Microbes on building stone: for good or ill?

Microorganisms play a crucial role in mineral transformation in the natural environment, notably in the formation of soils from rocks and the cycling of elements such as nitrogen and sulphur. It is therefore not surprising that a wide variety of micro-organisms, especially bacteria and fungi, have been isolated from rocks and the stonework of historic monuments and buildings such as Portchester Castle (Figure 1). The complex interaction of numerous microbial types at a microscopic level in intimate association with the mineral substrate is readily observed1 often reaching deeper than 3cm into the stone. Microorganisms can be on or inside stone, as endolithic communities. In some circumstances their longterm surface growth establishes a coloured, varied patina, which can sometimes be protective to the underlying stone. Often, however, some types of patina growth leads to damage caused by erosion, biopitting and exfoliation (Figure 2). Research has highlighted a possible role for microbes in stone deterioration due to one or more mechanisms: their presence as undesirable surface growths (aesthetic), mechanical damage (biogeophysical change) by biofilms or penetrating hyphae and corrosive effects (biogeochemical change) due to metabolic activity (Table 1). Scientific investigation can present severe problems with objects of cultural value. Phototrophic organisms such as higher plants, lichens and mosses, together with algae and cyanobacteria, cause obvious surface effects. The impact of most bacteria and fungi is more difficult to appreciate and separate from purely physical and chemical phenomena that are acknowledged threats to the integrity of building stone.