Nitrogen ion beam mutagenesis improves sophorolipid production and properties in Starmerella bombicola

Sophorolipids (SLs) are microbial biosurfactants with broad industrial potential, but their large-scale application relies on robust high-producing strains with desirable functional properties. In this study, Starmerella bombicola CGMCC 1576 was subjected to nitrogen (N+) ion beam mutagenesis to improve SL production and product performance. A mutant library containing 124 isolates was constructed, and two stable high-producing mutants, 2–21 and 3–5, were obtained through high-throughput screening and shake-flask validation. Mutant 2–21 preferentially accumulated acidic sophorolipids (ASL) and showed the highest total sophorolipid (TSL) production, whereas mutant 3–5 produced more lactonic sophorolipids (LSL). HPLC profiling showed that the mutants retained the major SL components of the parental strain but differed markedly in component abundance and product distribution. Transcriptional analysis revealed upregulation of genes associated with fatty acid utilization, SL biosynthesis and metabolite transport, particularly in mutant 2–21 at 96 h, suggesting enhanced precursor conversion and product accumulation. Functional assays further showed composition-dependent differences in surface activity and mildness. LSL-rich fractions exhibited stronger surface activity, while ASL-rich fractions showed lower zein solubilization and hemolytic activity. In 100-L bioreactor fermentation, mutant 2–21 produced 224.31 g/L TSL, confirming its scalability and production robustness. These results demonstrate that N+ ion beam mutagenesis is an effective strategy for developing high-producing S. bombicola strains and tailoring SL composition for functional applications. Key points Nitrogen ion beam mutagenesis improved SL production in S. bombicola.ASL-rich and LSL-rich mutants showed distinct SL profiles.Mutant 2–21 produced 224.31 g/L TSL in a 100-L bioreactor.