Microfluidic crystal tracking reveals an ionic-strength driven nucleation-growth switch in MICP

Kinetics of microbially induced carbonate precipitation (MICP) in hypersaline matrices are difficult to resolve because bulk assays collapse nucleation and growth into endpoint metrics, thereby masking interfacial heterogeneity. Here, we use an in situ microfluidic platform with time-lapse imaging and automated crystal tracking to probe how ionic strength reshapes bacteria-mineral biointerface interactions and thereby regulates CaCO₃ precipitation by Staphylococcus succinus J3 across 0-100 g L^-1 NaCl. Microfluidic observations reveal an ionic-strength-dependent nucleation-growth switch: low salinity is associated with the rapid appearance of abundant microcrystals and early growth saturation, whereas high salinity yields fewer observable nuclei but sustained post-nucleation growth, producing sparse yet much larger, calcite-dominated crystals. These trends were interpreted qualitatively using literature-based concepts from classical nucleation theory and double-layer interactions, suggesting that low ionic strength favors nucleation, whereas high ionic strength suppresses nucleation but promotes growth on existing surfaces. To test this mechanism, we introduced low salinity biogenic nuclei into hypersaline produced water, thereby increasing hardness removal from 91.32% to 98.67% and generating larger particles with improved separability. Overall, this work provides a biointerface-based mechanistic rationale for tuning MICP under high ionic strength and highlights microfluidics as a practical tool for resolving biomineralization kinetics in complex fluids.