Buildings and monuments are often colonized by microorganisms that can result in colour change and aesthetical and physico-chemical damages. This bio-colonization is dependent of the material and on the environment. In order to better understand and correlate the microbial development at the surface of buildings with meteorological parameters, concentration of green algae and cyanobacteria have been measured using an in situ instrument on the wall of a private habitation in the Parisian region during two periods: spring and fall-winter. Different locations were also chosen to assess the influence of the position (horizontal or vertical) and of the situation (shaded vs. sunny microclimate). The results show that the microorganism development rapidly responds to rainfall events but the response is more intense in winter as temperature is lower and relative humidity (RH) higher. Cyanobacteria are less sensitive to this seasonal effect as they are more resistant to desiccation than green algae. Based on all the data, different dose-response functions have been established to correlate RH, rain and temperature to the green algae concentration. The influence of the microclimate is considered via specific fitting parameters. This approach has to be extended to new campaign measurements but could be very useful to anticipate the effect of climate change.

Limestone buildings in urban areas are weathered due to climatic factors, to pollution but also to biological activity. Many studies have focused on microbially-mediated precipitation of calcite but few on their influence on limestone dissolution rates. In this study, a cultivable approach for studying bacterial dissolution of limestone is proposed. The results show, for the first time, that limestone has selected a specific structure in the bacterial communities and that each bacterial class has its own metabolism inducing a different efficiency on the alteration of limestone grains. Cultivable bacterial and fungal strains in our study permit to considerably increase (by 100 to 1,000,000 times) the chemical weathering rates compared to laboratory or field experiments. Individually, the results bring information on the ability to alter limestone by dissolution. Moreover, taken together, a functional ecological adaptation of bacterial and fungal classes to the alteration of the limestone monument has been highlighted. In order to release calcium into solution, these strains slightly acidify the medium and produce low molecular mass organic acids during experiments, especially lactic and oxalic acids.