A case study of the ancient wall in Shou County, Mid-China
The preservation and protection of cultural heritage are closely linked to the types and functions of microorganisms forming biofilms on its surface, which affect the biodegradation of cultural heritage through chemical element cycling. This study used metagenomic methods to investigate the microbial diversity and taxonomic composition of microbial communities on the millennium-old city wall of Shouxian, a material cultural heritage site in China. Taxonomic annotations of metagenomic data revealed dominant microbial groups that are associated with the biocorrosion of the city walls, revealing the impact of the microbial communities inhabiting the ancient and modern bricks on the biocorrosion of the walls. Additionally, sensitive microbial groups that are responsive to environmental factors were identified. Discrepancies in the distribution of taxonomic groups and metabolic functions within the metagenomic data indicated active elemental biogeochemical cycling in well-preserved city walls. Notably, metagenomic analysis indicated nitrogen cycling processes involving ammonia oxidation and nitrite to nitrate reduction on the well-preserved city walls, suggesting the potential role of nitrate in the weathering of city walls. Proximity to a national highway was found to directly result in severe bioweathering of the northern city walls due to vehicular exhaust emissions, as evidenced by the significant enrichment of the SoxYin the metagenomic data, indicating severe wall biocorrosion caused by the conversion of thiosulfate to sulfate. In terms of protective bacteria, active glycolysis, the ED pathway and the absence of the TCA cycle were identified as significant factors contributing to damage to the eastern ancient city walls. In summary, our study provides metagenomic evidence for microbially driven biogeochemical cycling on Earth, which leads to microbial deterioration of city walls.