Microbial induced stone discoloration in Alcobaça Monastery: a comprehensive study

Abstract

The Alcobaça Monastery (Portugal), a UNESCO World Heritage Site, currently exhibits a high degree of surface alterations of the stone architectural elements inside the church, including an extensive pink coloration in the walls and columns (Fig. 1 A-B), bacteria biofilms, and salt efflorescences (Fig. 1C). The main goal was to identify the microbiota that colonizes the walls and columns of this monument, to help custodians and conservators-restorers in the selection of the correct cleaning procedure to be adopted for the conservation of the monument. Regarding the observed pink discoloration, and considering previous studies, we hypothesized that it is caused by biofilms formed by bacteria or other microrganisms that produce pigments of the same color, particularly carotenoids. Curiously, a distinct phenomenon was noticed (Fig. 1A): the pink discoloration always seems to appear at a very similar height in most of the columns and walls, starting at 40 cm to the floor and associated with the presence of salts on the walls. Using high-throughput sequencing approaches, we were able to characterize the microbial community present. We identified several bacteria that are producers of pink pigments and halotolerant such as Bacillus aryabhattai, Rubrobacter radiotolerans and Halalkalicoccus sp. and thus develop in areas of high salinity [1-4].

Figure 1: (A) distinct phenomenon observed at the altar of the monastery church; (B) pink colored biofilms on the walls; (C) evidence of the presence of salts on the walls.

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The authors acknowledg to FCT – Foundation for Science and Technology, I.P., through the projects: UIDB/04449/2020, MICROCENO (PTDC/CTA-AMB/0608/2020), ART3mis (2022.07303.PTDC), I. Silva (UI/BD/153582/2022) and C. Salvador (DL 57/2016/CP1372/CT0019) to individual supports, and Directorate-General for Cultural Heritage (DGPC).