Biofilm: A biofilm forms when certain microorganisms adhere to a surface in a moist environment and begin to reproduce. In nature biofilms almost always consist of mixtures of many species of bacteria, as well as fungi, algae, yeasts, protozoa, and other microorganisms, along with non living debris and corrosion products. Biofilms are aerial – on the surface – and sub-aerial – below the surface.
Biocide: A solution killing the biofilm.
Metabolite: A non living substance produced by the biofilm generally to protect the organism from light. On buildings they are often red or black. Sometimes yellow or dark brown depending on the producing species. Metabolites are water soluble and undergo photolysis – damages by UV light.
Phototrophic organisms (algae, cyanobacteria) use carbon dioxide (CO2) from the atmosphere and sunlight as their carbon and energy source.
Heterotrophic organisms (most bacteria and all fungi) need some organic source for their growth, and this is provided by the metabolites of phototrophic organisms or by airborne deposition.
Cortex: The external skin of a fungal growth. In stone the cortex is often black and hard, sometimes harder than the stone host.
Photolysis: The chemical decomposition of materials under the influence of light.
Examples of fungal colonisation of render related to thermal transmission