## 2 Materials and Methods
### The Determination of the Fungal Contamination  
The Koch sedimentation method was used to evaluate the fungal contamination of the air. For the Koch method, Sabouraud agar plates consisting of 40 g/L glucose, 10 g/L peptone, 15 g/L agar, and 50 ug/mL chloramphenicol at pH 5.6 were used [43]. Three plates were placed in four distinct points in the investigated cave and left open for 10 min. swabs were also employed to collect samples from the rupestrian church (Figure 3b). After collection, all samples were sealed using parafilm. All investigations performed on the fungal samples took place in the Molecular Biology Center at the Institute of Interdisciplinary Research on Bio-Nano-Sciences of Babes-Bolyai University in Cluj-Napoca, Romania, according to standard procedures applied to microbiological sample handling [44]. The plates used for the Koch method were incubated at 20 \({}^{\circ}\)C for 10 days. The Omelianski formula (CFU/m\({}^{3}\) air = n\({}^{\ast}\)10,000/S\({}^{\ast}\)k, where n = number of colonies developed on the plate; S = surface of the Petri dish (\(\varnothing\)90 mm); k = air exposure time coefficient: k = 1 for 5 min, k = 2 for 10 min, k = 3 for 15 min, etc.) was used to estimate the fungal colony-forming units (CFUs) present in cave's air. The swabs were washed in 1 mL of saline solution and 150 \(\upmu\)L of the resuspended cells were inoculated on Sabouraud agar plates. For each swab, the inoculations were run in triplicates. The plates were incubated at 20 \({}^{\circ}\)C for 10 days. For every 10-day-old independent colony obtained on plates, the DNA was extracted using the Animal and Fungi DNA Preparation Kit(r) (Jena Bioscience, Jena, Germany) according to the manufacturer's instructions. The polymerase chain reaction (PCR) amplifications were carried out in a 25 \(\upmu\)L final volume consisting of 1\(\times\) MyTaq Reaction Buffer (Meridian Bioscience(r), London, UK), 0.5 mM of each primer (Macrogen Inc., Seoul, South Korea), 1.25 U MyTaq Red DNA Polymerase (Meridian Bioscience(r), London, UK), and 50-100 ng of DNA template. The PCR program consisted of the following conditions: initial denaturation at 95 \({}^{\circ}\)C for 5 min; 35 cycles of 95 \({}^{\circ}\)C for 30 s (denaturation); 56 \({}^{\circ}\)C for 30 s (annealing); 72 \({}^{\circ}\)C for 30 s (elongation); and final elongation at 72 \({}^{\circ}\)C for 5 min. The identification of the isolated fungi was based on the ITS region, and the utilized primers for the PCR amplifications were ITS 1 (5\({}^{\prime}\)-TCCGTAGGTGGAACCTGCGG-3\({}^{\prime}\)) and ITS 4 (5\({}^{\prime}\)-TCCCTCCGTTATTGATAGC-3\({}^{\prime}\)). The DNA fragments generated by PCR processing were extracted from the agarose gel using the commercial kit NucleSpin(tm) Gel and PCR Clean-up (Macherey-Nagel, Ping-Tung, Taiwan) and sequenced using the standard sequencing commercial service at Macrogen (MacrogenEurope, Amsterdam, The Netherlands). Several genera were identified through DNA and metagenomics, including bacteria commonly found in environmental and human-associated habitats. Understanding the specific genera present helps in tailoring conservation strategies to mitigate microbial-induced deterioration [45]. The DNA sequences were submitted to GenBank under the accession numbers PP949376-PP949378. In addition to the determination of microbiological contamination from the air and from the surfaces inside the rupestrian church, the rock samples were also analyzed with an electronic microscope to highlight the fungi that colonize the rocks and their possible action on the base material. For this, the Phenom ProX scanning electron microscope was used, which is particularly useful in studying different types of materials and their micro- and nano-level structural changes. It allows for the selection of an appropriate magnitude to observe fine surface details and is equipped with an energy-dispersive analysis system, which allows for the identification and quantification of the chemical elements present on the surface of the samples.