Carbohydrate-binding modules facilitate the enzymatic hydrolysis of lignocellulosic biomass: Releasing reducing sugars and dissociative lignin available for producing biofuels and chemicals.

The microbial decomposition and utilization of lignocellulosic biomass present in the plant tissues are driven by a series of carbohydrate active enzymes (CAZymes) acting in concert. As the non-catalytic domains widely found in the modular CAZymes, carbohydrate-binding modules (CBMs) are intimately associated with catalytic domains (CDs) that effect the diverse hydrolytic reactions. The CBMs function as auxiliary components for the recognition, adhesion, and depolymerization of the complex substrate mediated by the associated CDs. Therefore, CBMs are deemed as significant biotools available for enzyme engineering, especially to facilitate the enzymatic hydrolysis of dense and insoluble plant tissues to acquire more fermentable sugars. This review aims at presenting the taxonomies and biological properties of the CBMs currently curated in the CAZy database. The molecular mechanisms that CBMs use in assisting the enzymatic hydrolysis of plant polysaccharides and the regulatory factors of CBM-substrate interactions are outlined in detail. In addition, guidelines for the rational designs of CBM-fused CAZymes are proposed. Furthermore, the potential to harness CBMs for industrial applications, especially in enzymatic pretreatment of the recalcitrant lignocellulose, is evaluated. It is envisaged that the ideas outlined herein will aid in the engineering and production of novel CBM-fused enzymes to facilitate efficient degradation of lignocellulosic biomass to easily fermentable sugars for production of value-added products, including biofuels.

Interactions between Anaerobic Fungi and Methanogens in the Rumen and Their Biotechnological Potential in Biogas Production from Lignocellulosic Materials.

Anaerobic fungi in the digestive tract of herbivores are one of the critical types of fiber-degrading microorganisms present in the rumen. They degrade lignocellulosic materials using unique rhizoid structures and a diverse range of fiber-degrading enzymes, producing metabolic products such as H2/CO2, formate, lactate, acetate, and ethanol. Methanogens in the rumen utilize some of these products (e.g., H2 and formate) to produce methane. An investigation of the interactions between anaerobic fungi and methanogens is helpful as it provides valuable insight into the microbial interactions within the rumen. During the last few decades, research has demonstrated that anaerobic fungi stimulate the growth of methanogens and maintain methanogenic diversity. Meanwhile, methanogens increase the fiber-degrading capability of anaerobic fungi and stimulate metabolic pathways in the fungal hydrogenosome. The ability of co-cultures of anaerobic fungi and methanogens to degrade fiber and produce methane could potentially be a valuable method for the degradation of lignocellulosic materials and methane production.

Methane Production From Different Parts of Corn Stover via a Simple Co-culture of an Anaerobic Fungus and Methanogen.

To determine ways to improve the utilization of corn stover, this study investigated methane production from different parts of corn stover using a simple co-culture of an anaerobic fungus (Pecoramyces species) and methanogen (Methanobrevibacter species). The simple co-culture was incubated with the stem pith, leaf blade, or stem bark of corn stover (as substrates) at 39°C for 72 h. The results showed that the stem bark had the lowest (P < 0.05) digestibility (38.0 ± 1.36%) and neutral detergent solubles, that is, cell solubles (31.6 ± 0.45%), and the highest (P < 0.05) lignin content (4.8 ± 0.56%). The leaf blade had a significantly higher methane conversion rate (56.6 ± 0.76 mL/g digested substrate) than the stem pith (49.2 ± 1.60 mL/g digested substrate), even though they showed similar levels of methane production (42.4 ± 1.0 mL and 40.9 ± 1.35 mL, respectively). Both the leaf blade and stem pith of corn stover have the potential to produce methane in a simple co-culture of an anaerobic fungus and methanogen.

Effects of steam explosion on lignocellulosic degradation of, and methane production from, corn stover by a co-cultured anaerobic fungus and methanogen.

The aim of this study was to investigate the effects of steam explosion on lignocellulose digestibility of, and methane production from corn stover by a co-culture of anaerobic fungus and methanogen. The cumulative methane production at 72 h of incubation from the steam-exploded corn stover was 32.2 ±â€¯1.74 mL, which not significantly different (P > 0.05) from that of the untreated corn stover (37.1 ±â€¯1.09 mL). However, steam explosion decreased the hemicellulose contents of corn stover by 28.0 ±â€¯0.39% and increased the neutral detergent solute by 23.5 ±â€¯0.25%. While this treatment did not affect the dry matter digestibility (64.1 ±â€¯0.26%, and 64.1 ±â€¯0.28%, respectively). In conclusion, the co-culture of anaerobic fungus and methanogen can degrade the crude fibrous portion of corn stover without any pretreatments. It possesses promising biotechnological prospects for conversion of crop residue based straw resources to obtain biofuel in the form of methane.

The biotechnological potential of anaerobic fungi on fiber degradation and methane production.

Anaerobic fungi (phylum Neocallimastigomycota), an early branching family of fungi, are commonly encountered in the digestive tract of mammalian herbivores. To date, isolates from ten described genera have been reported, and several novel taxonomic groupings are detected using culture-independent molecular methods. Anaerobic fungi are recognized as playing key roles in the decomposition of lignocellulose (up to 50% of the ingested and untreated lignocellulose), with their physical penetration and extracellular enzymatical secretion of an unbiased diverse repertoire of cell-wall-degrading enzymes. The secreted cell-wall-degrading enzymes of anaerobic fungi include both free enzymes and extracellular multi-enzyme complexes called cellulosomes, both of which have potential as fiber degraders in industries. In addition, anaerobic fungi can provide large amounts of substrates such as hydrogen, formate, and acetate for their co-cultured methanogens. Consequently, large amounts of methane can be produced. And thus, it is promising to use the co-culture of anaerobic fungi and methanogens in the biogas process to intensify the biogas yield owing to the efficient and robust degradation of recalcitrant biomass by anaerobic fungi and improved methane production from co-cultures of anaerobic fungi and methanogens.