Effect of γ-polyglutamic Acid Produced by Drought Tolerant Bacillus subtilis FSO3 on Soil Moisture Retention

Drought stress poses significant risks to agricultural productivity by reducing soil moisture availability. Bacterial polymers, such as γ-polyglutamic acid (γ-PGA), have effectively enhanced soil moisture retention and promoted agricultural resilience. This study aimed to investigate the effects of drought stress on Bacillus subtilis FSO3, a glutamate-independent γ-polyglutamic acid (γ-PGA) producer, and to evaluate the potential of its γ-PGA-rich fermentation medium for soil moisture retention. The strain produced up to 2.24 g/L γ-PGA without external glutamate supplementation. Under polyethylene glycol-6000 (PEG-6000)-induced water-deficit conditions, B. subtilis FSO3 demonstrated moderate drought tolerance, with a 50% reduction in maximum biomass at 20% PEG-6000. Interestingly, γ-PGA yield increased from 1.6 g/L at 5% PEG-6000 to 2.3 g/L at 20%. The γ-PGA-rich fermentation medium, applied at 100 mg γ-PGA/kg of soil sustained soil moisture levels above 60% for approximately 33% longer than the control. These findings highlight the potential of the γ-PGA-rich fermentation medium produced by B. subtilis FSO3 as a promising and cost-effective solution for enhancing soil water retention under drought conditions.