Fungal and algal biomass in biofilms on artificial surfaces quantified by ergosterol and chlorophyll a as biomarkers

Abstract

The occurrence of microbial colonisation on artificial surfaces in urban areas (building facades, roof tiles) and interiors (damp rooms: silicone sealings, wallpaper) causes not only an aesthetically unacceptable discolouration of the surface, it also represents a conspicuous problem in terms of biodeterioration, accelerated weathering and a potential health risk for humans by fungal mycotoxines. The most conspicuous organisms responsible for biofouling terrestrial surfaces are fungi and green microalgae as well as cyanobacteria. Microorganisms are often estimated by cultivating, but the biomass can be misjudged because of contaminations or the existence of non-culturable cells and spores. By using ergosterol as a specific biomarker for living fungi and yeasts in combination with chlorophyll a for aeroterrestrial microalgae it is possible to quantify fungal and algal infection independent from cultivation and undisturbed by surface (dis)colouration. Using HPLC and photometric methods, conversion factors for microbial biomass were determined in representative fungal and algal species: 5 mg ergosterol g−1 fungal dry biomass and 23 mg chlorophyll a g−1 algal dry biomass. The applied wipe technique allowed a non-invasive sampling and evaluating of infections per square meter. Very low detection limits for ergosterol and chlorophyll a permitted determining fungi and algae before they become macroscopically visible. Up to 36 mg ergosterol m−2 and 180 mg chlorophyll a m−2 were detected on outdoor artificial surfaces, equivalent to approximately 7 g m−2 fungal dry biomass and 8 g m−2 algal dry biomass. The distribution and the composition of the microbial communities varied strongly between the sampling locations. Fungi were observed above windows of damp rooms or at more sun-exposed locations, whereas algae covered more wet and shadowed surfaces. The established methods are well suited for the precise joint determination of fungal and algal biomass in microbial communities of natural biofilms on artificial surfaces as a pre-condition for the development of prevention strategies against microbial colonisation.