丝绸之路甘肃段石窟壁画色变的微生物学机制研究

Color change of ancient wall paintings is a universal phenomenon which has serious effects to the quality and aesthetic value of wall paintings preserved in The Silk Road Grottoes, Gansu. Most of the early study toward to the mechanisms of mural color change mainly focused on the physical and chemical aspects, although a few of studies showed that microorganisms have took part in and mediated the color change process of ancient wall paintings, little of reports referred microbial biodeterioration and biodegradation mechanisms of mural's color change progress. This project aimed at murals preserved in three different grottoes with different extent of color changing and in varied precipitation of The Silk Road grottoes, Mogao Grottoes (35mm/year), Wuwei Tiantishan grottoes (296mm/year) and Tianshui maijishan grottoes (960mm/year), The Metagenomic high-throughput sequencing techniques will be used for the study of microbial community structures in different grottoes, The statistical methods such as Canonical Correspondence Analysis(CCA) and Nonmetric Multi-dimensional Scaling(NMDS) will be used to find out the major factors and their contribution rate in the color change process of wall paintings. After simulated experiments, combined with the study of coupling relationship between dominant microorganisms and mineral pigments deterioration, it is possible to reveal microbial mediated biodeterioration of wall paintings; These results can provide technological support for the conservation of color changed murals. Findings of this project has significant demonstration values for the conservation of the similar cultural relics, and with some important practical significance for the prompt of cultural heritage protection internationalization degree in China.

色变是古代壁画劣化的常见现象，严重影响到壁画品质与美学价值。对壁画色变机理的研究主要集中在物理化学方面，尽管有研究表明微生物介导了壁画色变过程，但微生物导致壁画色变机理研究少有报道，壁画色变的微生物学机制尚不清楚。本项目针对丝绸之路甘肃段不同降水梯度下敦煌莫高窟（35mm/年）、武威天梯山石窟（296mm/年）和天水麦积山石窟（960mm/年）典型洞窟壁画的色变程度，运用宏基因组高通量测序技术，研究不同石窟色变壁画中微生物群落结构特征，比较不同降雨条件下壁画色变程度及微生物组成差异；运用典型对应分析及非度量多维尺度分析等方法，解析导致壁画色变的主因子及其贡献率；通过仿真模拟实验研究优势微生物与色变颜料退化进程间耦合关系，揭示微生物介导的壁画色变机理，为色变壁画的科学保护提供技术支撑。研究成果对同类文化遗产的保护具有重要的示范价值，对提升我国文化遗产保护水平的国际化程度具有重要的现实意义。

本项目针对丝绸之路甘肃段敦煌莫高窟、武威天梯山石窟和天水麦积山石窟典型洞窟壁画色变问题展开了系统研究。壁画色变主要表现为变白、变灰、变褐和变黑，三处石窟调查区域变色比例达16%-63%。高通量分析显示：莫高窟色变与未色变壁画中芽孢杆菌属和类芽孢杆菌属为优势属；发菌科未鉴定属成员为优势真菌（＞44%）；细菌群落组成上没有明显差异，但特定属百分比有所不同，如类芽孢杆菌属在第420窟色变壁画中占更高比例。麦积山石窟色变壁画假诺卡氏菌属和糖多孢菌属细菌占总序列的54.06%，Capnodiales_unclassified，Ascomycota_unclassified和Capnodiales_no rank真菌类群序列占总序列的76.3%；主成分分析（PCA）显示壁画样品与粉尘样品间细菌群落组成差异明显。天梯山石窟原址和馆藏色变壁画上土芽孢杆菌属、乳球菌属、假单胞菌属和链球菌属为优势细菌属，酵母目成员Saccharomycetales_incertae_sedis为优势真菌属。三处石窟温差和光强累计值表现为莫高窟最大，天梯山次之，麦积山最小；年均相对湿度变化趋势相反；CCA分析显示相对湿度、降雨量、温度是影响麦积山石窟和天梯山石窟空气细菌与真菌的主要环境因子。从壁画中分离到的新种萃英诺曼菌在不同含铅颜料培养基上的色变过程为菌体吸收铅离子并还原底物氨基酸中硫元素，形成硫化铅（PbS），最终导致黑褐色的色变。少动鞘氨醇单胞菌通过代谢形成了黑色的PbS。黑曲霉通过产酸形成草酸铅（PbC2O4）、醋酸铅Pb（CH3COO)2）等导致铅丹变白。仿真模拟实验还原了微生物介导的壁画色变过程，在胶浓度5%模拟试块单次浸湿后，接种嗜松青霉，在RH99%条件下，橘红色铅丹（Pb3O4）可变为白色的碳酸铅（PbCO3）和黑色的二氧化铅（PbO2）。色变壁画微生物群落组成和壁画色变过程类型多样，主要受太阳辐射、气候（降雨量）、壁画湿度、外源营养物、本土或机会微生物生长代谢等多因素影响。项目研究成果对我国乃至世界同类文化遗产的保护具有重要的现实意义。