UV-C as an Efficient Means to Combat Biofilm Formation in Cultural Heritage Monument. Biodiversity and Impact on Prehistoric Pigments?

Caves are considered oligotrophic habitats exhibiting constant temperature and relative humidity throughout the year. While darkness inhibits photosynthetic microorganism growth, introducing artificial lights to promote touristic activity can induce algae and cyanobacteria proliferation. Besides the aesthetic problem, microorganisms are responsible of physical and chemical degradation of limestone wall with possibly a degradation of prehistoric painting of cultural value. In our studies, we identified lampenflora with new-generation sequencing (NGS) in five French show caves and also on a 180, 000 years old contaminated bear bones (Ursus deningeroides). Afterward, we attempted to find an ecological and efficient method to eradicate photosynthetic biofilms. Our previous works showed the good efficiency of UV-C against biofilms. To ensure the safety of UV-C treatment, pigments used for prehistoric parietal painting were strongly irradiated around 4800 kJ m−2. Afterward, we treated all biofilms in La Glaciere show cave. Following the treatment, irradiated biofilm color evolution was monitored. Results of sequencing showed that biofilms were mainly composed of bacteria, fungi, cyanobacteria, chlorophyceae, and diatoms. Multicellular mosses were also present in each biofilm. Furthermore, X-ray crystallography, infrared spectroscopy, and colorimetric measurement showed no change in pigment color nor in their chemical structure. Finally, UV-C treatment results showed that biofilms were completely eradicated with no algae proliferation observed after 21 months. Moreover, a complete bleaching is obtained with UV-C, which is not always the case with chemical conventional methods. In conclusion, the use of UV-C could be considered as an environmentally friendly good alternative to chemical products. Moreover, microorganisms are killed by radiation, while no change in pigment color nor in their chemical structure has been observed.