Adaptation of Scheffersomyces stipitis to hardwood spent sulfite liquor by evolutionary engineering

BACKGROUND: Hardwood spent sulfite liquor (HSSL) is a by-product of acid sulfite pulping process that is rich in xylose, a monosaccharide that can be fermented to ethanol by Scheffersomyces stipitis. However, HSSL also contains acetic acid and lignosulfonates that are inhibitory compounds of yeast growth. The main objective of this study was the use of an evolutionary engineering strategy to obtain variants of S. stipitis with increased tolerance to HSSL inhibitors while maintaining the ability to ferment xylose to ethanol. RESULTS: A continuous reactor with gradually increasing HSSL concentrations, from 20% to 60% (v/v), was operated for 382 generations. From the final obtained population (POP), a stable clone (C(4)) was isolated and characterized in 60% undetoxified HSSL. C(4) isolate was then compared with both the parental strain (PAR) and POP. Both POP and C(4) were able to grow in 60% undetoxified HSSL, with a higher capability to withstand HSSL inhibitors than PAR. Higher substrate uptake rates, 7% higher ethanol efficiency and improved ethanol yield were obtained using C4. CONCLUSION: S. stipitis was successfully adapted to 60% (v/v) undetoxified eucalyptus HSSL. A stable isolate, C(4), with an improved performance in undetoxified HSSL compared to PAR was successfully obtained from POP. Owing to its improved tolerance to inhibitors, C(4) may represent a major advantage for the production of bioethanol using HSSL as substrate.