Green stone, hidden life: Bacterial and fungal communities on the weathered serpentinite cladding of Florence Cathedral

Serpentinite is a metamorphic rock widely employed in architecture for its aesthetic value. In Tuscany (Italy), it acquired significant symbolic and cultural relevance during the Gothic and Renaissance periods, becoming a distinctive element of regional architectural identity. Within the region, Florence stands out for its extensive use in the characteristic polychrome cladding of several major monuments, including the Cathedral of Santa Maria del Fiore (SMFC). While the physical and chemical decay processes of serpentinite have been extensively documented, biodeterioration in architectural contexts remains poorly understood. This study aims to characterise both the conservation state and the microbial communities inhabiting the external serpentinite cladding of the SMFC and to compare them with those colonising adjacent white marble surfaces. To this end, an integrated approach combining in situ observations and analyses and microbiological methods (both culture-dependent and independent) was adopted. Microscopic observations revealed surface features such as efflorescence, pitting, and microfracturing, indicative of advanced chemical alteration and mechanical weakening. In situ XRF analysis revealed a relatively high calcium content and appreciable amounts of iron, nickel, and chromium. Bacteria and black meristematic fungi (BMF) cultured from serpentinite showed viable counts of the same order of magnitude across the investigated areas. In addition, strains of BMF were isolated and identified as polyextremotolerant members of the rock-inhabiting genera. Metabarcoding analyses of the 16S rRNA gene, and ITS regions revealed the presence of extremotolerant bacteria, including Deinococcus, Rubrobacter, and Truepera, as well as BMF such as Coniosporium, Knufia, and Vermiconidia. These findings provide new insights into the poorly investigated biodeterioration of serpentinite in cultural heritage, showing that prolonged physicochemical weathering may increase stone bioreceptivity and facilitate the establishment of specialised microbial communities potentially involved in further mineral alteration. Historically weathered serpentinite used as architectural heritage stone can host specialised microbial communities, and we discuss how their functional traits may contribute to subsequent stages of biodeterioration under real urban outdoor conditions. The occurrence of shared rock-inhabiting genera across both lithotypes suggests the presence of a core microbiota adapted to the extreme conditions of exposed stone surfaces, with potential implications for biodeterioration processes. Overall, these findings shed initial light on the understudied status of serpentinite in the cultural heritage context.