Abstract
The properties of lignocellulosic substrates obtained from different pretreatments have a big impact on downstream saccharification based on both the fungal cellulase system and the cellulosome-based whole-cell biocatalysis system. However the corresponding effect of these two distinct saccharification strategies has not been comparatively analyzed. In this work, three ammonium lauryl sulfate (AS)-based pretreatment combinations were conducted to treat wheat straw. The obtained pretreated substrates with different properties were saccharified using fungal cellulase or an engineered Clostridium thermocellum strain as the whole-cell biocatalyst, and the ability to release sugar was comparatively evaluated. It was found that for the whole-cell saccharification, the total sugar digestibility of AS + HT/X pretreated wheat straw was 10% higher than that of HT + AS pretreated wheat straw.

Introduction
Agricultural straw is one of the most abundant lignocelluloses, and its annual production in China is over 600 million tons. However, previously most of this was burned directly in the field, which caused serious environmental issues and waste of natural resources. Currently, the burning of agricultural waste is strictly forbidden, and simultaneously energy demand is increasing. Therefore, the Chinese government plans to use gasoline blended with 10% bioethanol for motors nationwide by 2020, which would require more than 10 million metric tons (about 3.3 billion gallons) of bioethanol. Also, the US Energy Independence and Security Act requires 36 billion gallons of biofuel by 2022. Yet, currently, over 90% of bioethanol is produced from corn instead of lignocelluloses in the US and China. In this case, the development of cellulosic ethanol by making better use of agricultural waste has become an urgent matter.
Among these approaches, ammonium lauryl sulfate (AS) pretreatment can efficiently remove lignin of lignocellulose via ammonolysis and sulfonation of lignin, thus enhancing the accessibility of cellulose. Also, the spent liquor of AS pretreatment can be used to produce lignin-based fertilizer due to the high nitrogen contents,14 and the sulfonated lignin can be used to produce other products as well, such as concrete water-reducer, dye dispersant, or surfactant,7 to further improve the economic feasibility and sustainability of the entire process of AS pretreatment.

Conclusions
In this study, two distinct biological saccharification strategies using fungal enzymes and C. thermocellum cellulosome were compared to evaluate the effect of the properties of substrates obtained from different pretreatment combinations based on ammonium lauryl sulfate (AS) pretreatment, hydrothermal (HT) pretreatment, and xylanase (X) pretreatment on wheat straw sugar release, and to reveal the reason why the two saccharification systems have different ability to release sugars with the same pretreated substrates. Results showed that AS + HT/X pretreatment could remove more lignin, generating large external surface and less surface lignin coverage on the substrate, thus leading to a higher accessibility to cellulosome, in comparison with HT + AS pretreatment. Therefore, AS + HT/X pretreatment is more compatible with cellulosome-based whole-cell saccharification. In addition, HT pretreatment should be carefully adjusted to reduce re-arrangement and condensation of lignin on fibers to match the cellulosome system, because cellulosome has a relatively higher sensitivity to accessibility of substrate compared to free enzymes, and the generation of condensed lignin and pseudo-lignin could lower the external surface of substrate.