Effects of Plant Growth-Promoting Rhizobacteria (PGPR) on Saltwater Evaporation: A Case Study Using Bacillus Subtilis

Plant growth-promoting rhizobacteria (PGPR) are known for their ability to enhance plant tolerance to abiotic and biotic stresses, including drought and salinity. Additionally, PGPR have been shown to mediate changes in physical properties and hydrological functions of soil. In previous studies, we demonstrated that Bacillus subtilis FB17 (trade name UD1022, a PGPR), could increase soil water retention, preserve continuity of the liquid phase in drying soils, and decrease evaporation. These effects are attributed to production of extracellular polymeric substances (EPS), which are capable of mediating local/micro scale changes in water retention and flow dynamics in soil. We have since extended our study to investigate the potential influence of UD1022 on saltwater evaporation from sand. Specifically, we are comparing evaporation of saltwater, at concentrations 0 (pure water), 10 and 20 ppt, from UD1022-treated sand columns and controls (without treatment). Measurements include temporal changes in cumulative evaporation and evaporation rate, as well as recording surface salt precipitation patterns. Preliminary results from experiments with pure water and 20 ppt saltwater show significant differences in evaporation of pure water between the treated and control columns, however, treatment effects on the evaporation of 20-ppt saltwater were much less pronounced. A preliminary experiment evaluating effects of salt concentration on pellicle formation showed that biofilm formation was suppressed with increasing salinity, presumably, leading to the insignificant effect in reducing evaporation. Nevertheless, images from light and scanning electron microscopes show an earlier onset of salt precipitation on the surface of UD1022-treated sand than control sand, hinting on the potentially very complex interactions between UD1022 and salt precipitation and their effects on evaporation. Additional on-going experiments at lower salt concentrations will allow better mechanistic understanding on how PGPR may mediate changes in salt precipitation and saltwater evaporation in porous media.