Abstract
BackgroundConsolidated bioprocessing (CBP) by anaerobes, such as Clostridium thermocellum, which combine enzyme production, hydrolysis, and fermentation are promising alternatives to historical economic challenges of using fungal enzymes for biological conversion of lignocellulosic biomass. However, limited research has integrated CBP with real pretreated biomass, and understanding how pretreatment impacts subsequent deconstruction by CBP vs. fungal enzymes can provide valuable insights into CBP and suggest other novel biomass deconstruction strategies. This study focused on determining the effect of pretreatment by dilute sulfuric acid alone (DA) and with tetrahydrofuran (THF) addition via co-solvent-enhanced lignocellulosic fractionation (CELF) on deconstruction of corn stover and Populus with much different recalcitrance by C. thermocellum vs. fungal enzymes and changes in pretreated biomass related to these differences.ResultsCoupling CELF fractionation of corn stover and Populus with subsequent CBP by the anaerobe C. thermocellum completely solubilized polysaccharides left in the pretreated solids within only 48 h without adding enzymes. These results were better than those from the conventional DA followed by either CBP or fungal enzymes or CELF followed by fungal enzyme hydrolysis, especially at viable enzyme loadings. Enzyme adsorption on CELF-pretreated corn stover and CELF-pretreated Populus solids were virtually equal, while DA improved the enzyme accessibility for corn stover more than Populus. Confocal scanning light microscopy (CSLM), transmission electron microscopy (TEM), and NMR characterization of solids from both pretreatments revealed differences in cell wall structure and lignin composition, location, coalescence, and migration-enhanced digestibility of CELF-pretreated solids.ConclusionsAdding THF to DA pretreatment (CELF) greatly enhanced deconstruction of corn stover and Populus by fungal enzymes and C. thermocellum CBP, and the CELF–CBP tandem was agnostic to feedstock recalcitrance. Composition measurements, material balances, cellulase adsorption, and CSLM and TEM imaging revealed adding THF enhanced the enzyme accessibility, cell wall fractures, and cellular dislocation and cell wall delamination. Overall, enhanced deconstruction of CELF solids by enzymes and particularly by C. thermocellum could be related to lignin removal and alteration, thereby pointing to these factors being key contributors to biomass recalcitrance as a barrier to low-cost biological conversion to sustainable fuels.
Highlights
Consolidated bioprocessing (CBP) by anaerobes, such as Clostridium thermocellum, which combine enzyme production, hydrolysis, and fermentation are promising alternatives to historical economic challenges of using fungal enzymes for biological conversion of lignocellulosic biomass
co-solvent-enhanced lignocellulosic fractionation (CELF) and dilute sulfuric acid alone (DA) deconstruction of corn stover and poplar wood The BioEnergy Science Center (BESC) through Oak Ridge National Laboratory (ORNL, Oak Ridge, TN) and the National Renewable Energy Laboratory (NREL, Golden, CO) provided BESC standard Populus (Populus trichocarpa) and corn stover, Zea mays, respectively, with compositions of each being reported in the Additional file 1
CELF proved more efficacious than DA for deconstructions of both corn stover and Populus in combination with fungal enzymes or C. thermocellum
Summary
Consolidated bioprocessing (CBP) by anaerobes, such as Clostridium thermocellum, which combine enzyme production, hydrolysis, and fermentation are promising alternatives to historical economic challenges of using fungal enzymes for biological conversion of lignocellulosic biomass. Lignocellulosic biomass stands out as an inexpensive, widely available non-food sustainable resource from which enough liquid fuels could be derived to impact energy demands and reduce atmospheric carbon dioxide accumulation. Saccharification of lignocellulosic polysaccharide to sugars for fermentation to ethanol and other products remains more expensive than petroleum based liquid fuels due to plant cell wall recalcitrance to chemical, physical, or biological deconstruction [15,16,17]. Trichoderma reesei fungal enzymes have been historically applied to break down plant cell walls [18, 19], but enzymes are estimated to cost ~ $0.70–$1.50/gal ethanol at loadings that achieve viable yields from even relatively low-recalcitrant corn stover [20, 21]. Tradeoffs among pretreatment types and conditions and fungal enzyme cocktails and loadings have been researched for various lignocellulosic feedstocks [22,23,24,25,26], enzyme costs remain too high [20, 27, 28], and it is desirable to explore new routes to achieving high yields from biological deconstruction
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