# Black on White: Microbial Growth Darkens the External
## 5 Discussion
This would be the case for the plant pathogens _Bipolaris_ (strain SD), _Arthrinium_ (strain SF), _Hyplodernella_ (strain NE), and _Coprimellus_ (strain SE and NT-S-10). Concerning _Parengyodontium album_ (strain Nord A), it has been frequently isolated from deteriorated materials of cultural heritage, stone included. Although several authors have linked the development of _P. album_ on cultural heritage monuments to the presence of insects, its role in biodeterioration is debated [59]. A fungus frequently detected at the NW site is _Pithomyces charatum_ (or its telecomorph _Leptosphaerulina charatum_; strains NT-N-6, NT-N-13, NT-N-20 and NT-N-22). _Phitomyces_ strains are commonly isolated from a wide range of plant material; it has been previously isolated from marble [60], but its possible role in deterioration is unknown. For this reason, _Parengyodontium_ and _Pithomyces_ (strain NT-N-13) were included among the strains selected for the calcium carbonate dissolution test (Section 4.5). They resulted negative (Table S1), confirming what was found for _P. album_ by [PERSON] et al. [58], and indicating that these fungi do not carry out this kind of chemical attack on the stone. On the other hand, the ability to dissolve calcium carbonate by isolated strains belonging to _Aspergillus_, _Aureobasidium_, and _Cladosporium_ shows that such degradative potential is present among the SMFC microbial community members already known as stone deriogens. This chemical activity is attributed to the production and excretion of organic acids that dissolve the stone carbonates acting as chelators of calcium and other cations. Other members of the SMFC fungal community, such as _Phoma_ and _Alternaria_, are known to firmly attach to and penetrate deeper into the marble by a physical attack [30]. The combined action of chemical and mechanical processes allows fungi to actively penetrate marble forming euendolithic communities in the bulk of the substrate [8; 32]. Concerning the meristematic fungi, strain SM was identified as _Exophilia capensis_, a species never isolated from stone, and M1 was identified as _Lithophilia guttlulata_, a fungus previously detected as a new species from marble artworks of the Vatican City [57]. On the contrary, strain NT-S-5 seems to be a not yet identified fungus, since ITS rDNA sequence analysis allowed to classify it up the _Dothidomycetes_ class. Overall, the picture of the fungal community emerging from the current cultivation data shows a prevalence of dematiaceous hyphomycetes with respect to meristematic fungi, according to the climate conditions of Florence. In fact, hyphomycetes including species of _Alternaria_, _Cladosporium_, _Epicoccum_, _Aureobasidium_, and _Phoma_ dominate the fungal communities on monuments in moderate and humid climates, while microcolonial black fungi dominate the fungal community in arid and semi-arid environments [55]. However, since we used cultivation conditions favoring the growth of fast-growing fungal strains (Section 3.4), the presence of meristematic fungi on SMFC marble will be better assessed by using more appropriate cultivation conditions for their growth. Other than biodiversity, the colonization pattern seems to be different at the NW and SE sites. At the NW site, the community grows mainly as a thick epilithic biofilm completely covering the marble surface and strongly adhering to it by EPS (Figures 2, 4 and 5). The biofilm does not invade the inner parts of marble, given the compactness of marble grains (Figure 5). At the SE site, microorganisms grow mainly as epilithic colonies or biofilms along boundaries of marble grains, and as endolithic biofilms that penetrate through and surround the grains' boundaries up to 4 mm in depth (Figures 2, 4 and 5). This penetration pattern relates to the observed detachment of marble grains (Figure 5b).