Bacillus subtilis Changes Salt Precipitation Patterns and Affects Saltwater Evaporation via Contact Line Pinning

Plant growth-promoting rhizobacteria (PGPR) have been shown to mediate drought tolerance by inducing changes in soil physical properties, water retention and flow dynamics. However, PGPR’s potential and mechanisms in mediating salt tolerance through these biophysical controls remain poorly understood. To address this, we conducted saltwater evaporation experiments with 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 cells was compared in these systems. Results demonstrated the significant influence of bacterial deposition on water film configuration, air-water interface behavior, and patterns of salt accumulation/precipitation during evaporation. Images of evaporation of sessile droplets showed that bacterial cells pinned the contact line, resulting in salt precipitation along the perimeter, whereas in the absence of UD1022, salt precipitates were concentrated in the droplet center. In microchannel packed with sand particles, salt clusters formed on sand particle surfaces in controls whereas salt precipitation occurred in the pore space between sand particles in UD1022-treated samples, consistent with contact line pinning. The biophysical controls observed at the microscale were reflected in mesoscale column measurements, where UD1022 treatment increased water retention and reduced evaporation at 10 and 20 g/kg salt concentrations compared to controls. Light reflection imaging revealed earlier onset and more salt precipitation in UD1022-treated columns compared to the controls. Mechanistically, bacterial-induced contact line pinning led to (1) earlier onset of salt precipitates resulting in partial blocking of pores and thus increased capillary connection and evaporation at the early stage, and (2) complete pore blocking thus reduced evaporation at the later stage. The sequential processes contributed to the observed overall reduction in evaporation and higher water retention.