Biofilm colonization of metamorphic lithotypes of a renaissance cathedral exposed to urban atmosphere

Abstract

Stone architectural heritage exposed outdoor represents a challenging habitat for biological growths; nevertheless, biocolonization on heritage structure is ubiquitous and represents a major mechanism of alteration. However, the identification of specific microorganisms with known reactivity towards the stone substrate does not necessarily imply that a biodeterioration process is in progress and, in specific conditions, bioprotection effects have been highlighted as a result of colonization. The main objective of the present research is to evaluate the biofilm formation on different lithotypes exposed to similar environmental polluted conditions, and to investigate whether the presence of subaerial biofilms can be associated to an increased magnitude of deterioration of the colonized surfaces with respect to the not colonized ones. In particular, the research examines the extensive biological colonization of the stone surfaces of the façade of the Cathedral of Monza (Italy). Four metamorphic stones widely used in the façade and showing rather different compositional, mineralogical and microstructural features were studied. The state of conservation of the stones was characterized under the mineralogical and compositional point of view by X-ray diffraction and Fourier Transformed infrared analysis. The microstructure of colonized substrates and of reference not colonized ones was studied by means of optical and electron microscopy, to comparatively evaluate the damage extent and weathering patterns in both conservative conditions. The structure and the architecture of biofilms growing on different lithic surfaces were investigated by CLSM in both fluorescence and reflection modes. Captured images were analyzed for 3D reconstructions of biofilm samples. The biovolumes were also calculated to estimate the total biomass. The results indicate that the four lithotypes showed different colonization extents. However, even in presence of extensive biological growth, chemical-physical deterioration mechanisms caused by environmental exposure were largely responsible for deterioration. A relationship between compositional and surface morphological features and biocolonization was also observed.

Aesthetic Alteration of Marble Surfaces Caused by Biofilm Formation: Effects of Chemical Cleaning

“…Microscope observations and molecular analysis showed that Cyanobacteria, Proteobacteria and Actinobacteria were the main bacteria present on the untreated Candoglia marble. The microscopic investigations did not show the presence of hyphae and spores that might be correlated to the presence of fungi as also reported in our previous investigation [1]. Alphaproteobacteria are often the predominant taxa on stone buildings [37], and in the Altamira cave they have been found to be the most metabolically active class of the Proteobacteria [38].…”

“…In previous work by our team [1], we proved that despite the massive presence of biofilms causing aesthetic alteration to the façade of Monza Cathedral, the biocolonization was not a primary damaging factor if compared to chemical-physical deterioration due to the aggressive polluted atmosphere. Nonetheless, the conservators of the Monza Cathedral have tried to remove the sessile dwelling microorganisms to reduce discoloration based on their previous experiences in similar cases.…”

“…The main mineralogical phase is calcite, with minor quartz and presence of pyrite. The complete mineralogical and compositional characterization of Candoglia marble samples from the façade of the Monza cathedral has been reported in a previous work [1].…”

Development of Multifunctional Coatings for Protecting Stones and Lime Mortars of the Architectural Heritage

“…Built Heritage is subjected to deterioration caused by a number of factors, environmental and anthropogenic as well. Among the first type, blackening of surfaces due to carbon particles accumulation, nitrogen oxides action and bacteria-induced growing of microorganisms may be cited (Brimblecombe and Grossi, 2005;Gulotta et al 2018;Pozo-Antonio et al 2019;Vidal et al 2019) The water access to the inner part of the building materials might be commented as another way of deterioration (Brai et al, 2007;Piuzzi et al 2018). As a matter of fact, water can penetrate into the materials giving rise to soluble salts dissolution, efflorescences and the breaking of the materials due to freezing-thawing cycles.…”

Conservation of the Built Heritage: Pilot Site Approach to Design a Sustainable Process

“…This included optical and scanning electron microscopy, Fourier-transformed infrared spectroscopy, and X-ray diffraction analysis. Specific diagnostic techniques were also employed to analyze the extensive biocolonization affecting the crowning element of the façade, to better understand the potential damaging role of the biofilms [40]. This last phase of the preliminary knowledge of the site features highlighted, as expected, that the deposition of particulate matter and black crust formation were major damage mechanisms of the stone elements partially sheltered from the rain runoff.…”