Microbial-Induced Calcium Carbonate Precipitation (MICP) is an environmentally friendly, efficient, and sustainable new soil reinforcement technology. For this study, Bacillus pasteurii were domesticated and cultured in a natural seawater environment with multiple gradients and used for coral reef calcareous sand reinforcement, comparing the mineral composition of the generated precipitates and the reinforcement strength under different domestication gradient conditions. The results revealed that, while the natural seawater environment inhibits the growth of Bacillus pasteurii, the gradient domestication method allows the bacteria to gradually adapt to the natural seawater environment. Notably, their shape becomes thin and long under the seawater environment. Furthermore, the MICP mineralisation reaction rate is faster in the natural seawater environment and, with an increase in the domestication gradient, the mineralisation reaction precipitates increased. At the same time, in the seawater environment, a small amount of mineral components were generated in addition to ${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3$, such as ${\mathrm{M}\mathrm{g}}_{\mathrm{x}}{\mathrm{C}\mathrm{a}}_{\mathrm{y}}({\mathrm{C}\mathrm{O}}_3{)}_{\mathrm{z}}$, and the ${\mathrm{M}\mathrm{g}}^{2+}$ mineral content increased with an increase in the domestication gradient. When comparing the curing effect under different gradients in the natural seawater environment, it was found that the Bacillus pasteurii can effectively enhance the curing effect of the calcareous sand after multi-gradient domestication in the seawater environment, with the curing effect increasing with an increase in the domestication gradient. The results of this study provide new ideas for the application of MICP technology in seawater environments for the reinforcement of calcareous sand in the construction of South China Sea islands and reefs.

Combining the Microbial-Induced Calcium Carbonate Precipitation (MICP) technique with plants to reinforce calcareous sand slopes on reef islands is expected to achieve both reinforcement and economic benefits. In this study, MICP was combined with Festuca arundinacea (MICP-FA) for calcareous sand reinforcement. Based on water retention and scanning electron microscopy (SEM) tests, the water retention performance and mechanism of MICP-reinforced calcareous sand under different cementation solution concentrations and cementation cycles were analyzed. The growth adaptability of Festuca arundinacea was evaluated under different bacteria solution concentrations, cementation solution concentrations and cementation cycles. The engineering applicability of MICP-FA-reinforced calcareous sand was evaluated by wind erosion tests, and the synergistic reinforcement mechanism was analyzed. The results show that with the increase in the cementation solution concentration and cementation cycles, more calcium carbonate filled and adhered to the calcareous sand particles, significantly improving the water retention performance. MICP-FA can enhance the wind erosion resistance of calcareous sand. This synergistic mechanism lies in the surface cementation effect of MICP and the deep anchoring effect of plant roots. This study provides theoretical basis and technical guidance for engineering applications in calcareous sand areas.

Microbially Induced Calcium Carbonate Precipitation (MICP) technology has garnered significant attention in geotechnical engineering and environmental remediation due to its environmentally friendly and cost-effective advantages. However, the current MICP technology faces challenges in practical engineering applications, particularly the prolonged cementation time, which makes it difficult to meet the requirements for coastal slope reinforcement. Therefore, this study designed novel cultivation conditions for Sporosarcina pasteurii by regulating external nitrogen source concentration and evaluated its environmental adaptability by measuring OD600, urease activity, and bacterial length. By monitoring the changes in Ca²⁺ concentration, pH, and precipitation rate over time during the mineralization process, rapid cementation under MICP conditions was achieved. The engineering applicability of this approach in slope reinforcement was comprehensively assessed through simulated on-site scouring and penetration tests. The reinforcement mechanism and the microstructure of the cementation under novel cultivation conditions were analyzed using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). The results indicated that the activity of Sporosarcina pasteurii in the modified NH₄-YE medium significantly improved in freshwater environments, and the MICP mineralization reaction was rapid, completing within 4 h. The primary crystal form of the generated precipitate was rhombohedral calcite, which formed a tightly bonded microstructure with calcareous sand, achieving a maximum reinforcement strength of 13.61 MPa. The penetration strength increased by at least 20%, and the precipitation rate improved by at least 2-fold. The scouring morphology remained essentially unchanged within 6 h. The findings of this study provide foundational and theoretical data for the application of MICP reinforcement technology to coastal calcareous sand models.