Microscale analysis of in vitro anaerobic degradation of lignocellulosic wastes by rumen microorganisms

Enhancing biogas generation from lignocellulosic biomass through biological pretreatment: Exploring the role of ruminant microbes and anaerobic fungi

Biogas production from Lignocellulosic Biomass (LB) via anaerobic digestion (AD) has gained attention for its potential in self-sustainability. However, the recalcitrance of LB cell walls pose a challenge to its degradability and biogas generation. Therefore, pretreatment of LB is necessary to enhance lignin removal and increase degradability. Among the different approaches, environmentally friendly biological pretreatment ispromising as it avoids the production of inhibitors. The ruminal microbial community, including anaerobic fungi, bacteria, and protozoa, has shown an ability to effectively degrade LB through biomechanical and microbial penetration of refractory cell structures. In this review, we provide an overview of ruminant microbes dominating LB's AD, their degradation mechanism, and the bioaugmentation of the rumen. We also explore the potential cultivation of anaerobic fungi from the rumen, their enzyme potential, and their role in AD. The rumen ecosystem, comprising both bacteria and fungi, plays a crucial role in enhancing AD. This comprehensive review delves into the intricacies of ruminant microorganisms' adhesion to plant cells, elucidates degradation mechanisms, and explores integrated pretreatment approaches for the effective utilization of LB, minimizing the impact of inhibitors. The discussion underscores the considerable potential of ruminant microbes in pretreating LB, paving the way for sustainable biogas production. Optimizing fungal colonization and ligninolytic enzyme production, such as manganese peroxidase and laccase, significantly enhances the efficiency of fungal pretreatment. Integrating anaerobic fungi through bioaugmentation during mainstream processing demonstrably increases methane production. This study opens promising avenues for further research and development of these microorganisms for bioenergy production.

Thermal pretreatment of Lantana camara for improved biogas production: Process parameter studies for energy evaluation

The rigid lignocellulosic structure of Lantana camara impedes the hydrolysis phase and reduces the biogas production during anaerobic digestion of Lantana camara. Hence, the current study focuses on the impact of various heating pretreatment techniques, viz., hot air oven (HAO), autoclave (ATC), hot water bath (HWB), and microwave (MCW) on L. camara to speed up hydrolysis and boost up biogas production. ATC pretreatment of L. camara was witnessed to be most efficient compared to HAO, MCW, and HWB pretreatment. ATC pretreatment enhanced the solubilization (45.44%), and an increment in volatile fatty acids (VFA) was observed (56.75%) at 110 °C for 80 min when correlated to the untreated (control). Cumulative methane production following ATC pretreatment had risen to 3656 mL in 5 weeks from 2895 mL in 7 weeks. Thermal pretreatment of Lantana camara broke down the rigid lignocellulosic structure, accelerating the hydrolysis stage and improving biogas production simultaneously. To the best of our knowledge, this is the first thermal pretreatment study conducted on Lantana camara for biogas production.

Exploration of deep eutectic solvent-based biphasic system for furfural production and enhancing enzymatic hydrolysis: Chemical, topochemical, and morphological changes

Developing a biorefinery process for a highly integrated valorization and fractionation of lignocellulose is crucial for its utilization. Herein, a biphasic system comprising choline chloride/lactic acid and 2-methyltetrahydrofuran with Al₂(SO₄)₃ and H₂SO₄ as catalysts was applied to pretreat Eucalyptus. Results showed that under the optimized conditions (150 °C, 30 min, 0.2 M Al₂(SO₄)₃, 0.075 M H₂SO₄), the furfural yield and enzymatic hydrolysis efficiency could reach 54.7% and 97.0%, respectively. The efficient cellulose conversion was attributed to remarkable removal of lignin (91.0%) and hemicelluloses (100.0%), thereby causing the disruption of cell wall structure and enhancement of cellulose accessibility. Meanwhile, confocal Raman microscope and atomic force microscope displayed that the pretreatment resulted in the decreasing intensities of carbohydrates and lignin different regions of cell walls, and exposing of the embedded microfibers from noncellulosic polymers. Overall, the deep eutectic solvent-based biphasic system displayed high performance for effective utilization of carbohydrate components in lignocellulose.

