The Causation And Prevention Of Microbe Outbreak On Ancient Wall Paintings In The Maijishan Grottoes

Ancient wall paintings are a form of painting with decorative and beautifying functions created by human beings in different periods on natural or artificial walls,whose contents can reflect the level of productivity,science and technology and the creative ability of people of the society at that time.As an important part of cultural heritage,it has important historical,artistic,scientific,social and cultural values However,a large number of ancient wall paintings have facing or are suffering from deterioration and destruction by various factors such as physical,chemical and biological due to their age and the change of preservation environment.In recent years,microbial degradation of wall paintings has gradually received the attention of cultural heritage conservationists and the biodegradation and biodegradation of wall paintings has gradually become a research hotspot in the field of conservation of cultural heritage.The Maijishan Grottoes is located in the Xiaolongshan area in the western part of the Qinling Mountains in China.It is the first batch of major historical and cultural sites protected at the national level and was listed in the UNESCO World Heritage in 2014.Unfortunately,a mass of microbial plaques grew on the surface of ancient wall paintings and statues in several caves during June to July in 2018.The current state of the microbial diseases was investigated and the result showed that the microbial diseases gradually increased from south to north and widely distributed on the north wall of the cave.What factors are responsible for microbe outbreak?What are the microbial communities consist of biodeterioration?How to cleaning?How to prevent?These problem needs to be solved urgently.Therefore,given the above problems,we carried out this research and obtained the following research results.1.In order to reveal the influence of environmental changes inside and outside the grottoes on cultural relics and the relationship between them and the occurrence of diseases,we used HOBOU23-001 and i Button DS1923 miniature embedded temperature and humidity monitors to conduct simultaneous real-time monitoring of the internal and external environments and the micro-environment of the grottoes from January 1st to December 31st of the year 2019.The results showed that the wall paintings are rich in organic matter and microbial plaques are concentrated on the north side of the cave.The seasonal variation of ambient air relative humidity（RH）outside maijishan Grottoes is small due to the influence of forest climate,and the RH inside the grottoes is over 70%for a long time.The monthly maximum air RH near the northern wall was higher than that near the southern wall between April and October 2019.The number of days with RH higher than 90%in the base layer of northern wall was 127,while that in the base layer of southern wall was lower than 90%,and the monthly maximum of RH in the base layer of northern wall was higher than that of the southern wall.These results indicate that long-term high RH in the air near the north wall of Cave 127 and in the base layer is correlated with the disease of cave wall and fresco,which is an important environmental factor leading to the disease.The results provide a scientific basis for the preventive protection of cultural relics in the Maijishan Grottoes.2.To prevent and control microbial damage to the wall paintings,we analyzed the bacterial and fungal communities in the normal wall samples and those with microbial plaques using high-throughput sequencing and the environmental factors associated with the microbe outbreak.The results showed that the normal wall samples were dominated by bacterial phyla Actinobacteria（71.7%）,Proteobacteria（17.8%）,Firmicutes（6.5%）,Chloroflexi（2.4%）,and Bacteroidetes（0.9%）,and fungal families Trichocomaceae（43.4%）,unclassified Capnodiales（23.2%）,unclassified Ascomycota（12.1%）,Teratosphaeriaceae（10.2%）,and unclassified Eurotiomycetes（6.2%）.The wall samples with microbial plaques were dominated by bacterial Actinobacteria（92.9%）,Proteobacteria（5.3%）,Firmicutes（1.1%）,and Chloroflexi（0.3%）and fungal families unclassified Eurotiomycetes（93.3%）,unclassified Ascomycota（3.3%）,and Microascaceae（2.9%）.The dominant fungus in the wall samples with microbial plaques was isolated has 92.28%ITS sequence similarity with Arachnomyces sp.,and the mycelium produced tan pigments during growth on PDA.These results indicated the great changes in bacterial and fungal communities during the microbe outbreak.The dominant fungus identified as a potential novel species and may cause contaminating to the wall paintings.3.The mycelia grown on the surface of wall paintings were removed with soft brushes under a negative pressure provided by a modified vacuum pump,and the pre-screened biocides were then selected and applied.Using a portable microscope,a microbial ATP bioluminescence detector,and a colorimeter,the effectiveness of cleaning and controlling against fungal mycelia on the wall paintings was evaluated.The microscopic examination indicated the surface of the wall paintings was nearly free of visible fungal mycelia after the manual removal.There was a significant reduction of ATP bioluminescence before（43,303±10,280 RLU/cm²）and after（2,833±388,RLU/cm²）the mechanical removal.The color difference(ΔE_ab^* value)were less than 2.0 before and after biocide treatment,such difference could almost be ignored.Additionally,2-Octyl-4-isothiazolin-3-one（OIT）and 4,5-Dichloro-2-n-octyl-4-isothiazolin-3-one（DCOIT）were found to be the most efficient biocide for controlling fungal growth and could be an emergency protection measure.Furthermore,the key environmental factors that triggered periodic breakouts of the microorganisms on wall paintings were long-term higher relative humidity at the interface of earthen plaster layer and supporting sandstone during raining seasons and poor ventilation of the cave.This study provides a reference for the scientific conservation of biodeteriorated ancient wall paintings.In summary,this study screened and explored a set of combined removal techniques consisting of mechanical removal and chemical killing based on the disease survey,environmental monitoring,microbial community analysis,and the isolation and purification of dominant culturable fungi.A set of scientific and effective control methods has been developed for the microbial diseases of the wall paintings in the Maijishan Grottoes.The results of present study provide a scientific validation for the prevention of microbial degradation of cultural relics in caves,and have important implications for the salvage treatment of sudden microbial diseases in other types of cultural heritage.