Biomass Conversion Research Laboratory (BCRL), Department of Chemical Engineering and Materials Science, Michigan State University, 3900 Collins Road, Lansing, MI 48910, USA ; DOE Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI 48824, USA.
DOE Great Lakes Bioenergy Research Center, University of Wisconsin-Madison, 1552 University Ave, Madison, WI 53726, USA.
Biotechnol Biofuels. 2013 Jul 27;6:108. doi: 10.1186/1754-6834-6-108. eCollection 2013.
Simultaneous saccharification and co-fermentation (SSCF) process involves enzymatic hydrolysis of pretreated lignocellulosic biomass and fermentation of glucose and xylose in one bioreactor. The optimal temperatures for enzymatic hydrolysis are higher than the standard fermentation temperature of ethanologenic Saccharomyces cerevisiae. Moreover, degradation products resulting from biomass pretreatment impair fermentation of sugars, especially xylose, and can synergize with high temperature stress. One approach to resolve both concerns is to utilize a strain background with innate tolerance to both elevated temperatures and degradation products.
In this study, we screened a panel of 108 wild and domesticated Saccharomyces cerevisiae strains isolated from a wide range of environmental niches. One wild strain was selected based on its growth tolerance to simultaneous elevated temperature and AFEX™ (Ammonia Fiber Expansion) degradation products. After engineering the strain with two copies of the Scheffersomyces stipitis xylose reductase, xylitol dehydrogenase and xylulokinase genes, we compared the ability of this engineered strain to the benchmark 424A(LNH-ST) strain in ethanol production and xylose fermentation in standard lab medium and AFEX pretreated corn stover (ACS) hydrolysates, as well as in SSCF of ACS at different temperatures. In SSCF of 9% (w/w) glucan loading ACS at 35°C, the engineered strain showed higher cell viabilities and produced a similar amount of ethanol (51.3 g/L) compared to the benchmark 424A(LNH-ST) strain.
These results validate our approach in the selection of wild Saccharomyces cerevisiae strains with thermo-tolerance and degradation products tolerance properties for lignocellulosic biofuel production. The wild and domesticated yeast strains phenotyped in this work are publically available for others to use as genetic backgrounds for fermentation of their pretreated biomass at elevated temperatures.
同步糖化和共发酵（SSCF）工艺涉及酶解预处理木质纤维素生物质和在一个生物反应器中发酵葡萄糖和木糖。酶解的最佳温度高于产乙醇酿酒酵母的标准发酵温度。此外，生物质预处理产生的降解产物会损害糖的发酵，尤其是木糖的发酵，并且会与高温应激协同作用。解决这两个问题的一种方法是利用一种具有对高温和降解产物固有耐受性的菌株背景。
在本研究中，我们筛选了来自广泛环境小生境的 108 株野生和驯化酿酒酵母菌株。根据其对同时升高的温度和 AFEX（氨纤维膨胀）降解产物的生长耐受性，选择了一株野生菌株。在用两拷贝酿酒酵母木糖还原酶、木酮糖脱氢酶和木酮糖激酶基因对该菌株进行工程改造后，我们比较了该工程菌株与基准 424A(LNH-ST)菌株在标准实验室培养基和 AFEX 预处理玉米秸秆（ACS）水解物中的乙醇生产和木糖发酵能力，以及在不同温度下的 SSCF 中的能力。在 35°C 的 9%（w/w）葡聚糖加载 ACS 的 SSCF 中，与基准 424A(LNH-ST)菌株相比，该工程菌株表现出更高的细胞活力，并产生了相似量的乙醇（51.3 g/L）。
这些结果验证了我们在选择具有耐热性和降解产物耐受性的野生酿酒酵母菌株用于木质纤维素生物燃料生产的方法。本工作中表型分析的野生和驯化酵母菌株可供公众使用，作为在高温下发酵其预处理生物质的遗传背景。
