Alterative microorganisms living on stone artworks exposed to environmental stress conditions

Many historic, cultural and artistic objects and buildings are made of stone. Stone, like all materials, is subject to inexorable deterioration leading to cultural and economical loss. In this regard, a critical important role is played by biological agents which can cause chemical, physical and aesthetic damage, specially on outdoor stone surfaces. Many biodeteriogens, organisms capable of causing deterioration, can be found on outdoor stone artwork: bacteria, algae, fungi and lichens (Polo et al., 2010). In these systems microorganisms are continually exposed to stress condition due to UV solar radiation, thermal stress, atmospheric pollution (particularly if in urban areas), moisture deficit, limitation of nutrients and weather, making rock substrates one of the most inhospitable habitats for microbial life (Cappitelli et al., 2007; Burford et al., 2003). We studied microbial diversity on coloured biological alterations found on two stone statues exposed to stress conditions, decorating the courtyard of the Buonconsiglio Castle in Trento (Italy). Here, biodeteriogen agents caused microbial contamination of different colours: green, green–black and black staining. As color of microbial contamination depends on the pigments produced by the microorganisms, an understanding of the complex microbial ecosystem of cultural heritage surfaces is also a prerequisite for controlling the growth of microbial species that cause biodeterioration. Both traditional cultural methods and biomolecular techniques like denaturing gradient gel electrophoresis and fluorescent in situ hybridization were used to study microbial communities. We found that cyanobacteria and green algae were responsible for the green staining whereas the black microbial contamination was due to dematiaceous fungi. Many identified biodeteriogen species are well known for adaptation properties to condition of environmental stress. Cyanobacteria and green algae are photoautotrophic microorganisms and dematiaceous fungi are characterised by the production of the black pigment melanin in cell walls that protects against UV irradiation, and manifest meristematic growth that is advantageous in drought condition (Tomaselli et al., 2000; Wollenzien et al., 1995). The identification and knowledge of biodeteriogen agents allowed us the selection of a suitable biocide for the removal of the coloured microbial contamination.