This study examines the microbial colonization of three fronts of an abandoned dolostone quarry (Redueña, Madrid, Spain) exposed to atmospheric conditions for different time periods since Roman times to the present. Through scanning electron microscopy in backscattered electron mode (SEM-BSE), endolithic colonization was predominantly detected in the most recently exposed front, while in the longer exposed quarry fronts, epilithic forms of growth were most often observed. These observations were confirmed by denaturing gradient gel electrophoresis (DGGE) analysis. Based on the distribution pattern of microbial colonization in the different quarry fronts, we then established a sequence of colonization events that took place over this long time frame. Bioalteration processes related to this sequential colonization were also identified. Characterizing these sequential processes can be useful for interpreting biodeterioration processes in historic dolostone monuments, especially those affecting constructions in the area of the Redueña stone quarry. In a second experimental stage, different biocide treatments were tested on this quarry rock to find the best way to avoid the microbial colonization effects identified. Through combined SEM-BSE/DGGE analysis, the efficacy of several biocides against the microorganisms inhabiting the dolostones was assessed after 4 and 16 months treatment. In general, all treatments were effective at reducing around 80% of the lichen cover, although effects on endolithic lithobiontic communities were dependant on how well the rock surface had been mechanically cleaned prior to treatment and gradually disappeared over time.
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The authors would like to thank the following persons for their contributions to this study: Fernando Pinto from the Electron Microscopy Service of the Institute of Agricultural Sciences (CSIC) and Teresa Carnota and Maria Jose Malo from the National Museum of Natural Sciences (CSIC) for their technical assistance; Manolo Castillejo and José Manuel Hontoria for polishing the samples for microscopy (MNCN, CSIC); Dr. Silvia Matesanz for help with the statistical analysis; Dr. Sergio Perez-Ortega for help with identifying the lichens; Ana Burton for editorial assistance; and reviewers for their constructive comments. This study was supported by grants GEOMATERIALES (S2009/MAT-1629) from the CAM and CTM2009-122838-C04-03 from the Spanish Ministry of Science and Innovation, and by a predoctoral fellowship (FPI program, BES-2007-15145) awarded by the Spanish Ministry of Science and Education.