In this review some case studies are presented to illustrate the complex interactions between organisms, substratum, and environment as an indication of the biogeochemical processes originating in monuments subjected to different environmental conditions. Building materials in urban environments develop black crusts, composed mainly of gypsum and polycyclic aromatic hydrocarbons. Gypsum is used by cyanobacteria as a sulfate source, whereas heterotrophic bacteria are able to use polycyclic aromatic hydrocarbons as a sole carbon and energy source. On the other hand, in hypogean environments with natural openings to sunlight, calcifying cyanobacteria mobilize calcium from the substratum to produce calcite crystals, which are deposited on the sheath and form a mineral coat. In rural environments, most conspicuous biogeochemical processes derive from natural colonization of building materials by cyanobacteria, algae, and lichens. Although cyanobacteria and algae dissolve calcium carbonate, lichens in some cases confer protection through the formation of oxalate patina, which protects the surfaces from weathering.
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