Bacterial diversity associated with saline efflorescences damaging the walls of a French decorated prehistoric cave registered as a World Cultural Heritage Site

This study focused on a French prehistoric decorated cave which suffers from the development of saline efflorescences. The goal was to determine the ecology of microorganisms associated with efflorescences, and determine if the sessile microbiota could have a role in the deterioration process. Samples from 2 sampling campaigns were analyzed. Elemental, mineralogical and isotopic analyses showed gypsum containing sulfur originating mainly from the host-rock. Microscopic observations, direct cell counting, culture-dependent assays and high-throughput sequencing (HTS) of bacterial 16S rDNA sequences indicated an abundant sessile microbiota, where Actinobacteria, Alphaprotebacteria, Gammaproteobacteria, Bacteroidetes and Planctomycetes were the main phylogenetic groups. Although a relatively low abundance of genera associated with sulfur cycling were identified, the presence of dense biofilms at the interface rock/efflorescences composed of a high relative abundance of Actinobacteria may reflect previously described indirect biomineralization. An increase of microbial abundance and bacterial richness between October 2014 and April 2015 might be attributed to fluctuations in environmental factors. This study gives insight into origin and bacterial composition of efflorescences that threatened Sorcerer's cave engravings. HTS has rarely been used in the field of cultural heritage and thus represents one of the originalities of this work. •Gypsum efflorescences damage the walls of the Sorcerers' cave.•Gypsum efflorescences formation is mainly due to geochemical phenomena.•An abundant sessile biomass is associated to gypsum efflorescences.•Bacteria belonging to the actinobacterial phylum are dominant.•A high abundance of halotolerant and halophilic bacterial genera are found.