Lichens and their impacts on cultural heritage: current methods and future strategies for their removal

BACKGROUND AND OBJECTIVESLichens as one of the important biodeteriorating organisms have a main role in the biodeterioration of cultural heritage monuments, although in some conditions, they show protective impacts and have a conservative effect to protect substrate damages. Since lichens mainly have deleterious effects, it is necessary to remove or mitigate with proper methods.Lichens with fungal hyphae penetration can able to cause to physical damage and with material secretion by mycobionts and photobionts can lead to chemical damage. Lichen produce encrustations in the stone material and can be cause biodeterioration in cultural heritage. Oxalic acid secreted by lichens mycobiont leads to chemical damage and chelate metal ions such as calcium and therefore widespread erosion in their substrata. The aim of this study was to overview on dual role of lichens in cultural heritage and strategies for control and remove of destructive lichen.MATERIALS AND METHODSArticles related to the subject were searched in Scopus, Science Direct and Web of Science databases and articles that evaluated the effects of lichens in biodeterioration and bio-protective of cultural heritage substrate and strategies for their mitigation were included in the study.RESULTS AND DISCUSSIONAccording to studies, in cases where lichens have protective effects or have slight deteriorative effects, create aesthetic in the substrate or are of biodiversity value, the removal of lichens is questionable. But in general, in the protection of cultural heritage, the connection between lichen and the deterioration of the building is always investigated and the removal of lichen in restoration is usually considered.Currently, different cleaning methods are used to eliminate lichens’ colonies and thus reduce their negative effects. These methods were categorized in three groups. Mechanical methods including brushing with metal tips and the scalpel, chemical methods using biocides and physical methods using laser ray irradiation are three main approaches to clean the monuments. Today, natural biocides such as essential oil are used in many indoor sites to control the biodeteriorant agents, which are friendly environment approach. The combination of this with other methods can increase the efficiency of those methods and reduce the dosage of the chemicals and irradiation.CONCLUSIONLichens have innately deleterious effect on stone integrity. Deterioration mechanism by lichens is mostly due to adhesion, penetration and volume changes of hyphal structures and the release of acidic or chelating metabolites. Lichens via protection and installation facilitation of degradative microorganisms can play role in the biodeterioration of the substrate.Although there are different methods for remove or mitigation in cultural heritage in order to avoid undesirable effects to some of them it is necessary to investigate on improving technology or using the combination of cleaning methods to reach a more favorable effect.

کلیدواژه ها:

Lichen ، Biodeterioration ، Protective ، Cultural heritage ، Conservation