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Cyanobacteria cause black staining of the National Museum of the American Indian Building, Washington, DC, USA

Microbial deterioration of stone is a widely recognised problem affecting monuments and buildings all over the world. In this paper, dark-coloured staining, putatively attributed to microorganisms, on areas of the National Museum of the American Indian Building, Washington, DC, USA, were studied. Observations by optical and electron microscopy of surfaces and cross sections of limestone indicated that biofilms, which penetrated up to a maximum depth of about 1 mm, were mainly composed of cyanobacteria, with the predominance of Gloeocapsa and Lyngbya. Denaturing gradient gel electrophoresis analysis revealed that the microbial community also included eukaryotic algae (Trebouxiophyceae) and fungi (Ascomycota), along with a consortium of bacteria. Energy-dispersive X-ray spectroscopy analysis showed the same elemental composition in stained and unstained areas of the samples, indicating that the discolouration was not due to abiotic chemical changes within the stone. The dark pigmentation of the stone was correlated with the high content of scytonemin, which was found in all samples.