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International audience; Nostoc is a genus of filamentous Cyanobacteria common in both terrestrial and aquatic habitats. On land, Nostoc species can also form biofilms on stone monuments, due to their ability to remain desiccated for months or years, and fully recover metabolic activity within hours to days after rehydration with liquid water. We performed in vitro experiments inoculating a Nostoc sp. strain collected from the UNESCO site of archaeological park of Pompeii on five different stone materials, namely tuff, brick, limestone, porphyry, and glass paste to assess the role of water availability on early stage of biofilm formation on stones. The stone tiles, incubated in sealed glass chambers at four different relative humidity values (100%, 92%, 85%, 76%), were observed weekly in a 12-weeks interval to determine surface colonization and growth rate. Confocal laser scanning microscopy (CLSM) was then used to assess thickness, cellular volume and exopolysaccharides (EPS) production on each material. We found that in humidity driven in vitro colonization of Nostoc sp. water absorption coefficient (WAC) determines the propagation of the colonies; in particular, tuff and brick showed the best bioreceptivity, and a greater Nostoc sp. colonization rate, which was attributed to the higher values of WAC.

The Bay of Naples, Italy, is renowned for its archaeological heritage. However, this heritage is threatened by the combination of weathering and the biological activity of microorganisms. Fungi are among the major agents of microbial deterioration of cultural heritage since they can cause cracks and lesions in monuments due to the penetrating force of their hyphae. Such biodeterioration may weaken the stone structures and threaten the longevity of these culturally important monuments. To address this, we collected, identified, and maintained in culture filamentous fungi that colonize the external surface of monuments at five important archaeological sites near Naples, namely Cuma, Ercolano, Nola, Oplonti, and Pompei. We isolated a total of 27 fungal taxa, all of which can be cultivated in the laboratory, and form a part of our reference collection. Many of the described fungal taxa we found belong to groups that are involved in stone biodeterioration and can thus be considered as model organisms for in vitro studies. These results emphasize the importance of identifying and cultivating fungal stock cultures for non-invasive studies on biodeterioration. Our newly developed reference collection represents a useful resource that is available to other researchers to rapidly identify potentially hazardous fungi on other monuments.