Lichen communities on cultural heritage assets: a sight on and beneath the surface

The outdoor stone cultural heritage (CH) faces material deterioration over years, including biodeterioration phenomena caused by biological growths. Between lithobionts, lichens are the most common and widespread in different environmental conditions. Restorers are often required for their removal, and the most used approach is the application of biocides followed by mechanical cleaning. However, recolonization often occurs within few years, and treatments have to be repeated. In this context, there is a need for a broader understanding of lichen biodeterioration phenomena, in order to address a sustainable conservation strategy that respects the principle of minimum intervention. This PhD thesis presents a wide view on lichen community dynamics and tracks on natural and artificial carbonatic stone surfaces in UNESCO heritage sites in Italy (Villa della Regina in Torino, UNESCO-WHS 823bis-003; Savoy Castle in Govone, UNESCO WHS-823bis-011; House of the Ancient Hunt in Pompeii, UNESCO WHS-829). The study may help restorers and other professionals to correctly approach the lichen (re-)growth on stone surface, giving information to eventually maintain the status of clean surfaces for a longer time. The sight beneath the surface was oriented to investigate lichen-stone interactions and how biocidal treatments spread and affect their endolithic components, while the sight on the surface (and around the monuments) aimed to characterize favorable (micro)climate conditions and other key factors that promote lithobiontic development and, in particular, lichen (re-)colonization. A final laboratory evaluation of the most used techniques to assess biocidal efficacy was also performed. These topics, considered in different chapters of the thesis, were investigated by the innovative application of already-known technologies in the field of conservation of CH. Patterns of different bioreceptivity, and their connection with past lichen colonization, were investigated using an image-analysis approach and its correlation with biomineralization products was assessed by X-ray powder diffraction (XRPD). Biocidal spread and penetration within the stone, with and without lichen cover, were visualized using fluorescein (uranine dye, C20H10Na2O5), and then with methylene blue dye, mimicking biocidal intervention by applying the solution with techniques commonly used in restoration procedures (spray, brush Lichen communities on cultural heritage assets: a sight on and beneath the surface 4 and poultice). The relationship between microclimatic conditions and the photosynthetic efficiency of lichens and other lithobionts was investigated with a long-term in-situ monitoring and under the actual microclimate conditions, by measurements of chlorophyll a fluorescence. The last section described the use of flow cytometry applied for a long-term monitoring on cultures of photobionts treated with different concentrations of commonly used biocides. The evaluation of the variation of specific parameters (size, granularity, presence of pigments) in relation to the treatment allowed the interpretation of the initial fluorescence increase often registered -in the field- during the assessment of biocide-driven lichen devitalization by chlorophyll a fluorescence measurements. In this thesis the presence of some lichen-derived protective effects is confirmed, and other results suggest the non-sustainability of most of the common approach based on the repetition of biocidal treatments. The fundamental role of microclimate conditions (water, in particular) on lichen activity is also proved, and suggested as a primary parameter to perform effective cleaning interventions and to prevent re- and new colonization processes.