Ecological impact of mechanical cleaning method to curb black stain alterations on Paleolithic cave walls

Abstract

Anthropization of Paleolithic karstic caves can cause an imbalance of cave microbiota and may trigger formation of wall alterations including black stains. In Lascaux Cave, a previous attempt to mechanically remove black stains was followed by reformation of the stain in months, suggesting that microbial recolonization had taken place. On this basis, we hypothesized that mechanical cleaning (a routine cleaning method for conservation of heritage sites) leaves a residual microbial community that can also serve as pioneer community, i.e. a community of early microbial residents that triggers subsequent microbial successions involved in the reformation of black stains. We monitored post-cleaning microbial recolonization over 19 months in the Apse of Lascaux Cave (France), after using two methods of mechanical cleaning (scalpel alone, or scalpel + sponge). Illumina MiSeq metabarcoding evidenced various taxa i.e. the bacteria Pseudomonas, Pedomicrobium and black-melanized fungi Ochroconis (=Scolecobasidium) during early recolonization of cleaned surfaces, and at later stages the establishment of several other taxa including the bacteria Luteimonas, Chitinophaga and the black fungus Exophiala. Surfaces at 19 months after cleaning were visually and microbiologically different from stained surfaces immediately after cleaned and from unstained surfaces, but also from non-cleaned stained surfaces, probably because of a particular microbial succession, distinct from the original succession during stain formation. Variations in relative abundance of Bacteroidota and Eurotiomycetes classes and Exophiala genus were higher when the sponge was used in addition to the scalpel. The bacteria Filomicrobium and the fungi Isaria and Cephalotrichum were identified on sponge-cleaned surfaces and on the sponge itself, pointing to a contaminant status due to the cleaning method. Overall, it suggests that post-cleaning pioneer communities may play an important role in orienting stain reformation in caves. Sponges routinely used by restorers to curb microbial stains may bring microbial contaminants, which questions current cleaning practices in show caves.