Cyanobacteria cause black staining of the National Museum of the American Indian Building (Washington, D.C., USA)

Microbial biofilms and their deteriorating activities contribute significantly to the acceleration of weathering processes of historical monuments (Gorbushina, 2007). The aim of this work was to confirm the microbial deterioration on areas of the National Museum of the American Indian (Washington, D.C., USA), to investigate the relationships of microorganisms with stone, and to identify the cause of the black staining of the wall surfaces. The NMAI Building opened in 2004 after several years’ construction. The stone is principally Monkato Kasota stone, a dolomitic limestone quarried in Monkato, Minnesota. Soon after the building completion, some areas of the external stone surface were covered by black staining, putatively attributed to microbial growth by conservators. The detection and identification of the deteriogenic microorganisms and their products are extremely important for the development of proper prevention and control practices. Optical and electron microscopy observations of limestone surfaces and cross sections indicated the presence of biofilms mainly composed of cyanobacteria, with the predominance of Gloeocapsa and Lyngbya. The observation of cross-sections showed that cyanobacteria developed not only on the surface but also underneath, reaching a maximum depth of about 1 mm. The greater penetration into the stone was generally observed underneath the thicker surface colonisation and along pre-existing microcracks and fissures. In extreme habitats, as outdoor stone surfaces, endolithic growth protects from high and low temperatures, UV radiation and desiccation (Giacomucci et al., 2011). Denaturing gradient gel electrophoresis analysis revealed that the rest of the microbial community included eukaryotic Trebouxiophyceae algae and Ascomycota fungi along with consortia of bacteria. Principal component analysis of the PCR-DGGE fingerprinting data showed that both in the case of the total community, including both prokaryotes and eukaryotes, and the case of the bacterial community taken alone, the communities of the samples were different. In addition, diversity indexes indicated diversity in species and richness among the samples despite the stone of the three samples was the same limestone. This fact highlighted the important role played by environmental conditions and substratum orientation in determining the microbial community structure (Macedo et al., 2009). EDX analysis suggested that the origin of the black staining on the NMAI building was not chemical, but biological in nature as control, stained and unstained areas showed a similar elemental composition. The discolouration could be ascribed to the presence of the sheath yellow-brown pigment scytonemin, which acts as ultraviolet sunscreen in habitats exposed to intense solar radiation (Balskus et al., 2011). The need to clean the NMAI building has been perceived as being critical for its proper conservation. However, this treatment is rather more difficult if photosynthetic microorganisms, like those retrieved in this study, penetrate deeply into the stone (Cappitelli et al., 2012).