Estimated to be around 17,000 years old, the Paleolithic paintings in the Lascaux cave of southwestern France give us a rare insight into the history and culture of communities that existed long before modern society. The conservation of caves such as Lascaux is a high priority for historians, scientists, and the general public. The anthropization, or human use, of caves may have dramatic effects on cave-dwelling macro- and micro-organisms, though few studies have been conducted on this topic. By comparing ‘pristine’ caves with anthropized caves frequently visited by humans, Dr. Lise Alonso and colleagues demonstrate that the anthropization of caves is associated with reduced microbial diversity for bacteria and archaea living on cave walls, though microeukaryotes and arthropods were not as strongly affected. In this post, we go behind-the-scenes with Dr. Yvan Moënne-Loccoz on their recent publication in Molecular Ecology and talk about the importance and challenges of working in cave ecosystems.

Link to the study: https://onlinelibrary.wiley.com/doi/10.1111/mec.15144

What led to your interest in this topic / what was the motivation for this study? 
Cave conservation is an important issue, especially when dealing with caves displaying Paleolithic artwork, as engravings and particularly paintings can be very fragile. There are many of these caves in Dordogne (South-West of France), some of them listed on the UNESCO World Heritage List (https://whc.unesco.org/en/list/85). The most famous Paleolithic cave in Dordogne is the Lascaux Cave, which was closed to the public in the 1960s for conservation reasons. To guide conservation efforts, it is important to understand the ecology and functioning of these caves, especially at the levels of microorganisms and arthropods, which form the main communities present. Against this background, the project was carried out to understand better the biotic communities residing in Lascaux Cave.

What difficulties did you run into along the way? 
When dealing with microorganisms and arthropods populating soils, sediments or water, in a majority of cases it is rather straightforward to collect samples and there is no restriction on sample size. In caves, taking samples from walls for microbial analyses, using a scalpel, may leave long-lasting marks. This is an issue in all caves, and particularly so in Paleolithic caves. In the Lascaux Cave, the sample list was prepared after discussions with the cave staff and approved by the cave conservator, and the samples were collected (away from ornate surfaces) by qualified restorers, under the guidance of microbial ecologists, so as to avoid any marks on the wall. It also means that only minute samples were available. Restrictions also apply for the type and location of arthropods traps, as sediments at the bottom of caves might contain historical artefacts.

What is the biggest or most surprising finding from this study? 
Caves are oligotrophic environments, so it is always a surprise to find diversified, rather large microbial communities on cave walls. In this study, the Lascaux Cave was compared with eight other caves from the same region, and these caves were quite different from one another in terms of size, architecture, distance from the soil surface, presence/absence of stream underground, human frequentation patterns, etc. Yet, there were clear distinctions in terms of microbial and arthropod communities when comparing anthropized caves versus non-anthropized (almost pristine) caves, which suggests that anthropization was more influential than these cave-specific features. Finally, we were rather surprised to find that prokaryotes (bacteria and archaea) were comparatively more impacted than eukaryotic residents (fungi, other micro-eukaryotes, arthropods) by cave anthropization.

Moving forward, what are the next steps for this research?
This work was carried out with the Lascaux Cave and eight other caves from Dordogne, which corresponds to a relatively small area. There were at the most 35 km between two caves in this study. Therefore, it remains to be seen whether the results of the current investigation are also relevant elsewhere. At a larger geographic scale, several differences in cave properties can be expected, for instance in geological features (e.g. limestone type) and climatic conditions, which have the potential to influence cave biotic communities. In addition, we evidenced parallel variations in the diversity of microbial and arthropod communities, and it will be important to explore and understand better the ecological interactions between both types of cave inhabitants.

What would your message be for students about to start their first research projects in this topic?
First of all, the underground world and the interface between ecology and artwork conservation issues are fascinating, so welcome to the field! More importantly, each cave is different and represents a complex situation of its own, so one can be very busy focusing on a single cave only. This is reflected by the literature on cave microbial ecology, where often a single cave is considered at a time. However, we found that the comparison of different caves, following the path of various groups (e.g. Campbell et al. 2011 J Cave Karst Stud 73:75 ; Hathaway et al. 2014 Geomicrobiol J 31:205 ; De Mandal et al. 2017 BMC Microbiol 17:90 ; Pfendler et al. 2018 Sci Tot Environ 615:1207), brought very interesting insights, so comparative assessments are worth the effort.

What have you learned about science over the course of this project? 
The majority of participants to this project usually work on soil or aquatic ecosystems, and we found (once again) that concepts and methodology are applicable across different types of ecosystems. More specifically, we realized that underground systems represent interesting models to investigate ecological perturbations, because they are rather confined environments, where community fluctuations in response to mild environmental variations can be documented.

Describe the significance of this research for the general scientific community in one sentence.
This research shows that microbiome diversity can be used as a bioindicator of the level of cave anthropization.

Citation
Alonso L, Pommier T, Kaufmann B, Dubost A, Chapulliot D, Doré J, Douady CJ, Moënne‐Loccoz Y. Anthropization level of Lascaux Cave microbiome shown by regional‐scale comparisons of pristine and anthropized caves. Molecular Ecology, 28(14), 3383-3394. https://onlinelibrary.wiley.com/doi/10.1111/mec.15144