## 4 Discussion
[PERSON] and colleagues established that humidity can activate numerous metabolic pathways, including those regulating the production of primary and secondary metabolites [54]. Microbiological metabolites responsible for the biodeterioration of stone surfaces include siderophores, inorganic acids (e.g., sulfuric, carbonic) and organic acids (e.g., citric, oxalic) [53,55]. Therefore, it is not excluded that an increase in humidity or seasonality may affect the secretion of specific metabolites, including organic acids. This is important because organic acids have a different mode of action than carbonic acid (large number of carbonates are sparingly soluble in water). Many organic acids exhibit chelating activity in contrast to carbonic acid, which is a well-known feature described in chemistry handbooks. Also, the rock type determines the type of metabolic pathways of microorganisms. Shotgun metagenomics analyses of bacterial communities on granite revealed a high number of genes associated with acid tolerance and chemotaxis, while the bacteria inhabiting limestone had a high proportion of genes involved in photosynthesis and radiation resistance which seems to be related to the occurrence of lichens [48]. In addition, the domination of acid tolerant microbial taxa (Acidobacteriaceae, Beijerniciaceae and Methylocystaceae) was found on granite tombstones, while alkaliophilic (_Spirosoma, Rubbillinicchioum_ and _Truepera_) constituted most abundant microbial taxa on limestone tombstones surfaces. The above observations allowed the authors to hypothesize that microorganisms that inhabit different types of stones may be predisposed to different (symbiotic versus free-living lifestyles) ecological strategies and that the main factor influencing the diversity of bacterial taxa are pH preferences of these microorganisms [48]. Our observations, in turn, may indicate the adaptation of microorganisms to the changing seasons, which results in the intensification of sandstone weathering in summer (strongly acidifying properties of bacterial strains), while in winter the alkalization of the substrate may be related with the biomineralization process, especially precipitation of amorphous or microcrystalline forms of some minerals by hydrolysis [56]. Unfortunately, there are only a few studies concerning sandstone surfaces. [PERSON] et al. [47] observed that both sandstone pH, as well as nitrate concentration depends on sample location. The above may be due to the fact that sandstone is not a homogeneous rock and, in addition, weathered sandstone materials may have contained acidifying and alkalizing compounds secreted by microorganisms during their growth. The authors also confirmed that alkalizing bacteria (Nitrospirae) were presented in exfoliated sandstone materials loosely on sandstone block of two Angkor temples [47]. The accumulation of polluting particles on the stone surfaces, especially as an effect of London smog during heating period and photochemical smog during summer, results in the formation of crust enriching the substratum for colonization of microorganisms and thereby could influence the microbial biodiversity pattern [57]. One of the most important microbial factors influencing colonization of exposed stone surfaces is the presence of chemoorganotrophs that trap minerals and organic substances from the air or nutrients from the stone [57]. Microorganisms utilizing organic substances from the air or nutrients from the stone can often be found in subaerial biofilms [58] and our data corroborate these findings. The presence of _Methylobacterium_ in bioenoses located in sites 1-3 was observed, which corresponds well with the presence of methane in atmospheric air. Also, Nocardioidaceae, which can use hydrocarbons compounds from air pollution, were detected on the Pergola surfaces.