Biochemical weathering mechanisms carried out by Penicillium frequentans and Cladosporium cladosporoides on unaltered sandstone, granite and limestone were studied using FTIR, X-ray diffraction, atomic absorption and flame photometry. Strains belonging to both fungal species, isolated from the facades of two Spanish Cathedrals, were used.Large amounts of oxalic, citric and gluconic acids were produced by P. frequentans in broth cultures. These metabolites caused extensive deterioration of clay silicates, micas and feldspars from both sandstone and granite and also of calcite and dolomite from limestone, as a result of high cation release and organic salts formation such as calcium, magnesium and ferric oxalates and calcium citrates. Comparatively, the biodegradative effect brought about by C. cladosporoides was much less than that caused by P. frequentans. Neither organic acids nor organic salts were formed by C. cladosporiodes samples.It is concluded that filamentous fungi are able to cause an extensive weathering of stone, due principally to organic acid excretion, although other metabolites participate to a lesser extent in these deteriorative processes. Ecological adaptative mechanisms, such micronutrients uptake and trivalent cations chelation (Fe3+ and A13+) are derived from fungal growth on stone monuments.

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Acid and nonacid generating fungal strains isolated from weathered sandstone, limestone, and granite of Spanish cathedrals were assayed for their ability to oxidize iron and manganese. In general, the concentration of the different cations present in the mineral salt media directly affected Mn(IV) oxide formation, although in some cases, the addition of glucose and nitrate to the culture media was necessary. Mn(II) oxidation in acidogenic strains was greater in a medium containing the highest concentrations of glucose, nitrate, and manganese. High concentrations of Fe(II), glucose, and mineral salts were optimal for iron oxidation. Mn(IV) precipitated as oxides or hydroxides adhered to the mycelium. Most of the Fe(III) remained in solution by chelation with organic acids excreted by acidogenic strains. Other metabolites acted as Fe(III) chelators in nonacidogenic strains, although Fe(III) deposits around the mycelium were also detected. Both iron and manganese oxidation were shown to involve extracellular, hydrosoluble enzymes, with maximum specific activities during exponential growth. Strains able to oxidize manganese were also able to oxidize iron. It is concluded that iron and manganese oxidation reported in this work were biologically induced by filamentous fungi mainly by direct (enzymatic) mechanisms.

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