Microbial biofilm and deterioration of a thousand-years-old dark-brick pagodas with nitrogen-cycling potentials in Southwest China

Bricks are widely used in architectural heritage, yet their microbial biodeterioration remains poorly understood compared with stone monuments. Here, two millennium-years-old dark brick pagodas in Leshan of Sichuan, China, were investigated using structural, chemical, and microbial analyses. Scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed porous microstructures and enrichment of carbon, calcium, iron, and sulfur relative to fresh local references. High-throughput sequencing showed diverse microbial communities dominated by cyanobacteria, actinobacteria, and stress-tolerant taxa. Nitrogen-cycle reaction analysis identified Nitrospira and trace Nitrosomonadaceae , suggesting nitrification potential (NH₄⁺/NH₃ → NO₂⁻ → NO₃⁻) within brick biofilms with possible localized acidification. Functional predictions further revealed enrichment of carbohydrate degradation, fermentation, and redox-associated pathways in Sanjiang samples, whereas Lingbao samples showed higher biosynthesis. Together, biodeterioration may involve biofilm-driven nitrogen transformation and acid-producing metabolism, contributing to microstructural weakening and elemental alteration. These findings identify microbial nitrification-induced acidification as a plausible contributor to brick deterioration.