Stochastic and deterministic assembly processes of microbial communities in relation to natural attenuation of black stains in Lascaux Cave.

Community assembly processes are complex and understanding them represents a challenge in microbial ecology. Here, we used Lascaux Cave as a stable, confined environment to quantify the importance of stochastic vs deterministic processes during microbial community dynamics across the three domains of life in relation to an anthropogenic disturbance that had resulted in the side-by-side occurrence of a resistant community (unstained limestone), an impacted community (present in black stains), and a resilient community (attenuated stains). Metabarcoding data showed that the microbial communities of attenuated stains, black stains, and unstained surfaces differed, with attenuated stains being in an intermediate position. We found four scenarios to explain community response to disturbance in stable conditions for the three domains of life. Specifically, we proposed the existence of a fourth, not-documented yet scenario that concerns the always-rare microbial taxa, where stochastic processes predominate even after disturbance but are replaced by deterministic processes during post-disturbance recovery. This suggests a major role of always-rare taxa in resilience, perhaps because they might provide key functions required for ecosystem recovery.IMPORTANCEThe importance of stochastic vs deterministic processes in cave microbial ecology has been a neglected topic so far, and this work provided an opportunity to do so in a context related to the dynamics of black-stain alterations in Lascaux, a UNESCO Paleolithic cave. Of particular significance was the discovery of a novel scenario for always-rare microbial taxa in relation to disturbance, in which stochastic processes are replaced later by deterministic processes during post-disturbance recovery, i.e., during attenuation of black stains.