Characterization of a biofilm and the pattern outlined by its growth on a granite-built cloister in the Monastery of San Martino Pinario (Santiago de Compostela, NW Spain)

The upper zone of the background walls of the processional cloister of the Monastery of San Martino Pinario (Santiago de Compostela, Galicia, Spain) is affected by a deep green, highly hydrophobic subaerial biofilm. The pattern that the biofilm follows the walls suggests that particular microclimatic conditions induce changes in the biofilm properties. To test this hypothesis, taxonomic and structural identification of the biofilm was carried out by respectively light microscopy and confocal laser scanning microscopy. In addition, the chemical composition was determined by quantification of extracellular polymeric substances in the extracellular matrix of the biofilm, and hydrophobicity was determined by contact angle and water drop penetration time of biofilm and cells. Furthermore, the bioprotective or biodeteriorative role of the biofilm on the cloister is discussed on the basis of the results of stereoscopic microscope observations, X-ray diffraction and attenuated total reflectance-Fourier transform infrared spectroscopy analysis of granite samples from uncolonized areas and from underneath the biofilm. The findings showed that Apatococcus lobatus is the predominant algae in the biofilm. The presence of this alga is favoured by water condensation and it neither damages nor protects the substrate, only causing an aesthetic impact.