DETERIORATION OF SILICEOUS STONE MONUMENTS IN LATIN AMERICA: MICROORGANISMS AND MECHANISMS

摘要

The microbial community present on and within stone monuments is highly varied and is subject to high and low temperatures, UV irradiation, and repeated desiccation. The mechanisms of the biodegradation of stone commonly thought to be important are acid- and base-induced dissolution of silica and mobilization of cations by chelation. The endolithic and epilithic microbial communities produce polyols as osmotic protectants (osmolytes) in response to desiccation. Low molecular weight polyols and polysaccharides (high molecular weight polyols) bind to the siloxane layers within layered siliceous minerals, such as micas and soapstone, by hydrogen bonding. These interlaminar complexes cause expansion of the crystalline layer, weakening the structure, and may allow entry of chelating agents, which mobilise the ions stabilising the crystal structure. During periods of desiccation, polyols become concentrated, forming non-aqueous systems where the chemistry of Lewis acids and bases must be considered. Basic catalysis in such water deficient ecosystems will favour the formation of water soluble organosilicon compounds, principally organic siloxanes. Such ecosystems with low water activity are common in all dry environments. Polyols and complex organic acids can attack siliceous minerals under alkaline conditions. Extracellular carbohydrate polymers released by fungi and, especially, bacteria can react with inorganic siloxanes to form water soluble organic siloxanes. Extracellular polymers on the surface also prolong water residence time, increasing the duration of the chemical reactions causing silicate weathering. The final result of all of these activities is the expansion of the rock or stone and spalling of the surface layers from the weakened material behind the spalling layer.