**3. Stone artifacts**

A significant percentage of world cultural heritage objects is represented by stone artworks, such as architectural monuments, statues and tombstones, to name just a few [26]. Stone is considered an extreme environment for microbial growth and proliferation, mostly due to the intensive oscillations of diurnal and seasonal microclimatic parameters and low nutrient and water content [27]. Stone surfaces directly exposed to sunlight could achieve temperatures above 60°C, while being simultaneously susceptible to freezing–thawing cycles [28, 29]. Regardless, certain groups of microorganisms, the so called lithobionts, are able to colonize such environments. Primary colonizers are photoautothrophic organisms – cyanobacteria, algae and lichens, while hemolithotrophic and hemoorganotrophic bacteria and fungi are considered secondary. The latter are oligotrophic or poikilotrophic organisms which adapted to survive and grow in harsh or variable environmental conditions [29].

Microbial dwellers of stone surfaces frequently form biofilm, highly structurised microbial consortium embedded in a mutual extracellular matrix. Biofilm formation starts with unspecific, reversible interactions, followed by stable interactions which are initiated as a consequence of the formation of specific molecules and structures (lipopolysaccharides, membrane proteins, flagellae). After initial adhesion, extracellular polymeric substances are produced and excreted, enhancing the adhesion and cohesion of cells [8]. Evolutionary advantages of biofilm are to provide protection, resistance to physical and chemical stressors, metabolic cooperation and mutually regulated gene expression [30]. All groups of microbial dwellers of stone are characterized with a high phenotypic plasticity, which is reflected in polymorphism – the change of growth or sporulation forms with regard to external conditions. Therefore, micromycetes, which colonize this environment, are able to form different somatic and reproductive structures - sclerotia, chlamidospores,

conidial clusters, perithecia and pycnidia [29, 31]. Moreover, stone interior is a specific microenvironment for the growth of certain microorganisms such are endolithic fungi. The microclimatic conditions in this environment are a little more favorable – water retention is higher and solar radiation and air current intensities are lower [27].

Among fungal colonizers of stone, micorcolonial fungi constitute a specific ecological group characterized by its slow growth and formation of irregular shaped cells, often packed in aggregates. They rarely form specialized reproductive structures and in turn, by active growth, form thick, pigmented cell walls which enable transition to the state of dormancy during prolonged, unfavorable environmental conditions. The ability to secrete extracellular polymeric substances and thick cell walls, enable water retention, nutrient absorption, desiccation reduction and cellular adhesion/cohesion. These organisms are able to survive for long periods without metabolic activities and their metabolic rates are low even during optimal environmental conditions [27].

Dematiaceous fungi are considered the most important agents of stone deterioration. All representatives intensively produce dark colored melanins which provide protection from excessive environmental radiation (UV radiation, x- and γ-rays) and chemical stressors. This group encompasses microcolonial fungi and black yeasts, species of genera: *Acrodictys*, *Aureobasidium*, *Capnobotryella*, *Coniosporium*, *Exophiala*, *Hormonema*, *Hortaea*, *Knuffia*, *Lichenothelia*, *Monodictys*, *Phaeococcus*, *Phaeococcomyces*, *Phaeosclera*, *Sarcinomyces* and *Trimmatostroma*, which are very important stone colonizers in arid and semiarid environments [27, 32]. Additionally, deatiaceous fungi include cosmopolitan filamentous species of genera *Alternaria*, *Cladosporium*, *Ulocladium*, *Epicoccum* etc. which are main colonizers of stone in more favorable conditions of temperate and humid environments [32]. They are especially important deteriogens of restored stone artifacts [27].

Fungi can deteriorate stone via physical and chemical mechanisms. Physical mechanisms include hyphal penetration of the rock surface which causes its fragmentation, while chemical ones include secretion of acidic metabolites and pigments and oxidation of mineral forming cations. Although many microorganisms are able to produce acids, fungi are considered as the most potent ones in nature that degrade rocks and minerals. The production of various acidic metabolites leads to the biocorrosion - dissolution of the mineral substrate, resulting in the formation of various secondary mycogenic minerals [26]. *In vitro* acid production and formation of calcium oxalate and calcium carbonate minerals have been reported by autochthonous isolates from limestone monuments such as ancient Roman stela and Portuguese king tomb [26, 33]. Synthesis and excretion of extracellular pigments mostly affect the stone aesthetically, although studies concerning pigment production on stone monuments are generally scarce [26]. Due to the mentioned processes, symptoms such as biopitting, biogenic patina and colorations can occur [11]. The growth of dematiaceous fungi results in the presence of dark stains while microcolonial fungi are the main culprits of biopitting phenomena on limestone and marble artworks, [32, 34]. Sanctuary of Delos in Greece is an example of mentioned biopitting phenomena [35].

Lichenized fungi are important colonizers of stone substrates. These organisms have a high tolerance to variations of environmental factors, especially temperature, insolation and water availability, which is responsible for their cosmopolitan distribution and ability to colonize extreme environments [36]. These organisms are poikilohydric, i.e. they are capable of enduring cycles of desiccation and rehydration due to their ability of lowering their metabolic rate and enter cryptobiosis under conditions of low water availability [37]. Endolithic lichens are of special importance to stone deterioration, since they are capable of the deepest penetration *Fungal Deterioration of Cultural Heritage Objects DOI: http://dx.doi.org/10.5772/intechopen.98620*

into the stone compared to other microorganisms [38]. Apart from the fact that hyphae of the mycobiont can penetrate the rock surface (**Figure 1E** and **F**), perithecia formation by some endolithic species can penetrate the surface from the inside out, which leads to biopitting [27]. Additionally, lichen growth could cause exfoliations, encrustations and disaggregation of the stone surface [11, 39]. Conversely, lichen morphology and its adhesive capability aren't always in correlation with its capacity to alter the substrate and physiological differences between the species are considered to be more significant [40, 41]. In fact, synthesis of different chemical deterioration agents is done by the mycobiont. Apart from carboxylic, lichens have the ability to produce lichen acids, semisoluble polyphenolic compounds which are able to form complexes with metal cations. The capability of lichens to absorb and maintain water enhances the duration of chemical reactions and therefore facilitates deterioration process [39, 41]. Lastly, some authors have reported the presence of orange-brownish patinas (*scialbatura*) on stone monuments made from limestone and marble. These colorations mainly consist of calcium carbonate and calcium oxalate minerals, sometimes intermixed with fragmented lichen thalli [42, 43]. Although this symptom is associated with deterioration, it is hypothesized that it may have a protective role to the monuments [27].
