Biodeterioration of Roman hypogea: the case study of the Catacombs of SS. Marcellino and Pietro (Rome, Italy)

No information exists on phototrophs growing on the stone surfaces of the Catacombs of SS. Marcellino and Pietro (the site was only recently opened to the public in 2014). Therefore, it was decided to characterise the microbial communities and to compare them with those of the other previously studied catacombs. Moreover, a new non-invasive strategy to reduce the phototrophic growth was tested. Phototrophic microorganisms were investigated under light and confocal laser scanning microscopes from samples collected non-invasively in situ. Tests were carried out to determine the effect of the application of two essential oils (from L. angustifolia and T. vulgaris) on biofilm photosynthetic activity. Laser-induced fluorescence (LIF) and reflectance measurements in the visible range have been used to evaluate respectively, any chemical modification and discolouration on a frescoed stone that may occur after the application of the essential oils. At all the concentrations of essential oils, there was a quasi-immediate, large reduction in photosynthetic activity of the biofilms. At 10% essential oil concentration, there was no detectable photosynthetic activity after 15 min. At 1%, there was a need for two applications and after 5-day activity was undetectable. No effect of the essential oils on the substrate surface properties or colour modification of the fresco has been observed with the LIF prototype. Cyanobacterial typical of Roman catacombs were present in the sites investigated. Innovative and non-destructive strategies, involving the application of a combination of two essential oils, have been successfully tested and developed to prevent biodeterioration of these sites.