Algae

Algae are micro-organisms that colonize the surface of organic materials such as stone or wood, forming powdery or viscous deposits ranging from one tenth of a millimeter to several millimeters in thickness. Algae form green, red, brown, or black veil like zones and can be found mainly on situations where the substrate remains moistened for long periods of time. Depending on the environmental conditions and substrate type, algae may form solid layers or smooth films. On monuments, algae are comprised of unicellular to pluricellular clusters, they never form macro-organisms.

Algae generally constitute a superficial surface layer and not adherent layers that detach upon drying. They may also be found deeper into the substrate (e.g. under scales, in cracks, etc.). Several groups of algae may grow on and in the substrate depending on climate and the characteristics of the colonized material. Algal blooms in green, yellow, brown, and in rare cases, red, may occur. Cyanobacteria (formerly called blue-green algae) are commonly found in stone and can cause black, bluish or even violet stains.

In some cases the substrate (i.e. stone, wood, etc.) serves as a source of nutrients. However the surface is usually only a solid host for growth.