## 2 Materials and Methods  
### Study Location and Sampling  
Two petroglyph sites in the Negev desert were considered for this study (Figure 1a,b): one is located in the central highlands, close to the ruins of the ancient city of Avdat (N 30\({}^{\prime\prime}\)47\({}^{\prime}\)10\({}^{\circ}\); E 34\({}^{\prime\prime}\)46\({}^{\prime}\)20\({}^{\circ}\)), and the second one is in the west highlands close to the community settlement of Ezuz (N 30\({}^{\prime\prime}\)48\({}^{\prime}\)5\({}^{\circ}\); E 34\({}^{\prime\prime}\)28\({}^{\prime}\)30\({}^{\circ}\)).  
Both areas are considered an arid climatic zone. The annual temperature in this region ranges between \(-3.5\)\({}^{\circ}\)C in winter and \(40\)\({}^{\circ}\)C in summer (Israel Meteorological Service, Sde Boker station), while precipitation ranges from 90 to 100 mm, mostly during the winter season [34,35]. There are about 200 nights of dew/year, providing abundant liquid water for microbial lithobiontic colonization in this desert [36].  
Sampling was carried out during winter (February and March) 2021. In these months, meteorological measurements were implemented with an in situ monitoring system (situated at Carney Avdat farm, about 100 m from the petoglyph sites) to define local environmental factors. The monitoring system contained a Campbell CR6 data logger (Campbell Scientific Inc., Logan, UT, USA) with sensors for air temperature (\({}^{\circ}\)C), air relative humidity (RH), and rock surface temperature (thermocouples). In addition, it can also measure the amount of rain received in the area. The data are recorded every 15 min and every whole hour. Thus, it was found that the air temperature from January-April 2021 ranged from a minimum of 1.9 \({}^{\circ}\)C to a maximum of 38.5 \({}^{\circ}\)C, with daily temperature fluctuation up to 20 \({}^{\circ}\)C, and the area received 22.5 mm of rain (Supplementary Figure S1a,b). Additionally, previous measurements revealed that the average rock surface temperature was relatively higher than the average air temperature, reaching up to 56.3 \({}^{\circ}\)C in summer.  
The stone samples were collected from similar rock types (limestone covered with desert varnish) in close proximity to the petoglyphs using a hammer and chisel previously sterilized with 70% ethanol (Figure 1c,d). Seven sampling points were chosen, and from each one different slabs of rock surface, about 3 cm \(\times\) 3 cm in size, were taken and placed in sterile plastic bags (Table 1). For each sampling point, part of the slabs was transferred to the Ben Gurion University of the Negev, Be'er Sheeva, Israel where Raman and XRF analysis were done, and part to the microbiology laboratory of the Academy of Fine Arts of Vienna, Austria, where the rest of the analyses were performed.  
Figure 1: Map of the Negev desert indicating the location of the two petoglyph sites (**a**); view of the rock art site in Ezuz with examples of rock engravings (**b**); image of a sampling point (**c**); and the sample observed by stereomicroscope (6\(\times\)) (**d**).  
Six of the samples (AV1, AV2, AV3, EZ1, EZ2, EZ4) were prepared as polished petrographic thin sections. The samples were vacuum-impregnated with epoxy resin from Struers, Denmark. For better visualization of the porosity of clear minerals, the resin was dyed with a blue dye (EpoBlue from Buehler GmbH, Dusseldorf, Germany). After impregnation, the samples were cut into thin sections, mounted on glass slides and polished to approximately 30 \(\upmu\)m in thickness. The samples were left uncovered for analysis by transmitted and reflected light optical microscopy and SEM-EDX.