# Black Fungi on Stone-Built Heritage: Current Knowledge and Future Outlook
## 2 Black Fungi and Stone Monuments: An Intimate Connection  
Beginning in the 1990s, black fungi were described as one of the most likely groups of microorganisms responsible for the biodeterioration of the stone monuments [22; 23; 24; 25] and it was confirmed in the following decades [6; 18; 26; 27]. The term \"black fungi\" refers to a very huge group of dematiaceous fungi, unrelated phylogenetically, which have in common the presence of melanin in the cell wall that confers an olive brown appearance to the colony [28]. Another common characteristic is the ability to withstand hostile environments such as scarcity of nutrients, high solar irradiation, scarcity of water, high osmolarity, and low pH [15; 19; 29]. As reported by [PERSON] et al. [30] the ancestors of black fungi were well adapted to live in oligotrophic environments such as rock surfaces or sub-surfaces, and currently they can also grow in anthropogenic habitats such as glass, silicon, organic surfaces, metals [31], or consolidants applied on the stone [9]. Their resilience is related to the extremotolerant or even polyextremotolerant characteristics of the species. The stress-tolerance is due to different factors such as: pigmentation, and in particular melanins production; myxosporine-like substances; morphological and metabolic versatility; meristematic development; and oligotrophy [32; 33; 34]. All these characteristics make them very suitable for colonizing outdoor rocks and built stones due to the fact that those surfaces can be exposed to extreme environments [17; 18; 19]. This group of fungi includes (a) fast growing hyphomycetes of epiphytic origin, recognizable under microscope by the presence of typical conidiophores and spores; (b) pleomorphic hyphomycetes that include the \"black yeasts\", showing a yeast-like form, and the so-called \"black meristematic fungi\" with a _Torula_-like growth pattern (Figure 1). Figure 1: Main morphological characteristic traits of MicroColonial Fungi, MCF. Dark black colonies due to the melanin production as seen (**a**) for the unidentified strain MC 655 on different cultural media after 1 month of incubation. (**b–d**) characteristic meristematic pattern of growth described also as _Torula_-like hyphae observed under Light Microscope. Bar is 10 \(\mu\)m.  
Hyphomycetes and black yeasts are ubiquitous and widespread all over the world in very different habitats (e.g., soil, fresh water, sea, plants, animals, and humans) [28; 35], while the meristematic black fungi, mostly isolated from stone or natural rocks, can be considered the true stone-inhabiting fungi [19; 20; 36; 37]. In the literature, many of black fungi are reported as RIF (rock inhabiting fungi) to emphasize that the \"rock\" is their preferred or exclusive habitat. However, this terminology does not include their main features such as melanin production, pleomorphism, or meristematic development; for this reason, we do not use it in this context. In the frame of cultural heritage the acronym MCF (MicroColonial Fungi) as first employed by [PERSON] [38] is widely used for their description. It refers to the typical black cauliform-like colonies visible on the rocks and stones. Humidity may affect the settlement of MCF on the stone artifacts as unique inhabitants or as associated with other stone colonizers. In fact, in lower or sheltered parts near the ground, where there is a sufficient availability of water, MCF are strictly associated with phototrophic microorganisms with whom, however, they do not establish a symbiotic relationship (Figure 2); in harsh, dry micro-environmental conditions, MCF become the unique colonizers [39; 40; 41]. Black fungi are currently classified in the Phylum of Ascomyca in the Class of Dothideomycetes and Eurotiomycetes, mainly in the order of Capnodiales, Dothideales, Chaetothyriales, Pleosporales and Cladosporiales, and Mycocaliciales [14; 20; 21; 42].