Biosynthesis of rhamnolipids from waste cooking oil by Pseudomonas aeruginosa RW9 with insights into stability and toxicity performance

Rhamnolipids (RLs) are eco-friendly surfactants mainly produced by Pseudomonas aeruginosa. This study explores the sustainable production of RLs using waste cooking oil (WCO) as a carbon source, with NaNO3 and yeast extract (YE) as nitrogen sources. Response surface methodology (RSM) based on central composite design (CCD) was applied to optimize the concentration of WCO (10–30 g/L), NaNO3 (0–0.05 mol/L), and YE (0–2 g/L). The optimized conditions were 25.95 g/L WCO, 0.04 mol/L NaNO3, and 0.41 g/L yeast extract, resulting in 7.93 g/L RLs concentration and 4.92 g/L biomass concentration, representing a 5.6-fold increase in RLs concentration using a similar carbon source and strain. Ten congeners of mono-RLs and di-RLs were detected by LC–MS/Q-TOF. The RLs stability was evaluated at different temperatures (4–121 °C), pH (4–12), and salinity (5%–25% w/v) by measuring the emulsification capacity, where 50% were maintained up to 100 °C, pH 4–8, and salinity up to 25% (w/v), indicating good physicochemical stability towards harsh conditions. Phytotoxicity tests on choy sum, cabbage, and mung bean seeds showed germination index (GI) values above 90% at 1 g/L RLs, indicating strong compatibility with crop growth. Meanwhile, aquatic toxicity test on zebrafish (Danio rerio) embryos showed an LC50 of 67.42 µg/mL RLs, demonstrating lower toxicity compared to the chemical surfactant. These findings highlight the feasibility of high-yield RLs production from WCO through a predictive process modelling, with a stable, highly functional, and low-ecotoxicity profile. The study introduces a resource-efficient strategy to support RLs’ applications in environmental remediation, and green products development.

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