SEM characterization of precipitated crystals for unseeded and seeded samples for high bacterial concentration (D1-C1) and lower bacterial cell concentration (D3-C1). CaCO₃ crystals that originated from MICP processes are marked with MICP in the SEM micrographs.

(a) MICP induced calcite crystals for D1-C1 in unseeded experiments. Imprints of bacteria can be identified on the surface. (b) Bottom of the crystal for D1-C1 (unseeded), where the amorphous phase is enclosed in the calcite crystal. (c) MICP induced calcite crystals for D3-C1 in unseeded experiments and (d) the bottom of the crystals for D3-C1 in unseeded experiments, where no amorphous phase was detected. (e,f) Seeded samples for D1-C1. Initial calcite seeds (that did not grow during the experiment) and the MICP induced CaCO₃ can be seen. (g,h) Initial calcite seeds (that did not grew during the experiment) and the precipitated CaCO₃ can be seen. The initial calcite seeds can be identified within the precipitated CaCO₃. Before SEM characterization, the crystals were transferred from the well plate to filter paper for drying. The crystals were not washed before SEM characterization. Needle-like crystals on the calcite surfaces are most likely calcium lactate, as this phase is expected in the precipitated samples.