| In the long history of human evolution, people constantly improve their living environment with their diligence and wisdom, in this process plenty of cultural heritages was produced and survived until now. These precious relics contain rich information about social productivity and living environment at that time of which had important research value is the common wealth of humans. However, many cultural heritages have been damaged for a long time by various physical, chemical and biological factors. Heritage preservers and researchers have already overcome many difficulties, and successfully protected many rare and precious cultural relics. In recent years much attention has been paid to biological factors causing biological corrosion, biological research has become a new research hotspot in the field of cultural relic’s preservation. The survival of each creatures needs to obtain energy from the outside world, and will inevitably pose a threat to the safety of the cultural heritage. No organism is existed alone, but connected directly or indirectly to other organisms or substance in a certain ecological system. For this reason, the biological corrosion research is difficult, and we need to learn the whole corrosion mechanism and corresponding prevention and control measures systematically.Mogao Grottoes of Dunhuang is famous for its long history and rich cultural connotation, including exquisite murals and beautiful statues that are really amazing. Mogao Grottoes also suffered serious damage as other heritages, including long-term sand erosion and salt injury and biological corrosion, etc. Airflow is the important carrier of microbial migration, which can transport all kinds of microorganisms and small dust particles. The airborne microbe could attach to the surface of the mural, and the mural microbial corrosion damage then occur. This study firstly selected the typical micro-environment as sampling site at Mogao Grottoes, the dynamic changes of airborne microorganism in these sites was monitored, at the same time, the influence of different management measures and visitors to the air microorganisms was assessed. This result will provide reference for protection and tourism management in Mogao Grottoes. Secondly, to analyzing microbial sources in painting surface, the dust sample from air, the substrate soil sample under painting layer, and the painting layer samples were prepared. All samples were treated for their microbial community analysis and comparison, the relationship was established between airborne microorganisms and mural microorganisms by analyzing similarities and differences between them. Thirdly, in order to analyzing the characteristics of microbial distribution in different painting surfaces, we sampled paintings with different pigments, different build ages, and different kinds of damaged paintings. Detailed microbial distribution characteristics were conducted, which lays the groundwork for the mural conservation research, providing the theory reference and scientific basis for prevention and control of microbial corrosion.The first part of this study is for the air bacterial diversity in Mogao Grottoes of Dunhuang, fungi part was studies by other researchers; therefore, this study is only focused on the bacteria part. Andersen FA-1 type air microorganism sampler was adopted in the experiment of sampling, sampling time was from September 2008 to August 2009, once every month and lasted one year. Four sites were chose for sampling, cave completely open to visitors (OC), temporary open cave (SC), completely closed cave (CC) and outdoor environment (OD). The experimental results were as follows:(1) The airborne cultivable bacteria concentration in Mogao Grottoes was in a range of (0.1-3.8)×103 CFU/m3. Obviously seasonal changes of airborne bacteria concentration was observed, showing high concentration in Summer and Autumn (May to October), low concentration in Spring and Winter (April to November). (2) The average concentrations in completely open cave (OC) and closed caves (CC) were higher than that in temporary open cave (SC) and outdoor (OD). The diameters of the particles carrying bacteria were mainly distributed in the first level (>8μm), third level (2.1-3.3μm), fourth level (3.3-4.7μm) and fifth level (1.0-2.0μm). (3) This study obtained 48 sequences of different bacteria species, belonging to 19 different bacteria genera. Predominant genera and its proportion in community were as following ordinally: Janthinobacterium 14.91%, Pseudomonas 13.40%, Bacillus 11.25%, Sphingomonas 11.21%, Micrococcus 10.31%, Microbacterium 6.92%, Caulobacter 6.31%, Roseomonas 5.85%. (4) Each bacterial species had unique intercept diameter in the air sampler, in each stage of the sampler layer appeared different bacterial distribution patterns. In the view of