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

Microbially-induced calcite precipitation (MICP) is ubiquitous in nature and has become an area of interest for environmental, geotechnical, and civil engineering applications. These include bioremediation, soil engineering, and self-healing of cementitious materials. To date, ureolytic bacteria have been favoured due to their ability to rapidly increase the pH of the environment through the hydrolysis of urea and thereby induce precipitation of calcite. 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 thus to induce rapid increases in pH. 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 new insights into the use of MICP for geotechnical and environmental engineering and will enable us to create a toolbox of microbial precipitators tailored for specific applications.