Changes in the biofilm microflora of limestone caused by atmospheric pollutants

Historic limestone materials in urban environments are continually exposed to air pollutants, including sulfur compounds and hydrocarbons. We investigated the effects of air pollution on the biofilm microflora of historic limestone gravestones located at two locations Massachusetts, USA. Our data showed that the culturable populations of chemolithotrophic and heterotrophic bacteria, and fungi were suppressed in the polluted habitat comparing with the unpolluted location. The diversity of the microflora was also reduced in the surface biofilms on gravestones in the city contaminated by air pollution. However, both the sulfur-oxidizing and hydrocarbon-utilizing microflora were enriched in the biofilms exposed to air pollution. In a laboratory study, low concentrations of the polluting chemicals stimulated growth of these bacteria, and resulted in rapid acid production. Scanning electron microscopy demonstrated that the biofilms of both the sulfur-oxidizing bacteria and the hydrocarbon-degrading microflora penetrated into the limestone. The enrichment of sulfur- and hydrocarbon-utilizing bacteria in the biofilms may contribute to dissolution of the stone. However, further research is required to determine the effects of specific metabolites of these microorganisms on stone deterioration.