The binding characteristics of sediment-derived dissolved organic matter with ceftazidime: a microstructural and spectroscopic correlation study

This research focused on the characterization of sediment-derived dissolved organic matter (SDOM) extracted from sediment of Yellow River and the binding behaviors of ceftazidime (CAZ) with the presence of SDOM. The morphology, surface composition and structure of SDOM and the complexation between SDOM and CAZ in terms of component features, binding capacity and sequence were studied by multiple approaches. Results showed that SDOM was in situ autochthonous-dominated with a low weight-average molecular weight and aromaticity (the value of S_R was 2.523). The multiple morphological characteristics, high surface oxygen contents (53.49%) and more aliphatic (H/C = 1.91) of SDOM were further confirmed. Studies on SDOM-CAZ interaction suggested that the functional groups and chemical compositions of SDOM were susceptible to CAZ. In more detail, the aromatic protons and aliphatic protons of CAZ impacted significantly, and the binding between CAZ and SDOM might relate to noncovalent. The protein-like fractions were considered to be the primary participant with 49% fractions lost and the aromatics and amides as mainly active site interaction with CAZ. These findings have significant implications on the environmental fate of cephalosporin antibiotics and that of sediment-derived DOM.

Biofilm mediated decolorization and degradation of reactive red 170 dye by the bacterial consortium isolated from the dyeing industry wastewater sediments

Reactive dyes are extensively used in a plethora of industries, which in turn release toxic wastes into the environment. The textile dye waste remediation is crucial as it may contain several toxic elements. The utilization of bacterial consortium for bioremediation has acquired consideration, over the utilization of single strains. In this study, a microbial consortium containing three bacterial sp. (Bacillus subtilis, Brevibacillus borstelensis and Bacillus firmus) was tested for its degrading ability of the textile RR 170 dye. The bacterial consortium degraded the dye effectively at lower concentrations and the efficiency decreased as the dye concentration increased. SEM analysis revealed that, with dye treatment, the consortium appeared as tightly packed clumps with rough cell surface and were able to produce EPS and biofilms. EPS production was higher at 40 mg/l, 100 mg/l and 200 mg/l of the dye treatment conditions. Interestingly, the maximum biofilm formation was observed only at 40 μg/ml of the dye treatment, which indicates that RR 170 dye concentration affects the biofilm formation independent of EPS levels. The UV-vis spectroscopy, HPLC, FTIR and 2D-FTIR analyses confirmed the decolorization and biodegradation of RR 170 dye by the bacterial consortium. Toxicological studies performed with the dye and their degraded products in Allium cepa root cells revealed that, whereas the RR 170 dye induced genotoxic stress, the degraded dye products showed no significant genotoxic effects in root cells. Together, the investigated bacterial consortium decolorized and degraded the RR 170 dye resulting in metabolites that are non-toxic to the living cells.

Characterization and structural analysis of a thermophilic GH11 xylanase from compost metatranscriptome

Xylanase is efficient for xylan degradation and widely applied in industries. We found a GH11 family xylanase (Xyn11A) with high thermostability and catalytic activity from compost metatranscriptome. This xylanase has the optimal reaction temperature at 80 °C with the activity of 2907.3 U/mg. The X-ray crystallographic structure shows a typical "right hand" architecture, which is the characteristics of the GH11 family enzymes. Comparing it with the mesophilic XYN II, a well-studied GH11 xylanase from Trichoderma reesei, Xyn11A is more compact with more H-bonds. Our mutagenic results show that the electrostatic interactions in the thumb and palm region of Xyn11A could result in its high thermostability and activity. Introducing a disulfide bond at the N-terminus further increased its optimal reaction temperature to 90 °C with augmented activity. KEY POINTS: • A hyperthermophilic xylanase with high activity was discovered using the metatranscriptomic method. • The mechanisms of thermophilicity and high activity were revealed using X-ray crystallography, mutagenesis, and molecular dynamics simulations. • The thermostability and activity were further improved by introducing a disulfide bond.

