# Role of Exposure on the Microbial Consortium on Historical Rural Granite Buildings
## 1 Introduction  
Granite is one of the most commonly used types of stone in construction worldwide and is known to have been in use since Egyptian times [1]. This igneous rock is widely used in construction because of its sustainability and durability, being a hard rock that is resistant to both heat and ultraviolet radiation. Nonetheless, all materials exposed to the environment gradually deteriorate as a result of physical, chemical, and biological processes [2, 3]. Interactions between the atmosphere and stone invariably lead to the formation of altered surface layers, which eventually results in damage to the original stone. This damage is of great concern in the case of stones in unique buildings and monuments [4]. Information about the biodeterioration of historical granite buildings is very limited relative to what is known about buildings constructed with limestone or sandstone [5], but it is known that the main factors leading to the granite biodeterioration process are the presence of soluble salts [6] and biodeterioration caused by colonizing organisms [7, 8, 9]. Microbial colonization alters the stone substrate causing physical-chemical and aesthetic damage. These negative effects have been widely studied [10, 11, 12, 13, 14, 15]. However, the idea of the protective effect of organisms on stone has recently gained relevance as their function as consolidants, binding particles, and protecting stone from erosion or aggressive substances, as well as from other abiotic factors, has been demonstrated [16, 17, 18]. Both deteriorative and protective actions can be caused by a single species, but can also be dueto the combined, inter-related effects of various different microorganisms, which also depends on the environmental conditions [19; 20; 21]. It is therefore important to understand the complexity, composition, and structure of the community inhabiting the stone of cultural heritage buildings, in order to foresee the risk deterioration degree and, thus, be able to carry out the most appropriate conservation tasks. Due to the interactions between organisms and substrate, with proved positive and negative effects, the development of techniques allowing the study and identification of microbial communities has gained importance as a key issue in the heritage conservation field. In this sense, different techniques have been used over time to identify organisms forming biofilms. Traditionally, their isolation, identification, and study have been carried out by means of classical culture techniques. These culture-dependent techniques have the advantage of being able to perform biochemical and physiological studies in parallel to identification; however, they greatly underestimate existing diversity [22] and favor the rapid growth of opportunistic species, making it difficult to identify other microorganisms that may play an important role [23]. With the emergence of metagenomics and, in particular, next generation sequencing (NGS) techniques, detection capacity has increased exponentially, thus improving the potential and level of detail of microbial ecology studies [24]. NGS techniques enable large amounts of data to be obtained more quickly than with any of the previous methods. Thus, the combination of these modern techniques together with traditional culture-dependent methods would enable the production of more data and a better understanding of specific findings [25]. Numerous studies on the microbial colonization of historical monuments have been carried out using these modern techniques, especially on sandstone, marble, and limestone substrates. In the case of historical granite buildings, studies characterizing microbial communities are scarce [12; 26; 27; 28] and most of them have been carried out using traditional methods [29; 30; 31], with only one using NGS techniques [26].