Natural biopatina on historical petroglyphs in the Austrian Alps: To clean or not to clean?

Some of Austria's most important petroglyphs are located in the alpine region of Dachstein, carved in the local limestone. Favored by the humid and shady environment, most of the engravings are covered by extensive biological colonization. In order to assess whether and to what extent the biopatinas present on the stone cause biodeterioration, we investigated their interactions with the rock substrate by microscopy methods (and PAS-staining), in combination with a metagenomic analysis to identify the biofilms' composition. The microscopy methods highlighted a general biodeterioration state of the stone and the great boring ability of the biofilms, especially of those with filamentous cyanobacteria (e.g., Nostocales and Oscillatoriales), and showed that a thick moss biofilm (with high abundance of Bacteroidota taxa) was the most harmful for the stone support, causing severe disruption of the underlying rock. Interestingly, the analysis of stone without visible biofilm revealed a microbiome dominated by ammonia-oxidizing archaea (AOAs), probably indicating the presence of a previous degraded biofilm (source of ammonia), and a current state of deterioration (dissolution of calcium carbonate). Removal of the biopatinas, or their alteration, would expose an already altered surface, potentially giving rise to a new colonization and new cycles of biodeterioration.