Essential oils to contrast biodeterioration of the external marble of Florence Cathedral

The search for more sustainable strategies to contrast biodeterioration of stone cultural heritage has been developing in recent years to find alternatives to synthetic biocides, since their toxicity and potential impact on the environment and health. In this study, the application of the oregano and thyme essential oils (EOs) was tested to control microbial growth on the external marble of Florence Cathedral affected by extended darkening. Before in situ application, preliminary tests were carried out to evaluate the interference of the EOs with marble (colorimetric and water absorption assays on marble specimens) and their efficacy in inhibiting marble microbiota (sensitivity test on nutrient media). EOs inhibited the whole cultivable microbiota sampled from the Cathedral marble at a very low concentration, while they did not interfere with colour and water absorption capability of uncolonised marble samples when applied as a 2 % solution. Then the two EOs and the commercial biocide Biotin T were used in in situ trials on marble in two outdoor study sites of Florence Cathedral. The effectiveness of the treatments was assessed through short- and mid-term evaluation by multidisciplinary in situ non-invasive (colorimetric and ATP assays, microscopy) and ex situ (microbial viable titer) tests. Concerning results, we found a good correspondence between parameters for evaluation of viability (bacterial and fungi viable titer) and activity (ATP determination) and some correspondence among these and microscopy and colorimetry. Considering the whole data, treatments with oregano and thyme EOs were effective against microbial community, in more cases comparably to the commercial biocide. Some differences found, particularly by viable titer, in the two study sites or in bacterial and fungal components of the microbiota can be partly attributed to differences in structure and colonization pattern of the microbial community due to the peculiar climatic conditions of the differently exposed study areas.