In-depth profiling of calcite precipitation by environmental bacteria reveals fundamental mechanistic differences with relevance to self-healing applications

Environmental, geotechnical, and civil engineering disciplines have utilized microbes in diverse applications, including bioremediation, soil engineering, and self-healing of cementitious materials. Concrete is one of the most used construction materials worldwide. However, it has considerable environmental and economic costs associated with both production and maintenance. Crack formation is a common phenomenon and leads to increased permeability, facilitating
ingress of aggressive substances that negatively impact durability. Microbially-induced calcite precipitation (MICP) is a process whereby an increase in pH and production of carbonate ions, resulting from microbial metabolism, leads to the precipitation of calcium carbonate (CaCO3). This has led to work investigating the use of bacteria to seal cracks in concrete structures by precipitating CaCO3. Many applications favour the use of ureolytic bacteria that are capable of
hydrolysing urea, resulting in a rapid increase in pH and subsequent precipitation of CaCO3. However, the requirement for urea can contribute to nitrogen-loading in the environment and prove to be incompatible in certain applications, such as in self-healing concrete where it delays setting. Non-ureolytic bacteria are thought to be less efficient at MICP as they lack the ability to hydrolyze urea and are thus incapable of inducing such rapid increase in pH. We here report that profiling of environmental bacteria has revealed the fundamentally different mechanisms that ureolytic and non-ureolytic bacteria utilize to precipitate calcite. These affect the timing of MICP and morphology of the crystals, but not necessarily the overall quantity of calcite precipitated. Furthermore, we show that MICP facilitated by non-ureolytic bacteria results in precipitates that contain significant organic components. These precipitates appear to have increased volume and
cohesiveness, which may prove advantageous in application. Our findings offer important insights into the use of MICP for geotechnical and environmental engineering and will enable us to create a toolbox of application-specific microbial precipitators.