The Dazu Thousand-handed and Thousand-eyed Avalokitesvara is widely regarded as a seminal example of Song Dynasty grotto art. Although the statue was restored in 2015, the preservation environment has not been effectively improved. It still suffers from issues such as gold foil delamination, cracking, and microbial erosion. Preliminary investigations by our research group into the microbial corrosion mechanisms on the gold foil indicated that the substrates generated from microbial metabolism could induce microbial colonization and alter the microenvironment of the gold foil surface. Nonetheless, the relationship between sealing materials and the microbiological colonization and corrosion processes on gold foil remains insufficiently studied. The present study established a Mucor fragilis-based culture system, incorporating raw lacquer, turpentine-modified lacquer, and tung oil-modified lacquer as sealing materials. These systems were combined with simulated corrosion experiments on gold foil surfaces to systematically evaluate the effects of different sealing materials on microbial physiological metabolism and the corrosion process. The results indicated that raw lacquer formed a stable biofilm structure with Mucor fragilis, providing both physical barrier and inhibition of organic acid secretion, thereby offering effective protection for the gold foil. Although the film morphology of turpentine-modified lacquer was relatively heterogeneous, its sustained mildly alkaline environment and low extracellular polymeric substances (EPS) concentration effectively suppressed microbial adhesion, suggesting a certain long-term protection potential. Tung oil-modified lacquer initially inhibited microbial growth through oxidative stress, but its protective effect diminished over time as fatty acid components were utilized as carbon sources.

In recent years, partial deterioration of the gilded layer on the Dazu Thousand-handed and Thousand-eyed Avalokitesvara has posed a serious threat to the preservation of this cultural heritage. In this study, Aspergillus versicolor and Mucor fragilis, previously isolated from the gold foil surface, were selected as model organisms. Laboratory-simulated conditions were employed to investigate their corrosive effects on the gold foil. Corrosion behavior was monitored through pH, open circuit potential, electrochemical impedance spectroscopy, and polarization curves. Additionally, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized to characterize morphological and elemental changes. The results demonstrated that both fungal strains significantly accelerated gold foil corrosion. A marked decrease in surface gold (Au) content was observed, accompanied by the enrichment of carbon (C), oxygen (O), and sulfur (S) elements. Electrochemical analyses revealed a negative shift in corrosion potential by 0.20-0.24 V, a nearly tenfold increase in corrosion current density, and corrosion efficiencies of 91.38% and 88.91% for Aspergillus versicolor and Mucor fragilis, respectively. The preliminary mechanistic inferences suggest that organic acids (e.g., citric acid and oxalic acid) produced by fungal metabolism rapidly reduced the local pH to 4.5-4.8, thereby disrupting the passive layer. Moreover, extracellular polymeric substances (EPS) secreted by the fungi facilitated biofilm formation and likely enhanced interfacial reactions through sulfide adsorption. The flake corrosion patterns induced by Aspergillus versicolor were attributed to the increased electrolyte penetration promoted by its hydrophilic polysaccharide-rich biofilm. Conversely, the crack propagation caused by Mucor fragilis was possibly driven by micro-galvanic effects associated with its saccharification products.

The Dazu Thousand-handed and Thousand-eyed Avalokitesvara is regarded as the largest and most completely preserved cliff carving of its kind in China. It is a remarkable integration of rock carving, polychromy, and gold foil application, and is regarded as the largest and most completely preserved Thousand-handed Avalokitesvara cliff carving extant in China. Recent continuous monitoring has revealed evident deterioration of the gold foil surface, primarily manifested as discolouration, cracking, and loss of the foil. A plethora of studies have examined the influence of environmental physicochemical factors and material characteristics on the corrosion of gold foil, however, the influence of microorganisms has received limited attention. The present study was conducted for the purpose of systematic characterisation of the microbial communities colonizing the gold foil surface using sterile swab sampling followed by high-throughput sequencing technology of the 16S rRNA gene and the Internal Transcribed Spacer (ITS). The results showed that bacterial communities were dominated by Firmicutes, Actinobacteriota and Proteobacteria at the phylum level, with Salinisphaera and Pseudonocardia identified as the core dominant genera. Fungal communities were primarily composed of Ascomycota and Mucoromycota, exhibiting dominance of Aspergillus, Filobasidium, and Mucor. Two dominant fungal strains (Aspergillus versicolor and Mucor fragilis) were successfully identified through selective media isolation, combined with colonial morphology observation and ITS sequence analysis. Based on the existing literature on microbial metabolism, these strains are hypothesized to facilitate gold foil corrosion via synergistic physical, chemical, and bioelectrochemical mechanisms.