Microscale dynamics of dark zone alterations in Lascaux Cave shows brutal microbial community collapse

Past anthropization of Paleolithic Lascaux Cave (France) resulted in recent formation of wall alterations termed dark zones, where Ochroconis fungi proliferate and Pseudomonas bacteria are counter-selected. Dark zones grow quickly and without gradual visual changes, suggesting a simple process rather than complex microbial successions, but this is counter-intuitive as underground microbial changes are slow and dark zones are microbiologically very distinct. We tested this hypothesis in situ, across two years of microscale sampling. Indeed, Illumina MiSeq metabarcoding evidenced only three community stages for bacteria, fungi and all microeukaryotes together (i.e. unmarked surfaces, newly-formed dark zones and intermediate/old dark zones) and just two stages for archaea (unmarked surfaces vs dark zones), indicating brutal community changes. The onset of dark zone formation coincides with the development of Bacteroidota and the bacterial genera Labrys, Nonomuraea and Sphingomonas, which is concomitant with Ochroconis proliferation and Pseudomonas counter-selection. Modeling of community assembly processes highlighted that the dynamics of rare taxa in unmarked surfaces adjacent to dark zones and in newly-formed dark zones were governed in part by deterministic processes. Taken together, these findings suggest a brutal community collapse as these new alterations form on Lascaux cave walls.