# Role of Exposure on the Microbial Consortium on Historical Rural Granite Buildings
## 3 Results and Discussion
### Next Generation Sequencing
The values of the Shannon index (Figure 6) indicated that diversity was generally highest among the bacteria, followed by the fungi and green algae. However, regarding Bacteria diversity, no significant differences were found between locations or orientations (\(p>0.05\)). In the case of fungi and green algae, a greater diversity of these organisms was found on the N-facing walls (\(p<0.05\)). However, the low diversity observed in the green algae group may be due to the use of the culture-dependent technique, which generally reduces the variability detected [22]. In terms of abundance, the orders of bacteria found in highest proportion were the Sphingomonadales and the Acidobacterialies. Most of the fungi present in the microbial consortium are lichenized fungi belonging to the order Lecanorales, although non-lichenized fungi from Capnoidales and Chateothyriales orders are also present in large proportion. Lichenized fungi were distributed differently in relation to orientation, and they were more abundant on the W-facing walls. In the case of green algae, one of the two most abundant species, _Trentepohlia_ cf. _umbrina_, mainly occurred on N-facing walls, while the other, _Desmococcus olivaceus_, was found on W-facing walls. Moreover, diatoms were only present in the microbial consortium on the N-facing wall of the BAR church, which confirms the damper conditions on this wall. The low abundance of cyanobacteria on the church walls was noteworthy, especially relative to their presence in biofilms formed on other types of stone. Thus, for instance, cyanobacteria were the most abundant microorganisms in biofilms developed on limestone and marble [12; 28]. However, the low abundance of cyanobacteria in the biofilms under study is consistent with the characteristics of biofilms on urban granite buildings in NW Spain [26]. In addition, they did not show any preferential distribution regarding orientation but did show a preferential distribution by location as they were only present on one church (ROC). The presence of cyanobacteria exclusively at ROC church may be due to the absence of close shade-producing structures, which are present in the case of the other churches. The absence of these structures results in a higher solar exposure of the west walls of ROC in relation to the west walls of SMV and BAR, favoring the development of cyanobacteria [87]. On comparing our results with those obtained by [PERSON] et al. [26], who studied the microbial diversity in granitic urban heritage, we found that apart from bacteria (not considered in the aforementioned study), fungi and green algae were still the predominant groups in the biofilms. The groups most commonly present in the microbiota colonizing the churches were green algae (Chlorophyta) and Ascomycota (fungi), with cyanobacteria occurring in lower proportions. The diversity of fungi was very similar in both studies, with average Shannon index values of 3.42 (present study) and 3.43 (previous study). Figure 6: Shannon diversity index for green algae, fungi and bacteria colonizing the three churches under study (SMV, ROC, and BAR). [PERSON] et al. [26] reported a higher diversity of algae, with Shannon index values between 1.75 and 1.54, in contrast to the values of less than 1 obtained in the present study.