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An evaluation of field data from historical buildings in Germany showed that chemoorganotrophic bacteria are the most numerous microorganisms in building stones, followed by fungi and nitrifying bacteria. Chemoorganotrophic bacteria and fungi were present in almost every sample. Ammonia and nitrite oxidizers were found in 55 and 62% of the samples, respectively. Within months, natural stone was colonized by chemoorganotrophic microorganisms. The highest cell numbers were usually found near the surface. The colonization of natural stone by nitrifying bacteria took several years. The highest cell numbers were in some cases found underneath the surface. Nitrifying bacteria showed a preference for calcareous material with a medium pore radius between 1 and 10 microns. Cell numbers of nitrifying bacteria did not correlate to the nitrate content of the stone material. We demonstrated that the stone inhabiting microflora can cause significant loss of nitrate by denitrification. Our data strongly suggested that microbial colonization of historical buildings was enhanced by anthropogenic air pollution. Samples taken from stone material with a pore radius < or = 1 micron had significantly higher cell numbers when they were covered with black crusts. A comparison of samples taken between 1990-1995 from buildings throughout Germany showed that in Eastern Germany a significantly stronger colonization with facultatively methylotrophic bacteria and nitrifying bacteria existed. The same was true for natural stone from an urban exposure site when compared to material from a rural exposure site. Data from outdoor exposure and laboratory simulation experiments indicated that the colonization of calcareous stone by nitrifying bacteria was enhanced by chemical weathering.

Microorganisms Involved in Biodeterioration Microbial Mechanisms Contributing to Bioweathering Materials, Properties, and Examples of Complex Weathering Processes Simulation of Microbial Weathering

A test system for the simulation of a combined chemically (gaseous pollutants) and microbiologically (nitrifying bacteria) influenced corrosion on natural sandstone is presented. A high stone moisture was essential for the growth of nitrifying bacteria on test stones. Under optimum conditions, a nitrifying biofilm developed on the calcareous Ihrlersteiner green sandstone, reducing the evaporation from the stone surface. Biofilm cells adapted well to high concentrations of gaseous pollutants. The mean metabolic activities of ammonia oxidizers were 11 and those of nitrite oxidizers 30 times higher than mean values of samples from historical buildings. The microbiologically influenced nitric acid corrosion alone was stronger than the chemically influenced corrosion by a smog atmosphere (1065-μg/m3 sulphur dioxide, 850-μg/m3 nitric oxide, and about 450-μg/m3 nitrogen dioxide). While 72 μmol of calcium were solubilized per week of exposure to the smog atmosphere, 161 μmol of calcium per week were solubilized by the nitric acid produced by nitrifying bacteria. If gaseous pollutants were added, the microbiologically produced nitrite and nitrate were removed by the action of sulphur dioxide. Thus, the combined attack of nitrifying bacteria and gaseous pollutants did not result in increased corrosion.