Bacterial mineralization of calcium carbonate for conservation of stone artworks

Calcareous stones have been widely used as artwork and building materials by human civilizations, especially in the Mediterranean Basin. Calcareous stone deterioration is a consequence of the weathering action of physical, chemical and biological factors, which causes a progressive dissolution of the mineral matrix and an increase in porosity, resulting in the weakening of the structure. Recently, increased environmental pollution and global warming are threatening stone cultural heritage more than ever. Inorganic or organic products have been using to slow down monument deterioration, but their use presents several drawbacks. Bacterial Calcium Carbonate Mineralization is a natural process widespread among bacteria and occurring in different environments. In the last decades it has been proposed as a new and environmentally friendly tool in conservation of monumental and ornamental calcareous stones. The aim is to develop a bacteria-mediated mineralization treatment providing a coherent calcium carbonate layer on the surface of deteriorated stone, able to protect it against the intake of water and chemicals and to consolidate the inner weakening structure. The advantage is to obtain a mineral product similar to the calcareous stone substrate, mimicking the natural process responsible for stone formation. This review introduces the mechanisms of bacterial mineralization and describes the current strategies based on this process to promote stone reinforcement in field tests. They include applications of selected bacterial strains and/or culture media as well as selected components of bacterial cells on stone. The review finally provides perspectives based on recent advances.