Artificial light pollution alters epilithic microbial communities and functional biodeterioration-related functional potentials in subterranean sandstone environments.

Artificial light pollution represents an important anthropogenic disturbance in subterranean heritage environments, yet its associations with epilithic microbial communities and biodeterioration processes remain poorly understood. In tourist-accessible subterranean caves, artificial light creates persistent illuminated areas on stone surfaces that may alter microbial community assembly and biogeochemical functions associated with biofilm development. Here, the Longyou Grottoes, a large underground sandstone cave complex in Zhejiang, China, were investigated using 16S rRNA gene sequencing and metagenomics. Bacterial communities dominated all samples (>98% relative abundance). Cyanobacteriota were enriched under artificial light and sunlight (>40%), whereas Pseudomonadota and Actinomycetota prevailed in darkness. Dark sites exhibited higher bacterial alpha diversity and more complex co-occurrence networks. Compared with sunlight and dark environments, the artificial-light environment was associated with greater stochasticity in bacterial community assembly, broader niche breadth, and lower niche overlap. Metabolic potential analysis further revealed light-dependent metabolic shifts: artificial light enhanced assimilatory nitrogen and sulfur metabolism, sunlight promoted nitrogen fixation, and darkness favored nitrification, denitrification, and sulfur oxidation. However, bacterial community patterns were also associated with temperature, humidity, pH, and soluble salts, indicating that the observed differentiation reflected the combined influence of light and environmental heterogeneity. These findings identify artificial lighting as an important and manageable environmental factor associated with epilithic bacterial communities and provide a basis for integrating lighting management with environmental control in the preventive conservation of subterranean sandstone heritage.