Increasing environmental pollution in urban areas has been endangering the survival of carbonate stones in monuments and statuary for many decades. Numerous conservation treatments have been applied for the protection and consolidation of these works of art. Most of them, however, either release dangerous gases during curing or show very little efficacy. Bacterially induced carbonate mineralization has been proposed as a novel and environmentally friendly strategy for the conservation of deteriorated ornamental stone. However, the method appeared to display insufficient consolidation and plugging of pores. Here we report that Myxococcus xanthus-induced calcium carbonate precipitation efficiently protects and consolidates porous ornamental limestone. The newly formed carbonate cements calcite grains by depositing on the walls of the pores without plugging them. Sonication tests demonstrate that these new carbonate crystals are strongly attached to the substratum, mostly due to epitaxial growth on preexisting calcite grains. The new crystals are more stress resistant than the calcite grains of the original stone because they are organic-inorganic composites. Variations in the phosphate concentrations of the culture medium lead to changes in local pH and bacterial productivity. These affect the structure of the new cement and the type of precipitated CaCO 3 polymorph (vaterite or calcite). The manipulation of culture medium composition creates new ways of controlling bacterial biomineralization that in the future could be applied to the conservation of ornamental stone.The study of bacterially induced and mediated mineralization is an emerging interdisciplinary research area (see references 6, 28, and 32 for recent reviews on the topic). Bacterially induced precipitation of calcium carbonate, the so-called "carbonatogenesis" (21, 51), has drawn much attention in recent decades because of its numerous implications. These include (i) atmospheric CO 2 fixation through carbonate sediment formation and lithification (22,29,36,48,49,59,65) and dolomite precipitation (76, 93), (ii) solid-phase capture of inorganic contaminants (95), (iii) the production of pathological concretions such as gallstones and kidney stones in humans (41,44,46), and (iv) understanding possible extraterrestrial biological processes like those of Martian carbonate-producing bacteria (58, 88). There is extensive literature on bacterial involvement in carbonate precipitation both in nature and in the laboratory

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Although it has already been shown that calcareous stone can be consolidated by 9 using a bacterially-inoculated culture media, a more user-friendly method is the in situ 10 application of a sterile culture media that is able to activate, among the microbial 11 community of the stone, those bacteria with a potential for calcium carbonate 12 precipitation. In order to test this new method for stone consolidation, non-sterilized 13 decayed porous limestone was immersed in sterile nutritional media. Results were 14 compared to those of the runs in which stone sterilized prior to the treatment was used. 15The effects of the microbial community on stone consolidation were determined by 16 recording the evolution of the culture media chemistry. The treated stone was tested for 17 mechanical resistance and porosity. Results demonstrate that the tested media were able 18 to activate bacteria from the microbial community of the stone. As a consequence of the 19 growth of these bacteria, an alkalinization occurred that resulted in calcium carbonate 20 precipitation. The new precipitate was compatible with the substrate and consolidated 21 the stone without pore plugging. Therefore, a good candidate to in situ consolidate 22 decayed porous limestone is the application of a sterile culture media with the 23 characteristics specified in the present study. 24

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