predominant genera, the first level was dominated by Janthinobacterium and Bacillus; The second level mainly for Janthinobacterium and Pseudomonas; The third and fourth levels had similar predominant genera, Janthinobacterium and Pseudomonas; The fifth level mainly for Janthinobacterium and sixth level for Micrococcus. (5) The environmental temperature (including parameters such as solar radiation intensity, surface temperature and air temperature), relative humidity and human activity (tourists) were the main factors affecting the cultivable bacteria community and composition. Bacteria concentration in the air had obvious correlation relationship with temperature, relative humidity and the number of visitors; Cultivable bacteria concentration in outdoor environment and small caves had positive correlation with temperature, and negative correlation with rainfall and relative humidity. The influence of visitor’s number to bacteria concentration in the Mogao Grottoes had an obvious threshold effect, the number of visitors exceed the threshold value will obviously influence air bacteria concentration and community composition.In the second part of this study, the microbial origin analysis from the painting surface was performed firstly. Three samples were collected respectively from painting layer, ground layer behind the painting layer and deposited air dust in wall painting corner. The microbial community was then explored after sample treatment. The results demonstrated that part of microbial community from painting surface was come from painting material itself, including some of the common microorganisms in the soils, such as Paenibacillns, Tumebacitllus, Halothermothrix, Gracilimanas, Alternaria, Cladosporium, etc. Another part had air source background, such as Chaetomium, Trichococcus, Cryptococcus, Phoma etc, indicated that a tight linlage existed between air microorganisms and painting microorganisms. This study also analyzed the microbial communities in different murals, including serious damaged mural frescoes, different color paintings and different build age murals. Experimental results obtained were as follows:(1) In the surface painting of Mogao Grottoes, the prevalence of bacterial taxa including Alphaproteobacteria, Gammaproteobacterium, Bacillus, Planococcus; Common fungi including Cladosporium, Penicillium. (2) In the view of time distribution, the proportions of Thermoanaerobacteraceae and Gammaproteobacterium increased from ancient to nowadays, but Alternaria decreased on the contrary. (3) In the view of spatial distribution, the proportion of Azospirillum increased from deep floor to high floor, as same as Filobasidiales, Eurotium, Aspergillus. (4) Some microbial genera closely related to damaged paintings, including Shigella, Peptostreptococcaceae, Shewanella, Aerococcus and Trichoderma (Fungus). These bacteria was associated with human activities, Trichoderma was a plant pathogen. (5) Different pigments represented the different contents of metal elements, if the metallic pigment toxicity was measured with microbial diversity growing on it, red pigment (Pb3O4) had the biggest biological toxicity, black pigment (PbO2) followed, blue pigment (Cu2+) was the weakest. (6) Mogao Grottoes was not belonging to oligotrophic environmental conditions, microbes growing in it primarily were heterotrophic microorganisms. The assembly of microbial community on mural paintings does not depend on autotrophic microbes to provide basic nutrition and ecological niche. (7) It wasn’t organic matter but water which limited microbial growth on the surface of Dunhuang mural, so microbial corrosion process of Dunhuang mural did not include the formation of biofilm.The above results show that the microorganisms involved in painting corrosion of Mogao Grottoes has different sources, including various organisms, is closely related with the nature of painting itself. Airflow is one source of microorganisms on murals, also the destinations of mural microbes. Air microbial monitoring could indicate the happening of mural corrosion, which has important significance in early warning. Dust is the main carriers of air microorganisms, appropriate measures should be adopt to stop the air dust into caves, but in the case of good air quality, promotion of the air circulation is beneficial to dilute microbes inside caves. Tourists had a greater influence on the quantity of air microorganisms, tourist capacity should be determined according to the characteristics of different caves. By limiting the amount of tourists the stability of the cave climate conditions maintained. The preliminary results of this study will be beneficial to preservation research progress in Mogao Grottoes, and lay the foundation for further in-depth study. |
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