Influence of atmospheric pollutants on the biodeterioration of stone

Studies have been carried out on specimens of Saltrio stone, Carrara and Candoglia marbles, exposed for some years to the polluted atmosphere of Milan. The test specimens, which were directly exposed to the washing away, showed alterations in the form of grey-black stains which were different in dimensions and quantity according to the stone material, while those sheltered showed only a uniform grey deposit of particulate matter. The decay on the exposed specimens has been tested by gravimetric analyses and scanning electron microscopy (SEM) observations. Microbiological investigations evidenced that microorganisms, either chemoautotrophic or photosynthetic, were not present in any sample. On the washed out test specimens, the presence of heterotrophic bacteria was considerable, while the sheltered specimens showed negligible values. Chemical analysis, carried out through gas chromatography–mass spectrometry (GC/MS), of the particulate deposited on the test specimens, evidenced the presence of a wide range of aromatic and aliphatic hydrocarbons and fatty acids. Cultural tests of mixed cultures and isolated strains were carried out by using a mineral medium added with 14 organic compounds, typical representatives of atmospheric pollution, as a sole source of carbon and energy. The results evidenced that the mixed cultures in particular, as well as some of the isolated strains, are capable of using many of the tested molecules (fatty acid, aliphatic compounds, p-xylene).