A comparative study of artificial cow and sheep rumen fermentation of corn straw and food waste: Batch and continuous operation

To optimize the artificial rumen microorganism sources and develop a stable artificial rumen system, batch and continuous operation were investigated with corn straw and food waste as substrates. The batch trials evaluated the volatile fatty acid (VFA) yield, biogas production, and lignocellulose degradation efficiency. The continuous test evaluated the performance of the artificial cow and sheep rumen systems using a dynamic membrane bioreactor (DMBR) with a stepwise organic loading rate at mesophilic temperature. The anaerobic digestion (AD) of the lignocellulose biomass after rumen fermentation pretreatment and of the permeate from the artificial rumen system were also evaluated for CH₄ production. The results indicated that the cow rumen microorganisms were more suitable than sheep rumen microorganisms for lignocellulosic biomass pretreatment and maximized the CH₄ yield through the AD process without inhibition. After approximately four months of continuous operation, a stable and continuous artificial rumen system for lignocellulosic biomass degradation was achieved with cow rumen fluid as inoculum. Based on analysis of the core lignocellulose-degrading enzyme levels and gel filtration chromatography, the cow rumen microorganisms could secrete more extracellular multienzyme complexes to hydrolyze lignocellulosic biomass than the sheep rumen microorganisms in vitro. During the batch and continuous operations, a high diversity and similar richness of bacteria and fungi demonstrated that the cow rumen microorganisms can be used as a preferred inoculum for the artificial rumen system. The use of an artificial cow rumen system with a DMBR is a promising way to construct a stable and continuous artificial rumen system to biodegrade lignocellulosic biomass for biogas production.

Cosubstrate strategy for enhancing lignocellulose degradation during rumen fermentation in vitro: Characteristics and microorganism composition

To enhance the degradation of wheat straw (WS) and corn straw (CS) in rumen fermentation, characterization of degradation and ruminal microorganisms of monosubstrate (WS/CS) groups and a cosubstrate strategy with food waste (FW) group was performed. The cellulose, hemicellulose, and lignin degradation efficiency of WS and CS; soluble chemical oxygen demand; volatile fatty acid yields; and activity of ligninolytic, cellulolytic, and hemicellulolytic enzymes for the cosubstrate group were improved compared with those for the corresponding monosubstrate groups. An accurate and a good of fit of the Weibull kinetic model, decreased crystallinity index values, and characteristic absorbance bands in the Fourier transform-infrared spectra further confirmed that cosubstrate addition with FW decreased the resistance of cellulose and hemicellulose to biodegradation. High-throughput sequencing results suggested that the bacterial diversity in CS rumen fermentation and fungal diversity and richness in WS rumen fermentation were promoted with FW as a cosubstrate. The cosubstrate addition with FW significantly affected the composition of the ruminal bacteria and fungi in rumen fermentation. The relative abundances (RAs) of rumen bacteria were increased in the cosubstrate CS/WS and FW fermentation conditions, and the enhancement of CS degradation with FW supplementation was stronger than that of WS rumen fermentation with FW supplementation. The RAs of the ruminal fungal genera Ustilago and Fusarium were promoted in CS and WS fermentation with FW, respectively. Moreover, the fermentation properties and rumen flora in the FW rumen fermentation also provided some evidence to suggest an enhancement of the cosubstrate strategy compared with the monosubstrate strategy.

Persistent action of cow rumen microorganisms in enhancing biodegradation of wheat straw by rumen fermentation

