Pink discoloration due to pigmented Archaea on the walls of the so-called Michelangelo's “secret room” (Medici Chapel, Florence, Italy)

Pink discolorations are often observed on heritage buildings mainly in areas affected by salt weathering, where the development of halotolerant and halophilic microorganisms is favored. Part of these extremophilic microorganisms contains carotenoids, reason for which their colonization becomes visible by naked eye on large surfaces. This work investigates the pink alteration of the walls with drawings attributed to Michelangelo, located in the basement room of the Medici Chapel (Florence, Italy). The results of in-depth multidisciplinary investigations are discussed in the context of a thorough literature review on pink alterations of heritage buildings. For the first time, we combined culture-based approaches with metagenomic analyses that revealed the pink pigmented archaea Halalkalicoccus sp. as dominant, pointing their role in the pink discoloration present on the wall. Raman spectroscopy was exploited for the characterization of the biogenic pigments and Fourier Transform infrared spectroscopy and scanning electron microscopy to analyze the salt formations present on the walls.