Non-destructive testing of stone biodeterioration and biocleaning effectiveness

Microorganisms (bacteria, green algae and fungi) may lead to complex problems in the conservation of cultural heritage assets due to their biodeteriorative potential. The biodeterioration phenomena observed on materials of cultural heritage is determined by several factors, such as climatic conditions, chemical composition and nature of the material itself, as well as biological colonizers. The combination of several non-destructive techniques is compulsory in the field of cultural heritage in order to develop and design new and effective conservation strategies to prevent, control and minimize the causes of decay. For instance, Confocal Laser Scanning Microscopy (CLSM) permits to detect the presence, the penetration depth and the spatial organization of different phototrophic microorganisms established on stone surfaces, as well as Digital Image Analysis allows quantifying the surface area covered by microorganisms without destroying the sample. In this study, these non-destructive instrumental analyses, together with in vitro chlorophyll a quantification, were applied in order to evaluate a new procedure of stone cleaning consisting of the application of secondary metabolites with biocide properties. Three different natural biocides were tested on Hontoria limestone samples, a biosparitic limestone used in many Spanish monuments, previously inoculated with a multi-species phototrophic culture. After 1 month of biofilm growth on Hontoria limestone samples, the following biocides were applied: - culture filtrates of the fungus Trichoderma harzianum T-22 strain, - culture filtrates of the bacterium Burkholderia gladioli pv. agaricicola ICMP (Bga) 11096 strain, - glycoalkaloids (GLAs) extract from unripe berries of Solanum nigrum. After one month of incubation, CLSM revealed that none of the treatments was efficient against all inoculated phototrophic species, probably due to different biocide resistance. This was also corroborated by spectrophotometric determination of chlorophyll a and digital image analysis. Among all treatments, the culture filtrates of T.harzianum and the GLAs’ extract showed higher biocidal efficiency than the Bga culture filtrate. It seems evident that non-destructive analysis and ecological cleaning methods can represent a innovative strategy for the protection of our stone cultural heritage. Further work on the biocidal effectiveness and durability of secondary metabolites in the medium and long-term is needed for the design of effective and sustainable treatments for minimizing or eradicate stone biodeteriorationPeer Reviewe