Rumen fermentation is known to be effective for lignocellulosic-wastes biodegradation to certain extent but it is still unclear if there exists a termination of the microorganisms' action to further degrade the bio-refractory fractions. In order to illuminate the related microbiological characteristics, experiments were conducted in a prolonged duration of rumen fermentation of mechanically ruptured wheat straw, with inoculation of cow rumen microorganisms in vitro. Although the organic wastes could not be biodegraded quickly, continuous conversion of the lignocellulosic contents to volatile fatty acids and biogas proceeded in the duration of more than three months, resulting in 96-97% cellulose and hemicellulose decomposition, and 42% lignin decomposition. X-ray diffraction and Fourier transform infrared spectroscopy further demonstrated the characteristics of lignocellulosic structure decomposition. Under the actions of cow rumen microorganisms, stable pH was maintained in the fermentation liquid, along with a steady NH₄⁺-N, volatile fatty acids accumulation, and a large buffering ability. It was identified by enzyme analysis and Illumina MiSeq sequencing that the rich core lignocellulolytic enzymes secreted by the abundant and diverse rumen bacteria and fungi contributed to the persistent degradation of lignocellulosic wastes. Members of the Clostridiales order and Basidiomycota phylum were found to be the dominant lignocellulolytic bacteria and fungi, respectively. It could thus be inferred that the main lignocellulose degradation processes were a series of catalytic reactions under the actions of lignocellulolytic enzymes secreted from bacteria and fungi. The dominant hydrogenotrophic methanogens (Methanomassiliicoccus, Methanobrevibacter, Methanosphaera, and Methanoculleus) in the rumen could also assist CH₄ production if the rumen fermentation was followed with anaerobic digestion.

Cow manure as additive to a DMBR for stable and high-rate digestion of food waste: Performance and microbial community

Cow manure (CM) was added to a dynamic membrane bioreactor (DMBR) operated under anaerobic condition for enhancing food waste (FW) digestion for over 300 days with stepwise increase of organic loading rates (OLRs) from 1.07 to 11.9 g COD/L/day. At a FW/CM ratio of 3.5:1 (based on volatile solids), the mixed liquor pH was always above 8.0 and no apparent volatile fatty acids (VFAs) accumulation occurred even at the highest OLR of 11.9 g COD/L/day (hydraulic retention time as 10 days and solid retention time as 15.5 days, correspondingly), indicating a very stable operation condition which resulted in an average CH₄ yield as high as 250 mL/g COD and CH₄ production as high as 2.71 L CH₄/L/day. The hardly biodegradable organic components, such as cellulose, hemicellulose, and lignin, were effectively degraded by 78.3%, 58.8%, and 47.5%, respectively. Significantly high anaerobic digestion reaction ratios, especially the hydrolysis ratio which is usually the limiting factor, were calculated based on experimental results. Furthermore, the high lignocellulase contents and coenzyme F₄₂₀ levels, along with the decrease of cellulose crystallinity from 72.6% to 16.4% in the feedstock, provided strong evidence of an enhanced biological activity by CM addition. By high-throughput sequencing analysis, more abundant and diverse bacterial, archaeal, and fungal genera were identified from the DMBR sludge. With CM addition, the biodegradation of lignocellulose might have produced sufficient H₂ and CO₂ for the hydrogenotrophic methanogens such as Methanoculleus, Methanomassiliicoccus, and Methanobacterium, which were highly tolerant to ammonium inhibition, and then the elevated ammonium level would have provided high buffering capacity in the DMBR thus ensuring a stable condition for high rate FW digestion and CH₄ production.

Metagenomic insights into the rumen microbial fibrolytic enzymes in Indian crossbred cattle fed finger millet straw

The rumen is a unique natural habitat, exhibiting an unparalleled genetic resource of fibrolytic enzymes of microbial origin that degrade plant polysaccharides. The objectives of this study were to identify the principal plant cell wall-degrading enzymes and the taxonomic profile of rumen microbial communities that are associated with it. The cattle rumen microflora and the carbohydrate-active enzymes were functionally classified through a whole metagenomic sequencing approach. Analysis of the assembled sequences by the Carbohydrate-active enzyme analysis Toolkit identified the candidate genes encoding fibrolytic enzymes belonging to different classes of glycoside hydrolases(11,010 contigs), glycosyltransferases (6366 contigs), carbohydrate esterases (4945 contigs), carbohydrate-binding modules (1975 contigs), polysaccharide lyases (480 contigs), and auxiliary activities (115 contigs). Phylogenetic analysis of CAZyme encoding contigs revealed that a significant proportion of CAZymes were contributed by bacteria belonging to genera Prevotella, Bacteroides, Fibrobacter, Clostridium, and Ruminococcus. The results indicated that the cattle rumen microbiome and the CAZymes are highly complex, structurally similar but compositionally distinct from other ruminants. The unique characteristics of rumen microbiota and the enzymes produced by resident microbes provide opportunities to improve the feed conversion efficiency in ruminants and serve as a reservoir of industrially important enzymes for cellulosic biofuel production.

