Biofilm-induced deterioration of marble at the Taj Mahal: spectroscopic and microscopic evidence of cyanobacterial colonisation

Abstract Microbial colonisation represents a persistent challenge to the conservation of marble-built heritage, particularly under semiarid climatic conditions. The Taj Mahal, a UNESCO World Heritage monument constructed of Makrana marble, has experienced progressive surface alteration associated with cyanobacterifal biofilm formation. This study aims to investigate the chemical composition, microstructural organisation, and deterioration mechanisms of cyanobacterial biofilms colonising the marble surfaces of the Taj Mahal. This investigation is based on the working hypothesis that biofilm-derived extracellular polymeric substances (EPS) actively contribute to marble deterioration through coupled chemical interactions with the calcite substrate and physical disruption of the stone microstructure. In this study, the biochemical composition, microstructural organisation, and deterioration mechanisms of cyanobacterial biofilms colonising the marble surfaces of the Taj Mahal were investigated using Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and optical microscopy. FTIR analysis revealed key EPS, including carbohydrates, proteins, lipids, cellulose, carotenoids, and photosystem II (PSII) membrane-bound proteins, highlighting the metabolic activity and structural complexity of the biofilm matrix. SEM observations revealed the penetration of cyanobacterial filaments and fungal hyphae into calcite crystals, leading to mineral dissolution, pore development, and the detachment of marble grains. Optical microscopy confirmed the dominance of Nostoc and Anabaena species, the presence of akinetes, and symbiotic interactions with fungal hyphae and green algae, contributing to biofilm resilience under environmental stress. The integration of biological polymers with externally sourced minerals such as silica and nano-iron oxides further increased the mechanical stability and persistence of the biofilm. These findings demonstrate that the biofilm-induced biodeterioration of marble is driven by coupled biological and physicochemical processes. Unlike previous studies that primarily report microbial colonisation, this study provides an integrated chemical–microstructural perspective linking biofilm composition to marble deterioration processes under real environmental conditions. This study provides material-based evidence that is essential for developing preventive conservation strategies for marble-built heritage structures exposed to biological colonisation and environmental stressors.