Novel Methods for the Assessment of Biodeterioration of Stone Monuments

El autor González Grau, Juan Miguel pertenece actualmente al Instituto de Recursos Naturales y Agrobiología de Sevilla CSICSeveral methods for material characterization and surface analysis such as scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM), energy dispersion X-ray analysis (EDX), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Mossbauer spectroscopy (MS) are used for assessing weathering and biodeterioration effects on stone monuments. Novel molecular biology techniques to identify the microbial components of biofilms are also described.Cultural heritage covers a wide diversity of archeological monuments and sites deemed to have artistic or historic significance by both the local and the international societies. Three main factors are generally involved in different extents in relation to materials decay : a) weathering of materials exposed to the environment; b) biodeterioration processes; c) natural and anthropogenic pollution. The environment plays a relevant role in the type and degree of deterioration experienced by cultural property. Relative humidity, high temperatures and pollution usually enhance biodeterioration hazard. Weathering, the breakdown of rocks resulting from their direct contact with the prevailing atmospheric conditions, may be due to mechanical or chemical effects. In the former case rock is broken by physical processes while in the latter the constituents of the rock undergo chemical changes which lead to their decomposition. Under natural conditions, mechanical and chemical processes of weathering usually take place concurrently. Biodeterioration is the physical or chemical damage caused by biological agents on structural materials and occurs through the formation of biofilms. These films are biological deposits of a highly hydrated gel of extracellular polymeric substances containing microbial cells and inorganic detritus. Biodeterioration has usually been considered to be a secondary degradation process following the initial deteriorating effects of inorganic agents. These effects include the aesthetically unacceptable appearance of staining of stone surface and the entrapment of atmospheric pollutants by biofilms. The bioreceptivity of stones is influenced by their chemical nature, physical structure and geological origin, such as from igneous, sedimentary or metamorphic rocks. Natural and anthropogenic pollution impact the structural materials according to the aggressiveness of each different environment. The former is due to natural contaminants such as sea salts and particulate matter from meteorization processes while the latter may be produced in diverse activities like energy generation, transportation and agriculture. Anthropogenic pollutants include compounds such as sulfur and nitrogen oxides, organic volatile compounds and many other organic and inorganic substances. These pollutants can react with materials, affecting their properties or they can act as catalysts of material decay by other chemical, physical or biological routes. There is a close inter-relationship between the three factors mentioned before as the main causes of deterioration. Weather directly influences the local environmental conditions enhancing biodeterioration through synergistic effects. The microflora improves the nutrient and moisture levels needed for microbial growth by forming surface-covering biofilms which are favoured by the anthropogenic euthrophication of the atmosphere. A wide diversity of microbiological, surface analysis and characterization techniques complemented with microscopic observations are needed to reach an appropriate assessment of biodeterioration and weathering effects on structural materials of cultur.N