Kothari Ninad, Holwerda Evert K, Cai Charles M, Kumar Rajeev, Wyman Charles E
1Dept. of Chemical and Environmental Engineering, Bourns College of Engineering, University of California Riverside (UCR), Riverside, CA USA.
2Center for Environmental Research and Technology (CE-CERT), Bourns College of Engineering, University of California Riverside, Riverside, CA USA.
Biotechnol Biofuels. 2018 Aug 4;11:219. doi: 10.1186/s13068-018-1216-7. eCollection 2018.
The thermophilic anaerobic bacterium is a multifunctional ethanol producer, capable of both saccharification and fermentation, that is central to the consolidated bioprocessing (CBP) approach of converting lignocellulosic biomass to ethanol without external enzyme supplementation. Although CBP organisms have evolved efficient machinery for biomass deconstruction, achieving complete solubilization requires targeted approaches, such as pretreatment, to prepare recalcitrant biomass feedstocks for further biological digestion. Here, differences between how and fungal cellulases respond to senescent switchgrass prepared by four different pretreatment techniques revealed relationships between biomass substrate composition and its digestion by the two biological approaches.
Alamo switchgrass was pretreated using hydrothermal, dilute acid, dilute alkali, and co-solvent-enhanced lignocellulosic fractionation (CELF) pretreatments to produce solids with varying glucan, xylan, and lignin compositions. achieved highest sugar release and metabolite production from de-lignified switchgrass prepared by CELF and dilute alkali pretreatments demonstrating greater resilience to the presence of hemicellulose sugars than fungal enzymes. 100% glucan solubilization and glucan plus xylan release from switchgrass were achieved using the CELF-CBP combination. Lower glucan solubilization and metabolite production by was observed on solids prepared by dilute acid and hydrothermal pretreatments with higher xylan removal from switchgrass than lignin removal. Further, (2% by volume inoculum) showed ~ 48% glucan solubilization compared to < 10% through fungal enzymatic hydrolysis (15 and 65 mg protein/g glucan loadings) of unpretreated switchgrass indicating the effectiveness of 's cellulosome. Overall, performed equivalent to 65 and better than 15 mg protein/g glucan fungal enzymatic hydrolysis on all substrates except CELF-pretreated substrates. CELF pretreatments of switchgrass produced solids that were highly digestible regardless of whether or fungal enzymes were chosen.
The unparalleled comprehensive nature of this work with a comparison of four pretreatment and two biological digestion techniques provides a strong platform for future integration of pretreatment with CBP. Lignin removal had a more positive impact on biological digestion of switchgrass than xylan removal from the biomass. However, the impact of switchgrass structural properties, including cellulose, hemicellulose, and lignin characterization, would provide a better understanding of lignocellulose deconstruction.
嗜热厌氧菌是一种多功能乙醇生产菌，能够进行糖化和发酵，是将木质纤维素生物质转化为乙醇的整合生物加工（CBP）方法的核心，该方法无需额外添加酶。尽管CBP微生物已经进化出高效的生物质解构机制，但要实现完全溶解需要采用有针对性的方法，如预处理，以准备难降解的生物质原料用于进一步的生物消化。在此，研究了嗜热厌氧菌和真菌纤维素酶对通过四种不同预处理技术制备的衰老柳枝稷的反应差异，揭示了生物质底物组成与其通过两种生物方法进行消化之间的关系。
使用水热、稀酸、稀碱和共溶剂强化木质纤维素分级分离（CELF）预处理对阿拉莫柳枝稷进行预处理，以产生具有不同葡聚糖、木聚糖和木质素组成的固体。嗜热厌氧菌从通过CELF和稀碱预处理制备的脱木质素柳枝稷中实现了最高的糖释放和代谢产物生成，表明其对半纤维素糖的存在具有比真菌酶更强的耐受性。使用CELF-CBP组合实现了柳枝稷100%的葡聚糖溶解以及葡聚糖和木聚糖的释放。在通过稀酸和水热预处理制备的固体上，观察到嗜热厌氧菌的葡聚糖溶解和代谢产物生成较低，这些预处理从柳枝稷中去除的木聚糖比木质素更多。此外，嗜热厌氧菌（接种量为2%体积）显示出约48%的葡聚糖溶解，相比之下，未经预处理的柳枝稷通过真菌酶水解（15和65mg蛋白质/g葡聚糖负载量）的溶解率小于10%，这表明嗜热厌氧菌的纤维小体具有有效性。总体而言，在除CELF预处理底物外的所有底物上，嗜热厌氧菌的表现等同于65mg蛋白质/g葡聚糖的真菌酶水解，且优于15mg蛋白质/g葡聚糖的真菌酶水解。柳枝稷的CELF预处理产生的固体无论选择嗜热厌氧菌还是真菌酶都具有高度可消化性。
这项将四种预处理和两种生物消化技术进行比较的工作具有无与伦比的全面性，为未来预处理与CBP的整合提供了强大的平台。去除木质素对柳枝稷生物消化的积极影响大于从生物质中去除木聚糖。然而，柳枝稷结构特性的影响，包括纤维素、半纤维素和木质素的表征，将有助于更好地理解木质纤维素的解构。
