# Black on White: Microbial Growth Darkens the External
## 5 Discussion
Bacteria were also detected [3], and their analysis is the subject of an ongoing work. Concerning the photoautotrophic community, the biodiversity seems to be very similar in the investigated areas. Lichen species richness is extremely low as it is expected in an area exposed to pollutants as the Cathedral is. Nonetheless, some species (_Myriolecis albescens_ and _M. dispersa_) show a high prevalence. This result can be likely due to the pollution resistance of these species as well as to microenvironmental conditions. They grow preferentially on biofilm black spots. Cyanobacteria and algae are considered primary colonizers of the bare stones. Cyanobacteria, in particular, can easily develop in harsh conditions, and they widely grow on NW and SE surfaces of the SMFC, being important contributors to the darkening alterations. Dark pigments such as mycosporine-like amino acids and carotenoids (common in cyanobacteria and some green algae) and scytonemin (found in sheathed cyanobacteria) contribute to the blackening phenomena even on other monuments [25; 26]. The coccoid-sheathed cyanobacteria are dominant on the SMFC marbles, but the green unicellular algae showed a broad presence as well. Both unicellular cyanobacteria and green algae may be part of the symbiotic process in the presence of mycobionts, therefore leading to the formation of lichens. On the other hand, fungi can be prevalent in urban conditions since they utilize the airborne anthropogenic compounds [7]. The isolated fungi showed a high biodiversity, with genera and species richness similar at both study sites. Although some differences in species composition at the NW and SE sites were detected, the strong presence of dematiaceous fungi (more than 70% of the total 21 isolated genera)--in particular, of _Alternaria_ and _Cladosporium_ (the most abundant genera)--are nevertheless the common denominator of the fungal community at both the study sites. Dematiaceous or black fungi are an artificial heterogeneous group of darkly pigmented fungi with different morphological characteristics (and often pleomorphic behavior) forming dark-brown, green-black, or black colonies due to the production of melanin and melanoid pigments [54]. Among them, meristematic fungi are well known as rock inhabitants able to deal with varying microclimatic conditions. They have cell walls strongly melanized and form small black colonies on and inside the stone, often occurring in close association with lichens [55]. Black fungi are considered as major agents of microbial deterioration of building stones. Their occurrence on marble produces well-documented effects such as aesthetical damage due to darkening (from black spots to black layers completely covering stone) and other color changes, as well as surface erosion and exfoliation [8; 20; 27; 30; 32; 55; 56; 57]. Their activities are favored by the urban \"air eutrophication\" that increases the occurrence of discolorations and crust formations [30]. Among the isolated dematiaceous fungi, _Alternaria_, _A Aspergillus_, _Aureobasitium_, _Cladosporium_, _Epicoccum_, and _Phoma_ are the most frequent genera present as airborne ubiquitous spores growing on stone in urban environments [16], and well known to cause blackening on marble and limestone [30; 58]. These genera were also found on darkened areas of the external marble of the Milan Cathedral, where their growth was enhanced by naturally aged acrylic resins used as stone protectives and consolidants [19]. Some of them, particularly _Phoma_ and _Alternaria_, are considered as ones of the most damaging organisms that attack and even penetrate the surfaces of stone monuments [29; 30; 56]. While for the above-mentioned dematiaceous genera the role in stone deterioration is well documented, other isolated fungi would be present as airborne spores occasionally deposited or entrapped in biofilms on marble, with an unclear role in stone colonization.