A Biofilm forms when certain microorganisms adhere to a surface in a moist environment and begin to reproduce.Biofilm formation on surfaces usually starts with phototrophic organisms (algae, cyanobacteria) which 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. The biofilm community is therefore sometimes formed by a single microbial species, but 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.
All biofilm forming microorganisms may cause biodeterioration and degrade stone or render mechanically, chemically and aesthetically through the metabolic activities and biomineralization process in these biofilms.
It is apparent that some species, in particular pigment-producing fungi (Alternaria, Penicillium, Aspergillus), can lead to deterioration of appearance which, even after removal of the species itself, can be difficult to remove.
Other species such as algae and lichens are also very brightly coloured and can rapidly develop and spoil the appearance of surfaces