmu\)m), dark coating highly enriched in manganese that forms on exposed rock surfaces in arid and hyper-arid environments. It is composed of clay minerals and amorphous silica (-70%) in a matrix of poorly crystallized manganese (Mn) and iron (Fe) oxides and hydroxides [17; 18; 19]. Despite the fact that geochemical aspects of rock varnish have been thoroughly investigated [19; 20; 21; 22; 23; 24] and it is believed that its primary source of material is airborne dust, there is an ongoing debate about the exact process of formation and specifically whether or not microorganisms are involved in this process [5; 25; 26]. In a recent study, [PERSON] et al. [27] proposed a new hypothesis for varnish formation. They state that Cyanobacteria accumulate manganese as a nonenzymatic antioxidant system and that, consequently to the cells death, the manganese-rich residue left behind is oxidized to generate the manganese oxides present in the varnish. Cyanobacteria are often reported as one of the dominant phyla present in rock varnish, along with Actinobacteria, Proteobacteria, Bacteroidetes and Chloroflexi [16; 28]. In fact, despite the harsh environmental conditions, microbial studies revealed a broad spectrum of stone-dwelling microorganism associated with desert varnish [29; 30].