Biodegradation and high-value utilization of waste cooking oil: Strategies and mechanisms for producing lipopeptide biosurfactants using Bacillus subtilis YZQ-2

Abstract

This study explored the strategies and mechanisms of Bacillus subtilis YZQ-2 in producing lipopeptide biosurfactants using waste cooking oil (WCO) as a raw material. The optimal conditions achieved a WCO utilization rate of 75.88% and a biosurfactant yield of 0.30g/L at pH 6.0, 30°C, with 20g/L WCO and 5g/L yeast extract. GC-MS analysis of degradation products at different intervals elucidated the mechanism of WCO degradation. After 72hours of fermentation, a reduction in linoleic and palmitic acid levels indicated the completion of lipid hydrolysis and the continuation of β-oxidation. The biosurfactants, identified as lipopeptides (surfactin and fengycin), exhibited stability in alkaline environments (pH 7.0-11.0) and high-salt concentration (5-45g/L). Genomic and transcriptomic analyses of Bacillus subtilis YZQ-2 revealed genes associated with biosurfactant production, which were upregulated in the presence of WCO. The differential expression values (log2 fold change) were SrfAA 3.30, SrfAB 3.31, SrfAC 3.63, FenA 2.42, FenB 2.55, FenC 1.99, FenD 2.03 and FenE 2.29, respectively. The proposed biosynthesis pathway of lipopeptides begins with the hydrolysis of WCO by lipase into glycerol and fatty acids. The glycerol is then transformed into pyruvate through glycolysis, entering the Tricarboxylic Acid cycle. Additionally, pyruvate contributes to synthesizing amino acids and branched-chain fatty acids, which serve as precursors for lipopeptide synthesis. These findings underscore the potential of WCO as a sustainable feedstock for biosurfactant production.