ARTP–UV mutagenesis and adaptive evolution to improve sophorolipid production and straw hydrolysate inhibitor tolerance in Starmerella bombicola

Abstract

Corn straw, rich in cellulose, can be hydrolyzed to produce fermentable saccharides, providing a cost-effective raw material for producing sophorolipids (SLs) as an alternative energy source. Enzymatic saccharification efficiency improves with pretreatment before cellulose hydrolysis. However, multiple inhibitors are released during pretreatment, hindering microbial growth and metabolism. Reducing this inhibitory effect is a crucial step in lignocellulose biorefinery. As SLs production relies on high-carbon-content sources, enhancing strain tolerance to inhibitors in concentrated hydrolysates is key to achieving efficient substrate conversion. This study employed a combined mutagenesis approach using atmospheric room-temperature plasma and ultraviolet irradiation on Starmerella bombicola, followed by adaptive evolution and breeding under stress. A mutant strain (U1–4) with significantly improved SLs production was identified, with a 36.32 % higher production than the original strain. The tolerance of the mutant to key hydrolysate inhibitors was evaluated, revealing notable improvements in resistance to formic acid, acetic acid, syringaldehyde, and vanillin. For instance, the SLs production inhibition and cell death rates decreased by 79.24 % and 68.13 %, respectively, in the presence of 1.5 g/L vanillin, compared with the original strains. This study investigated the impact of fermentation conditions on SLs production by the mutant strain using straw cellulose as the raw material. After optimizing fermentation conditions and scaling up the SLs process from shake flasks to a bioreactor, the SLs production reached 172.52 g/L, with a productivity of 0.80 g/(L · h). This study provides highly tolerant fermentation strains for SLs biorefinery using lignocellulose as a raw material, thereby enhancing the bioconversion efficiency of renewable resources.