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Chemoorganotrophic fungi are especially concentrated in stone crusts. They are able to penetrate into the rock material by hyphal growth and by biocorrosive activity, due to the excretion of organic acids or by the oxidation of mineral forming cations, preferably iron and manganese.
This can often leave a rust-like stain on paving slabs even after washing. The deterioration activities of dematiaceous fungi also include the discolouration of stone surfaces, due to the excretion of dark pigmented melanin.
Species capable of such staining are Alternaria, Aureobasidium, Cladosporium and Penicillium spp. The photographs below show a building wall covered with fungal growth (identified as Alternaria) sp.. The microscopic images show the development of fungal hyphae and on close-up, the pigmented spores can be seen.
Filamentous fungi are composed of many cells forming thread-like hyphae which form a mycelium, or network of hyphae. Most fungal species prefer to grow at acid to neutral pH (3-6) and only require 30% moisture to survive. From these dense mats, aerial reproductive hyphae (known as conidiophores) will release millions of spores into the space directly above and hence cause massive colonisation in suitable growing areas.
The most common contaminants on substrates such as roofs, pavements and walls are the pigment producing Aureobasidiumsp sp., and Cladosporium sp. and Alternaria sp.
These species will rapidly spread and colonise many surfaces causing disfigurement and decay by the release of pigments and acids respectively.
Indeed there are scientific studies that have identified over 22 fungal flora including filamentous micro fungi to be present on sandstone buildings. The major contaminant isolated was Aspergillus sp. which produced dark pigments and organic acids which were responsible for the decay of the sandstone historical monument.
The photographs below show the dark coloured species thought to be Aspergillus niger growing on sandstone paving stones.