Influence of pretreatment techniques on anaerobic digestion of pulp and paper mill sludge: A review

Pulp and paper industry is one of the most polluting, energy and water intensive industries in the world. Produced pulp and paper mill sludge (PPMS) faces a major problem for handling and its management. An anaerobic digestion has become an alternative source. This review provides a detailed summary of anaerobic digestion of PPMS - An overview of the developments and improvement opportunities. This paper explores the different pretreatment methods to enhance biogas production from the PPMS. First, the paper gives an overview of PPMS production, and then it reviews PPMS as a substrate for anaerobic digestion with or without pretreatment. Finally, it discuss the optimal condition and concentration of organic and inorganic compounds required for the anaerobic metabolic activity. Future research should focus on the combination of different pretreatment technologies, relationship between sludge composition, reactor design and its operation, and microbial community dynamics.

Effect of Feedstock Concentration on Biogas Production by Inoculating Rumen Microorganisms in Biomass Solid Waste

A methane production system with continuous stirred-tank reactor, rumen liquid as inoculate microorganisms, and paper mill excess sludge (PES) as feedstock was studied. The work mainly focused on revealing the effect of feedstock concentration on the biogas production, which was seldom reported previously for the current system. The optimal fermentation conditions were found as follows: pH = 7, T = 39 ± 1 °C, sludge retention time is 20 days, sludge with total solids (TS) are 1, 2, 3.5, 5, 10, and 13% in weight. Daily gas yields were measured, and biogas compositions were analyzed by gas chromatograph. Under such conditions, the optimum input TS was 10 wt%, and the biogas yield and volume gas productivity were 280.2 mL/g·TS and 1188.4 mL L^-1·d^-1, respectively. The proportions of CH₄ and CO₂ in the biogas were 65.1 and 34.2%. The CH₄ yield reached 182.7 mL/g VS (volatile suspended solid), which was higher than previously reported values. The findings of this work have a significant effect on promoting the application of digesting PES by rumen microorganisms and further identified the technical parameter.

Precise pretreatment of lignocellulose: relating substrate modification with subsequent hydrolysis and fermentation to products and by-products

BACKGROUND

Pretreatment is a crucial step for valorization of lignocellulosic biomass into valuable products such as H₂, ethanol, acids, and methane. As pretreatment can change several decisive factors concurrently, it is difficult to predict its effectiveness. Furthermore, the effectiveness of pretreatments is usually assessed by enzymatic digestibility or merely according to the yield of the target fermentation products. The present study proposed the concept of "precise pretreatment," distinguished the major decisive factors of lignocellulosic materials by precise pretreatment, and evaluated the complete profile of all fermentation products and by-products. In brief, hemicellulose and lignin were selectively removed from dewaxed rice straw, and the cellulose was further modified to alter the crystalline allomorphs. The subsequent fermentation performance of the selectively pretreated lignocellulose was assessed using the cellulolytic, ethanologenic, and hydrogenetic Clostridium thermocellum through a holistic characterization of the liquid, solid, and gaseous products and residues.

RESULTS

The transformation of crystalline cellulose forms from I to II and from I_α to I_β improved the production of H₂ and ethanol by 65 and 29%, respectively. At the same time, the hydrolysis efficiency was merely improved by 10%, revealing that the crystalline forms not only influenced the accessibility of cellulose but also affected the metabolic preferences and flux of the system. The fermentation efficiency was independent of the specific surface area and degree of polymerization. Furthermore, the pretreatments resulted in 43-45% of the carbon in the liquid hydrolysates unexplainable by forming ethanol and acetate products. A tandem pretreatment with peracetic acid and alkali improved ethanol production by 45.5%, but also increased the production of non-ethanolic low-value by-products by 136%, resulting in a huge burden on wastewater treatment requirements.

CONCLUSION

Cellulose allomorphs significantly affected fermentation metabolic pathway, except for hydrolysis efficiency. Furthermore, with the increasing effectiveness of the pretreatment for ethanol production, more non-ethanolic low-value by-products or contaminants were produced, intensifying environmental burden. Therefore, the effectiveness of the pretreatment should not only be determined on the basis of energy auditing and inhibitors generated, but should also be assessed in terms of the environmental benefits of the whole integrated system from a holistic view.

