# Role of Exposure on the Microbial Consortium on Historical Rural Granite Buildings
## 3 Results and Discussion
### Next Generation Sequencing  
Microbial community composition (of both bacteria and fungi) of the samples collected from both N- and W-facing walls of the SMV, BAR and ROC churches is represented in Figure 5A (at the phylum level) and Figure 5B (at the class level). The biofilms sampled include a large diversity of species, particularly in the bacterial domain, but also with a strong presence of lichenized fungi. The main species present are shown in Table 2. The value of sampling rural churches to analyze the biodiversity of granite colonizing organisms was previously reported [31; 45] and the substrate aspect, inclination, wetness, exposure to rain, degree of disaggregation of the rock, and proximity to mortar were indicated as important factors in determining the composition of the flora. Bacteria, which are known to discolor, disrupt, weaken and dissolve a wide variety of materials [46], proved the most diverse domain. The main phyla of bacteria that were found, which in several of the churches accounted for up to 50% of the colonization, were Proteobacteria and Bacteroidetes, followed by Acidobacteria, Plantomycetes, and Actinobacteria. The phyla Deinococcus-Thermus, Verrucomicrobia, Armatimonadetes and Cyanobacteria were also present, but were less abundant. Within the phylum Proteobacteria, the most abundant class was Alphaproteobacteria, with Sphingomonadales and Rhodospirillales being the most abundant orders on most of the walls and with fewer representatives of Caulobacterales, Rhizbiales and Rhodobacteralesorders. Sphingomonads have been reported in a wide diversity of rocky habitats, ranging from a preserved sandstone slab at an exposed site in Oxford, UK [47], to the stone surfaces of monuments on the UNESCO World Heritage List of the Hangzhou West Lake Cultural Landscape in China [48]. The most abundant genus of the order Sphingomonadales found on the church walls was _Sphingomonas_ (including the species _Sphingomonas prati, Sphingomonas hengshuiensis_ and _Sphingomonas echinoides_). It was identified from biofilms from the N-facing walls of the SMV and BAR churches (31.0 and 15%), as well as from the W-facing walls of SMV, BAR and ROC (4.9, 20.1 and 10.25%, respectively). Species of the genus _Sphingobium_ were also present on the W-facing wall of the BAR church (8.3%), and species of the genus _Sphingoaurantiacus_, specifically _Sphingoaurantiacus polygranulatus_ was also found in the N- and W-facing wall of BAR (1.3 and 11.3%) and in the N-facing wall of SMV (8.4%). These genera are often regular inhabitants of heritage walls, due to their ability to live in nutrient-poor environments. Their presence causes a visual deterioration of the heritage due to the change of color that their presence produces in the stone, as well as the production of sphingans, a rather viscous exopolysaccharide [49; 50; 51; 52]. All genera of the second most abundant order in the studied biofilms, Rhodospirillales, are part of the so-called acetic acid bacteria. This order was particularly abundant on the N-facing wall of the ROC church, where the genera _Gluconacetobacter_ and _Granulibacter_ occur (5.7 and 12.4%). The species _Roseomonas arcticisoli_ (5.7%) and _Dankookia rubra_ (1%) were identified on the N-facing wall of the SMV church. Other less abundant species of this order, such as _Acidalus organivorans_, occurred on the N- and W-facing walls of the BAR church (2.3 and 6.7%), as well as on the N-facing wall of the SMV church (5.2%) and on W-facing wall of the ROC church (1%). Moreover, _Roseomonas rgiuicci_, _Acidisphaera rubrificiens_ and _Acidisoma sibiricum_ also colonized the N-facing wall of the ROC church (4.1, 1.5 and 3%), but also the W-facing wall of this church (5%), the W-facing wall of the BAR church (1.2%) and the N-facing wall of the SMV church (1%). These species are highly tolerant of low pH conditions, which is advantageous to growth on granitic substrate [53; 54] because the substrate acidity is a key factor determining which organisms will develop most successfully.