PLANT PRODUCTS AS A GREEN SOLUTION IN THE FIGHT AGAINST BIODETERIORATION OF STONE MONUMENTS

Stone materials in archaeological sites and confined environments are exposed to biodeterioration due to the presence of living organisms that endanger their durability. Weathering and natural or artificial light favour the growth of phototrophic biofilms that are generally composed of cyanobacteria, bacteria, microalgae, mosses and fungi [1]. These microbial communities can cause aesthetic damage to valuable surfaces by causing discoloration and, in some cases, structural problems due to the ability of microorganisms to attack stone, by penetrating the material also through the painted layers. Nowadays, the restoration practices in use include the mechanical removal by brushes combined with spraying chemical biocides that are frequently hazardous towards human health and environment and the effect on the treated surface is not always well understood. Moreover, the employment of chemicals seems to be selective for resistant microorganisms [2]. As a result of these negative factors, more sustainable and safer practices are being investigated for their biocidal action on typical biofilms of lithic surfaces [3-5]. The site of interest for this study is the Domus Aurea (Rome, Italy), built after the fire of 64 AD, as the new residence of the Emperor Nero, the Domus’s walls were covered with precious marbles and vaults decorated with gold and precious stones, then Nero's successors buried the residence, in-filling it entirely with soil. To date, among the restoration efforts to recover the monument, the disinfection of biodeteriogens growing on the walls and ceilings has become a highly important aspect. Based on prior knowledge it was decided to carry out tests on biofilms using the essential oil from Lavandula angustifolia and alcohol extracts from leaves of Glycyrrhizza glabra (Trifolio-M GmbH) and Capsicum spp. These natural compounds were tested at different concentrations, as alternative biocides of phototrophic biofilms. Biofilms samples were collected in ‘room 93’ from an undecorated wall and were then inoculated in agarized BG11 growth medium, to reconstruct homogeneous biofilms to be tested in laboratory. Observations of the biofilms showed that Scytonema julianum was the dominant species. This cyanobacterium has often described from hypogea environments, such as the Roman Catacombs, and it is well known for its biodeteriogenic activity due to the precipitation of calcium carbonate in its sheaths after the dissolution of minerals from the substrate. The tested products were applied to the biofilms twice (day 1 and 5), and the photosynthetic response on the biofilm was followed for five days with a mini-PAM portable fluorometer. As photosynthesis is highly susceptible to this kind of treatment, measurements of rates were used as a proxy for cell health. Changes in photosynthetic activity of the samples treated with the extracts were compared to control biofilms receiving no treatment. Results show that the essential oil of L. angustifolia and G. glabra extract 30% had a higher inhibition potential, followed by G. glabra extract 10%. Capsicum extract was the least efficient. These first results are encouraging enough to continue beyond the laboratory trials in the hope to realise an eco-friendly, non-toxic and sustainable strategy for the conservation of lithic cultural heritage