On the other hand, chemical, physical, biological, and anthropogenic factors are the principal perpetrators of monumental stone decay. Some of the metabolic pathways involved in CaCO 3 precipitation are anaerobic sulfide oxidation, photosynthesis, methane oxidation, ammonification, denitrification, sulfate reduction, and ureolysis. Genetics and physiology involved in the process are quite a challenge to understand. Microorganisms use different metabolic pathways to induce CaCO 3 precipitation however, this process is not entirely defined yet. Microbiologically induced carbonate precipitation (MICP) is mainly driven by factors, such as pH, Ca 2+ concentration, dissolved inorganic carbon concentration, and availability of nucleation sites. Depending on the cell surface properties of bacteria, especially proteins and extracellular polymeric substances, the morphology and mineralogy of calcium carbonate can be varied, e.g., rhombohedral (calcite), hexagonal (vaterite), or needle-like crystal (aragonite), being calcite the most stable molecular structure. Calcium carbonate may precipitate through the attachment of the calcium ions to the microbial cell walls or to the extracellular polymeric substances, which act as crystal nucleation sites. The process of calcium carbonate precipitation is present in nature, commonly in marine environments, freshwater, and soil (e.g., solid surfaces). The different type of minerals that bacteria are able to produce includes nitrates, silicates, calcium oxalates, halides, apatite, gypsum, oxides, phosphates, and calcium carbonate. In addition, some macromolecules and cell structures can act as heterogeneous crystallization nuclei, inducing the precipitation. This releasing of molecules increases pH and elevates the supersaturation, resulting in the precipitation of minerals. The precipitation of minerals by microorganisms is obtained by the modification of the local environment as a result of the metabolites release. The second is distinguished by massive intracellular and/or extracellular mineral formation commonly in the form of teeth, skeletons, shells, etc. The first takes place in various animals in a process where the organism produces an organic framework to introduce ions for further crystallization and growth mediated by an organic matrix. This mineral formation occurs through two different processes. Minerals precipitation by living organisms activity, so-called biomineralization, is a process that occurs from bacteria to chordates. MICP is proposed as a potentially safe and powerful procedure for efficient conservation of worldwide heritage structures. The treatment has revealed best results on porous media matrixes nevertheless, it can also be applied on soil, marble, concrete, clay, rocks, and limestone. This method has shown to be successful as a restoration, consolidation, and conservation tool for improvement of mechanical properties and prevention of unwanted gas and fluid migration from historical materials. In this work, some published variations of a novel and ecological surface treatment of heritage structures based on MICP are presented and compared. Historic buildings and artwork, especially those present in open sites, are susceptible to enhanced weathering resulting from environmental agents, interaction with physical-chemical pollutants, and living organisms, among others. The high concentration of carbonate and calcium ions on the bacterial surface, which serves as nucleation sites, promotes the calcium carbonate precipitation filling and binding deteriorated materials. Microbiologically induced carbonate precipitation (MICP) is a well-known biogeochemical process that allows the formation of calcium carbonate deposits in the extracellular environment.
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