Microbial Contact Line Pinning: How <i>Bacillus subtilis</i> Reshapes Salt Precipitation and Evaporation

Abstract Plant growth‐promoting rhizobacteria (PGPR) have been shown to mediate drought stress by inducing changes in soil physical properties, including water retention and flow dynamics. However, the potential role and underlying mechanisms by which PGPR mediate salt stress through biophysical controls remain poorly understood. To address this gap, we conducted saltwater evaporation experiments using Bacillus subtilis FB17 (UD1022), a PGPR, across multiple scales, including microscale (sessile droplets on glass slides and microchannels packed with a thin layer of sand) and mesoscale (columns packed with sand). Evaporation of NaCl solutions (0, 10, and 20 g/kg) mixed with and without UD1022 was compared. Images of evaporating sessile droplets showed that bacterial cells pinned the contact line, resulting in salt precipitation along the droplet perimeter. In contrast, in the absence of UD1022, salt accumulated at the droplet center. In sand‐packed microchannels, salt clusters formed on sand particle surfaces in control samples, whereas in UD1022‐treated samples, salt precipitated within pore spaces between sand particles, consistent with contact line pinning. These microscale biophysical effects scaled up to the mesoscale, where column measurements showed that UD1022 increased water retention and reduced saltwater evaporation compared to controls. Mechanistically, whereas unrestricted salt precipitation in control systems spread across the surface and increased the effective evaporation area, bacterial‐induced contact line pinning (a) confined salt precipitation to air‐water‐solid interfaces at the contact line, resulting in partial pore blockage at an early stage, and (b) led to complete pore blockage at a later stage, further decreasing evaporation.