Microorganisms and Microbially Induced Fabrics in Cave Walls

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Abstract
Host-rock, speleothems, and rock art paintings from Altamira and Tito Bustillo Caves (northern Spain) are coated by dense biofilms of phototrophic microorganisms (cyanobacteria and algae) and networks of heterotrophic bacteria (mainly actinomycetes). These microorganisms induce constructive (calcification, crystalline precipitates) and destructive fabrics (irregular etching, spiky calcite). To assess the microbial impact on both caves - which contain valuable rock art paintings - mineralogical, petrographical, hydrochemical, microenvironmental, and microbiological studies were carried out. The identified damage includes: (1) covering (scattered colored spots, whitish powdery patinas)of paintings by the microbial communities themselves and/or by their metabolic activity (including biofilms and 'bioinduced' precipitates); (2) chemical alteration, such as microbially mediated dissolution; and (3) mechanical alteration, such as substratum breakdown and scaling. In addition, some types of speleothems, such as hydromagnesite coatings, traditionally attributed to inorganic precipitation, can be considered to be a product of microbial activity, as this activity could foster the precipitation of Ca–Mg-carbonates by creating conditions of high alkalinity with low PCO 2 values. Hydrochemical, geochemical, and microenvironmental data exclude an inorganic process in the precipitation of hydromagnesite and huntite in Altamira Cave.