Salt contamination is widespread and very difficult to eradicate from stone monuments. Indeed, desalination treatments may affect the endogenous microbial communities (Caneva et al. The presence of salts also affects stone-associated microbial communities, which are often rich in highly specialized microorganisms such as halophilic microorganisms (Schabereiter-Gurtner et al. The biological population associated with stone-built structures is determined by a combination of factors such as environmental conditions (e.g., humidity, temperature, light, and pollution), architectural design, the chemical and mineralogical composition, and petrophysical properties of the stone (e.g., roughness and pore structure) and anthropogenic influences (e.g., human occupancy and restoration processes) (Gorbushina and Broughton 2009 Gadd 2017). As geomicrobial agents on the built environment, stone colonizers are involved in elemental cycling, rock transformation, soil formation, organic matter decomposition, and cycling of elements, among other processes (Barton and Northup 2007 Gadd 2017). 2021 Ortega-Morales and Gaylarde 2021 Bosch-Roig et al. 2020), and even positive effects, such as bioprotection, biomineralization, and bio-desalination phenomena (Kembel et al. However, the presence of these communities can also have negligible effects, such as surface deposition with no substrate interaction and only an esthetic impact (Sanmartín et al. The presence of microbes on heritage stonework is generally considered negative owing to the physical and/or chemical damage that they cause via biodeterioration mechanisms. Stone monuments provide habitats for multispecies microbial communities, including bacteria, fungi, lichens, algae, and archaea (Hoppert et al. Microbial communities involved in nitrate and sulfate cycles were detected.Halophilic and mineral weathering microorganisms reveal ecological impacts of salts.Poorly (CaSO 4 ) and highly (Ca(NO 3 ) 2, Mg(NO 3 ) 2 ) soluble salts were detected.Further analysis regarding organic matter and recalcitrant elements in the stonework should be carried out.
By contrast, no direct relationship between the damp staining and the salt content or related microbiota was established. A well-defined relationship between microbial data and soluble salts was identified, suggesting that poorly soluble salts (CaSO 4) could fill the pores in the stone and lead to condensation and dissolution of highly soluble salts (Ca(NO 3) 2 and Mg(NO 3) 2) in the thin layer of water formed on the stonework. State-of-the-art technology was used for microbial identification, providing information about the microbial diversity and phylogenetic groups present and enabling us to gain some insight into the biological cycles occurring in the community key genes involved in the different geomicrobiological cycles. Soluble salt analysis and culture-dependent approaches combined with archaeal and bacterial 16S rRNA and fungal ITS fragment as well as with the functional genes nirK, dsr, and soxB long-amplicon MinION-based sequencing were performed.
To enhance understanding of the role of microorganisms in the presence of salt and damp stains, we determined the salt content and identified the microbial ecosystem in several paving slabs and inner wall slabs (untreated and previously bio-desalinated) and in the exterior surrounding soil. The irregular damp dark staining on the stonework of a salt-contaminated twelfth century granite-built chapel is thought to be related to a non-homogeneous distribution of salts and microbial communities.
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