Biodeterioration of Natural and Artificial Stones Caused by Lichens and Algae

摘要

This paper briefly illustrates the main features of the biodeterioration caused by lichens and, to a lesser extent, algae, on the different kinds of stones that are extensively employed in the building of historical monuments. The actions of two epilithic lichens belonging to the genera Caloplaca and Lecanora, and of the endolithic Verrucaria on substrate surfaces are discussed, with some remarks on the possible effects of the application of biocides to remove the biodeteriogens without damaging the monuments, particularly in the case of endolithic lichens. The analysis of substrate-lichen interaction has been carried out on samples of stones colonised or not colonised by the biodeteriogens, and on samples after biocide treatments. The samples were prepared for light and fluorescence microscopy, polarised light microscopy, scanning electron microscopy, X-ray microanalysis and X-ray diffraction. Our data show that the stone bioalteration is caused mainly by lichens and consists in physical and chemical modifications of the rock texture and composition in the layer where the lichen hypae and perithecia penetrate and eventually accumulate Ca-oxalate crystals and crusts due to calcite re-precipitation. The tests to remove algae and lichens without damaging the substrates were useful after biocide treatments.