Microbial melanins in stone monuments

In polluted environments, black deposits on the surface of building stones have traditionally been considered a result of fossil fuel combustion. This has been supported by SEM observations and EDX analyses of gypsum black crusts as well as by chemical characterization of the organic compounds. However, very recently, several authors have hypothesized that black colouration on marbles of historic buildings could have a biogenic origin and that it is related to microbial melanins. In order to avoid confusion, both in definitions and in terms of chemical nature, the possible occurrence of microbial melanins on building stones and their relationship with humic substances and related materials are discussed.