The lichen that threatens the monuments of Persepolis

Lichens are stable unions formed by fungi and algae. An important but little-known aspect of lichens is their ability to degrade rocks in the natural environment. Little by little, over centuries, lichens, mainly the hyphae of the fungus that penetrate the substrate, promote the disintegration of the rock they colonize. These natural processes can have a major, often damaging, effect when lichens grow on stones that form part of monuments or man-made buildings, contributing to their deterioration. This degradation of the rock is especially harmful in those monuments that show intricate bas-reliefs and sophisticated stonework that are susceptible to loss.

This is the case of Persepolis, the ancient capital of the Persian Empire during the Achaemenid era, located in the province of Fars, in Iran, just 50 kilometers from the city of Shiraz. Founded by Darius I in 521 BC. C. and later destroyed by Alexander the Great in 330 BC Persepolis, a UNESCO world heritage city, contains the remains of the splendor of that time, reflected in the archaeological remains of the ancient palaces, the cuneiform writing engravings and delicate bas-reliefs. . This invaluable heritage, preserved for millennia, has in some species of lichen, silent enemies that dent and pierce the walls of the ancient palaces and erode their magnificent bas-reliefs.

For more than a decade, Sergio Pérez-Ortega, researcher at the Royal Botanical Garden (RJB), and Asunción de los Ríos, researcher at the National Museum of Natural Sciences (MNCN), both centers of the Higher Council for Scientific Research (CSIC) in Spain, work in collaboration with the Iranian expert Mohammad Sohrabi, from the Iranian Organization for Research in Science and Technology (IROST), in the study of the effect of lichens on different monuments in Iran that are part of the world heritage.

As a result of this work, a species of lichen-forming fungus called Circinaria persepolitana has just been described, which, so far, is only present among the archaeological remains of Persepolis, where it is especially abundant. According to the new study, the new species, described by Sergio Pérez-Ortega and Mohammad Sohrabi, can cover large areas of the engravings and bas-reliefs of Persepolis, so its effect on the monumental rock has been analyzed by Asunción de los Ríos through application of scanning electron microscopy techniques.

Circinaria persepolitana on the bas-relief showing several warriors in the palace of Apadana, in Persepolis. (Photo: Sergio Pérez Ortega / RJB / CSIC)

“The new species has proven capable of exerting biogeophysical and biogeochemical changes on the rock surface associated with its establishment and, therefore, represents a potential threat to the conservation of this relevant heritage, which is why Sohrabi is already working on the possibility of eliminating the species on those surfaces of greatest artistic value,” says Pérez-Ortega.

The work carried out by Asunción de los Ríos has allowed us to calibrate in a detailed and precise way the real damage that this species can cause in bas-reliefs of high artistic value. “The decision to eliminate lichens and other organisms from cultural heritage is not always easy to make; it would have to be based on the impact that the different species present can exert, but there are also other factors to take into account such as the speed of growth and recolonization , the damage of the treatments per se on the monumental stone or the value of the diversity found as in this case,” says the MNCN researcher.

The study is titled “Circinaria persepolitana (Megasporaceae), a new lichen species from historic stone surfaces in Persepolis, a UNESCO World Heritage Site in Iran.” And it has been published in the academic journal The Lichenologist. (Source: CSIC)