Comparison of biofilm development on three building and restoration stones used in French monuments

This study investigated the bioreceptivity of two Lutetian limestones (Courville and Ditrupa), mainly used in the north-eastern region of France, and of their restoration stone (Savonnieres). Samples of the three stones were exposed outdoor for 2.5 years to determine if the replacement of Lutetian stones by Savonnieres was relevant regarding their intrinsic properties and their susceptibility to microbial colonisation. Cultivation assays revealed the presence of similar strains for all stones, but the number of bacteria changed and reached 703.5 CFU/cm2 for Courville, 964.5 CFU/cm2 for Ditrupa and 254.8 CFU/cm2 for Savonnieres. A significant colour change of the surfaces was noticed during the first winter, especially for Savonnieres (ΔE*ab = 18.3). It was associated with a net increase of the chl. a fluorescence and of the pigment content, which suggested a clear growth of phototrophic microorganisms. The Hg porosimetry showed that the porous network of Courville was only slightly impacted by biocolonisation, in contrast to Ditrupa and Savonnieres, where the macropores were obstructed. Savonnieres stone had a weaker bioreceptivity for bacteria than Lutetian stones, but it strongly promoted the colonisation by other microorganisms which could induce more severe bioweathering than on the Lutetian stones.