Primary bioreceptivity of limestones to phototrophic microorganisms: a laboratory-based stone colonization experiment

Abstract

The conservation of historic buildings and monuments from cultural heritage is a major issue in modern societies, both from economic and cultural viewpoints. This study aimed to evaluate the primary bioreceptivity of limestones widely used as building materials in Mediterranean countries to further establish an index of bioreceptivity. The experimental set up relied on: i) molecular and culture characterization of photosynthetic-based biofilms collected from five limestone monuments from Portugal, Spain and Italy, to select the most adequate culture to be used as inoculum for laboratory based bioreceptivity experiments; ii) inoculation of five limestone types with the phototrophic culture: Ançã limestone (CA), Lioz limestone (CL), San Cristobal stone (SC), Escúzar stone (PF) and Lecce stone (PL); iii) incubation for 90 days within a growth chamber and monitoring of photosynthetic biomass using different analytical approaches. DNA-based molecular analysis revealed that the biofilms from Orologio Tower (Lecce, Italy) and Santa Clara-a-Velha Monastery (Coimbra, Portugal) were dominated by microalgae, particularly Chlorella, whereas the cyanobacterium Chroococcidiopsis was the dominating genus from Ajuda National Palace (Lisbon, Portugal). The biofilms from Seville and Granada Cathedrals (Spain) were mainly composed of the cyanobacterium Pleurocapsa. DGGE analysis of the cultivated biofilms revealed a remarkable stability, of the microbial components from the Coimbra biofilm, which was further used as inoculum for the laboratory-based bioreceptivity experiment. The results from the petrophysical characterization of the lithotypes and from microbial growth monitored along the course of the bioreceptivity experiment were statistically analysed by means of PCA and ANOVA in an attempt to determine the primary bioreceptivity of the five lithotypes to phototrophic microorganisms and to evaluate the direct relationships between stone bioreceptivity and petrophysical properties. The limestones PF followed by SC showed the highest bioreceptivity, contrasting with the lowest bioreceptivity of CL. The CA and PL revealed moderate bioreceptivity. The petrophysical characteristics, water absorption by capillarity, surface roughness, as well as open porosity and water vapour permeability, were the keys for the microbial development on the stone samples. The high values of water absorption by capillarity and open porosity obtained for PF, SC and PL indicate great susceptibility to biodeterioration, which render them unsuitable for very humid outdoor environments if preventive treatments, such as the application of hydro-repellents, are not taking into account. The relative low values of water vapour permeability and open porosity of CA renders this lithotype less bioreceptive for microbial colonisation. The very compact nature of CL and its extremely low capillarity coefficient, open porosity, surface roughness and water vapour permeability constitute an impediment to microbial growth. The diffusion of these data to end-users (e.g. conservators/restorers, engineers, architects) may contribute to decision-making on the lithotypes to apply in future building constructions and to choose the best conservation strategy based on the potential bioreceptivity characteristics of the materials used in monuments.