Lichen colonization and associated biodeterioration processes on ancient bricks of the Gonbad-e Qābus tower, UNESCO World Heritage Site, Iran

The impact of lichens on ceramic building materials has received little attention, particularly in the case of bricks and with reference to (semi-)arid bioclimatic areas (i.e. with steppe and desert climates), where they are a remarkable component of built cultural heritage. In this study, lichens were examined on the outer brick walls of the ten-pointed star-shaped tower of Gonbad-e Qābus (N-Iran; UNESCO World Heritage), investigating their diversity and distribution with respect to microclimatic features related to wall orientations, as well as physical and chemical interactions of dominant species with the substrate. Although lichens occurred widely across all the tower walls, higher specific diversity and percentages of colonized bricks were found on sides with higher surface moisture. This was related not only to prevailing wind directions, and consequent wind-driven rain, but also to the lower average values of solar radiation. Light and electron microscopy highlighted the penetration of mycobiont hyphae down to millimetric depths, associated with substrate disaggregation. Energy-Dispersive Spectroscopy, X-Ray Diffraction and Raman analyses documented elemental absorption by hyphae in contact with the substrate minerals, and the formation of calcium oxalate deposits at the interface between some species and the bricks. Such findings provide initial evidence at the microscopic scale of the strong biodeteriogenic impact of lichens on bricks in (semi-)arid climate areas. Further investigations are needed to clarify the balance between these biodeterioration phenomena and the potential counteracting role of thalli in protecting bricks from atmospheric agents, in order to advise on the need for lichen removal.