## 3 Results  
### First Screening  
The first screening can be undertaken by naked eye or by using a portable microscope with about 20\(\times\) magnification which can help to choose the sampling area and to make a rough morphological discrimination, through the inspection of the color and morphological aspects of the biological colonization (mat aspect with visible filaments, patina aspect with uniform color, spots like growth, etc.). A good camera with a macro objective can also be very useful. In this step, it is difficult to assign the sample to a single phototrophic group, as on monumental fountains and other wet stone surfaces there are developing communities of microorganisms, which are a mix of different cyanobacteria and/or green algae and/or diatoms, as can be seen in some fresh samples observed under the microscope (Supplementary Materials S1-S5). Diatoms prefer high levels of humidity and are more often associated with green algae, and less only with cyanobacteria, but they can be found beside algae and cyanobacteria as well. Often, the color gives an idea about the possible phototrophic presence, but it is never conclusive [37]. * visible vivid green, light green, green or brownish phototrophic growth..  
Figure 1: Some simple cyanobacteria (blue-green algae): (**A**) _Gloewdhece_ (4a), (**B**) _Gloewdapsa_ (5a), (**C**) _Chroococcus_ (5b), (**D**) _Microcystis_ (7a), (**E**) _Aphanocapsa_ (7b), (**F**) _Aphanothece_ (8a), (**G**) _Chlorogloa_ (9a), (**H**) _Chroococcidiopsis_ (9b), (**I**) _Cyanosarcina_ (12a), (**J**) _Myasarcina_ (12b). Scale bar 10 μm. The number in the brackets corresponds to the one indicated in the identification key. Figure 2: Some filamentous cyanobacteria (blue-green algae): (**A**) _Chumaselphon_ (2a), (**B**) _Pleurocapsa_ (2b), (**C**) _Oscillatoria_ (5a), (**D**) _Nostoc_ (6a), (**E**) _Calothrix_ (9a), (**F**) _Tolypothrix_ (10b), (**G**) _Symploa_ (13b), (**H**) _Schizothrix_ (14a), (**I**) _Phormidium_ (16a), (**J**) _Microclous_ (17a). Scale bar 10 μm. The number in the brackets corresponds to the one indicated in the identification key. trichomes not clearly developed, compact groups of cells........................................2 trichomes clearly developed, isolated or forming mats or pellicles........................................3 short apical row of cells easily damaged, basal cells with colored sheath, forming macroscopic brown spots..  
Figure 3: Some simple green algae: (**A**) _Euglena_ (3b), (**B**) _Chlamydomonas_ (4a), (**C**) _Haematococcus_ (4b), (**D**) _Oocysts_ (8a), (**E**) _Cosmarium_ (12a), (**F**) _Chloroccecum_ (13b), (**G**) _Scenedesmus_ (17b), (**H**) _Chlorosarcinopsis_ (19a), (**I**) _Tetracystris_ (19b), (**J**) _Palmella_ (20a). Scale bar 10 μm. The number in the brackets corresponds to the one indicated in the identification key. Figure 4: Some filamentous green algae from the C2 group: (**A**) _Spirogyna_ (2a), (**B**) _Oedogonium_ (3a), (**C**) _Klebsormidium_ (4b), (**D**) _Illothrix_ (4a), (**E**) _Apatococcus_ (5a), (**F**) _Cladophora_ (6a), (**G**) _Gongrosira_ (7a), (**H**) _Leptosira_ (8a), (**I**) _Dilabiflum_ (9b), (**J**) _Pleurastrum_ (9b). Scale bar 10 \(\mu\)m. The number in the brackets corresponds to the one indicated in the identification key. Figure 5: Some diatoms: (**A**) _Diatoma_ (2a), (**B**) _Melosira_ (2b), (**C**) _Navicula_ (8a), (**D**) _Gomphonema_ (9b), (**E**) _Cymbella_ (10a), (**F**) _Amphora_ (10b), (**G**) _Surirella_ (11b), (**H**) _Nitzschia_ (12a), (**I**) _Epithemia_ (12b), (**J**) _Achranthets_ (13a). Scale bar 10 \(\upmu\)m. The number in the brackets corresponds to the one indicated in the identification key. cells with a colonial organization, united in filament-like structures, numerous discoidal plastids..................................2
* solitary cells, sometimes forming colonies, with valves of different shapes (elliptic, lancoelate, oblong, etc.