Microbiological and calorimetric investigations on degraded marbles from the Cà d'Oro facade (Venice)

Microbiological, physicochemical and biochemical investigations were carried out on degraded marble samples from the Ca d'Oro facade (Venice) in order to verify the presence of biodeterioration agents. The aerobic heterotrophic bacteria (mean value 10(5) CFU/g marble), which were also observed by scanning electron microscope (SEM), were characterized as belonging to the Micrococcus and Bacillus genera. Nitrifying and sulphooxidizing bacteria were not found in any samples. The fungi (mean value 10(2) CFU/g marble) were attributed to the Penicillium and Aspergillus genera. Enrichment cultures revealed the presence of photosynthetic microorganisms belonging to the div. Cyanophyta and Chlorophyta. The metabolic activity of the microorganisms derived from the cultural tests, was confirmed by the ATP content extracted directly from the powdered marble samples (average 3170 pg ATP per g marble). Microbiological alterations of stone are the result of the metabolic activity of the microorganisms, whose heat production is an expression of this activity. Confirmation of the presence of the microorganisms on the degraded marble samples was, therefore, obtained by calorimetric investigations, which revealed that the process is completely exothermic and can be divided into two phases: the first showed the attainment of a plateau, typical of a zero-order reaction; the second was characterized by a peak followed by an abrupt arrest.