# Biological Risk for Hypogea: Shared Data from
## 4 Discussion
### Biodeteriogenic Taxa, Phenomenology of Alteration and Biodeteriogenic Processes
As observed for the Mogao Grottoes (Dunhuang, China), biodeterioration of wall paintings suggests that only a small fraction of the rich microbial community may in effect grow and cause damage, whereas most of these microorganisms are dormant or metabolically slow [105]. Hence it is of the utmost importance to study the metabolic traits of isolates to associate them to a risk. For example, moonmilk which is present in Etruscan tombs, as in karst caverns or in other hypogeal environments [107] does not seem to greatly detrimental, and _Streptomyces_ species may actually exert a protective effect, since restorers have reported that under the patina the wall paintings were well preserved [42]. The most evident effect of microbial colonisation are the different chromatic alterations on the surfaces, often producing whitish or grayish patinas and spots which resemble salt efflorescence or carbonate deposits [26; 38; 80]. As shown in Tables 1 and 2, the same taxa can at times be associated withquite different biodeterioration patterns, probably due to the complexity of the microbial communities and subterranean trophic networks as recorded for example in _Acremonium_ sp. found as a minority component of a green phototrophic patina as well as in a whitish-gray patina (Table 2). Moreover, as observed in other subterranean environments, apparently similar alterations which were often attributed to different species of _Streptomyces_, which arise from the growth of different kinds of bacteria [12]. At times, purple pigmented patinas also due to Actinobacteria colonisation have been detected in the tombs of Tarquinia as well as in other Roman tombs [26; 108]. The phenomena of dark spots caused by various fungal colonisations have also been documented [19; 32; 104; 105]. Microbial patinas may not be easily observable at times, probably due to their growth in underground layers, though their potential aggressiveness should not be neglected. The physico-chemical interaction of the identified species is often underestimated, despite the fact that some species can give rise to biomineralisation phenomena as well as acid metabolite production, such as chelating compounds which can extract ions from the substrata and produce carbonate dissolution, penetrating inside crystal matrices [4; 13; 32]. Several authors also stress the role of microorganisms in biodeterioration processes, since different actinobacteria are involved in the bio-precipitation of minerals. In the Mercareccia Tomb, a relevant number (71) of bacterial strains capable of precipitating carbonates have been detected [39]. Mechanical interaction, such as the capacity of several fungi to dissolve minerals and mobilise metals, is well known. When colonising rock substrata, fungal hyphae can induce a chemical deterioration secreting siderophore-like compounds, whereas the greatest damage is due to mechanic activity. In fact, hyphae of some black fungi demonstrated their ability to penetrate silicate and carbonate rocks exerting strong mechanical pressure up to 12.39 bar gounging cavities at depths ranging from few hundreds of microns to several millimeters and producing pulverisation of the substrate through chemical processes [109; 110]. Recent studies showed that dark spots occurring in the Tomb of Tutankhamun were caused by _Penicillium chrysogenum_ colonisation, which would suggest their detrimental nature, due to the ability to produce malic acid [111]. On the light of some interesting metabolic features showed by soil microorganisms, the use of subterranean strains for biotechnological applications could be a possibility, but further analyses are required. In Etruscan tombs, a fair amount of mechanical damage was caused by root penetration and its development, even up to several meters in depth, which has led to the disintegration of the plaster and painted layers. In some cases, roots systems of different sizes and aggressiveness were highly developed.