Optimization of Culture Conditions to Enhance Biosurfactant Production by Bacteria Isolated from Manyspiked Flatsedge (Cyperus polystachyos) Rhizosphere Soil

Abstract

This study explored the emulsification capabilities and optimization of culture conditions of bacterial isolates obtained from the rhizosphere soil of Cyperus polystachyos (Manyspike Flatsedge), emphasizing their potential for biotechnological applications. Soil samples were subjected to detailed physicochemical analysis to determine pH, electrical conductivity, and concentrations of nutrients and metals. The soil exhibited a slightly acidic pH (5.90) and moderate electrical conductivity (328 µS/cm). Nutrient analysis revealed nitrate (7.290mg/kg) and phosphate (1.417mg/kg) availability, alongside the presence of heavy metals such as iron (115.352mg/kg), zinc (12.313 mg/kg), chromium (5.112 mg/kg), aluminum (2.456 mg/kg), and nickel (17.472 mg/kg). A total of seventeen bacterial isolates were obtained and subjected to a series of biochemical tests and Gram staining.  Preliminary screening for biosurfactant activity using emulsification index (E₂₄) revealed that Klebsiella sp. exhibited the highest activity, achieving an E₂₄ index of 51.6%, followed closely by Acinetobacter sp. with an E₂₄ index of 50%, Bacillus sp (E₂₄: 20 %), Pseudomonas sp (E₂₄: 5.9 % %; 16 %), Staphylococcus sp(E₂₄: 0 %; 24 %) and Micrococcus sp (E₂₄: 14.2 %) suggesting their strong potential for biosurfactant production. Optimization of physicochemical parameters showed that all isolates exhibited poor or no biosurfactant production across tested pH levels (3, 5, 7, and 9), except Pseudomonas sp , which displayed positive biosurfactant activity at pH 5 and 7, suggesting moderate tolerance to slightly acidic and neutral conditions. Similarly, salinity tolerance tests (1%, 4%, 8%, and 10% NaCl) indicated that all isolates were inhibited under saline conditions, showing no biosurfactant activity. The emulsification activity demonstrated by Klebsiella sp. and Acinetobacter sp. highlights their applicability in bioremediation of hydrocarbon-contaminated sites and industrial processes requiring eco-friendly surfactants. The ability of Pseudomonas sp to produce biosurfactant at pH 5 and 7 suggest production can be optimized at such pH. Further investigations into optimizing growth conditions and characterizing biosurfactant properties are warranted to maximize their practical utility.