Rosy Discolouration in an Alpine Chapel: Beyond Salt Dependence

Stone heritage is central to cultural identity, yet alpine monuments remain understudied and vulnerable to deterioration. In mountain environments, moisture and temperature fluctuations promote salt crystallisation in porous geomaterials, frequently associated with rosy discolouration attributed to pink biofilms, a phenomenon still poorly explored at higher elevations. Here, we investigated rosy discolouration at about 1000 m altitude on the walls of St. Cyprian Chapel (South Tyrol) using metabarcoding and chemical analyses. We compared two cases of rosy discolouration: one on the north wall, where salt efflorescence is present, and one on the south wall, where it is absent. The aim was to characterise how salt efflorescence modulates rosy discolouration by comparing the two walls and linking substrate chemistry and mineralogy to biofilm pigments, structure, and microbial community composition. Despite similar mineralogy, the north wall exhibited distinct surface chemistry consistent with salt efflorescence. Biofilm biomass was comparable between walls, and no photosynthetic organisms were detected in either biofilm, yet microbial communities differed markedly. The south wall hosted carotenoid-producing biomarkers, including Rubrobacter and Pontibacter, although Raman did not detect carotenoid signals, indicating low pigment expression. In contrast, the north wall showed stronger discolouration and clear detection of bacterioruberin. Notably, the north-wall biomarker Chryseobacterium may also contribute to the rosy discolouration through flexirubin-type pigments. Overall, salt efflorescence is not required for pink biofilm establishment, but salt-rich microhabitats appear to enhance pigment production. This study provides new insights into the drivers of rosy discolouration and contributes to the still-limited research on alpine cultural heritage.