Exploratory catalyst screening studies on the liquefaction of model humins from C6 sugars

A catalyst screening study is reported on the liquefaction of humins, the solid byproducts from C6 sugar biorefineries for levulinic acid and 5-hydroxymethylfurfural production.

Enhancing anaerobic digestion of lignocellulosic materials in excess sludge by bioaugmentation and pre-treatment

This study attempted to enhance anaerobic conversion of lignocellulosic materials in excess sludge by bioaugmentation and pretreatment. The results reveal that highly active lignocellulolytic microorganisms (Clostridium stercorarium and Bacteroides cellulosolvens) could be enriched from anaerobic sludge in ordinarily operated anaerobic digester (AD). Inoculating these microorganisms into AD could substantially enhance the degradation of cellulose and hemicellulose. However, this effect of bioaugmentation was shielded for raw excess sludge due to lignin incrustation in native biosolids. For this problem, pretreatments including acid, alkali, thermal and ultrasonic methods were effectively used to deconstruct the lignin incrustation, in which thermal pretreatment was demonstrated to be the most effective one. Then, pretreatment associated with bioaugmentation was successfully used to enhance the energy conversion of lignocellulosic materials, which resulted in the degradation of cellulose, hemicellulose and lignin to 68.8-78.2%, 77.4-89% and 15.4-33.7% respectively and thus increased the CH4 production by 210-246%, compared with ordinary AD.

A biologically inspired variable-pH strategy for enhancing short-chain fatty acids (SCFAs) accumulation in maize straw fermentation

This study investigates the feasibility of varying the pH to enhance the accumulation of short-chain fatty acids (SCFAs) in the in vitro fermentation of maize straw. The corresponding hydrolysis rate and the net SCFA yield increased as inoculum ratio (VSinoculum/VSsubstrate) increased from 0.09 to 0.79. The pH were maintained at 5.3, 5.8, 6.3, 6.8, 7.3, and 7.8, respectively. A neutral pH of approximately 6.8 was optimal for hydrolysis. The net SCFA yield decreased by 34.9% for a pH of less than 5.8, but remained constant at approximately 721±5mg/gvs for a pH between 5.8 and 7.8. In addition, results were obtained for variable and constant pH levels at initial substrate concentrations of 10, 30 and 50g/L. A variable pH increased the net SCFA yield by 23.6%, 29.0%, and 36.6% for concentrations of 10, 30 and 50g/L. Therefore, a variable pH enhanced SCFA accumulation in maize straw fermentation.

Effectiveness of bulking agents for co-composting penicillin mycelial dreg (PMD) and sewage sludge in pilot-scale system

Penicillin mycelial dreg (PMD) has a distinguishing characteristic of the high content of penicillin residue and nutrients. The existing handling of PMD used as feed additive of livestock and poultry is facing a direct challenge of penicillin transportation into environment due to the inadequate absorption through the digestive system. This work aims at examining the feasibility of co-composting of PMD with sewage sludge (SWS) in a pilot-scale system and evaluating the effect of four bulking agents. Seven treatments were co-composted over a 32-day period in 390-L reactors using the same PMD and SWS with different bulking agents, corresponding to the seven formulas (T-1: PMD + SWS + RS; T-2: PMD + SWS + WS; T-3: PMD + SWS + RS + SD; T-4: PMD + SWS + WS + SD; T-5: PMD + SWS + SD; T-6: PMD + SWS + RS + WS; control: PMD + SWS). The parameters monitored over this period included temperature, organic matter (OM), TN, NH4(+)-N, NO3(-)-N, pH, EC, penicillin residue, as well as germination index (GI). The results showed that co-composting PMD and SWS with BA is feasible. The highest rate of OM mineralization was observed in T-3, while below 30% for T-2, T-4, and T-5. Furthermore, the SD addition resulted in both the increase in the duration of thermophilic stage and maximum temperature and the decrease in TN losses, particularly in T-3, suggesting that the formula of the T-3 is very suitable option for the co-composting of PMD and SWS.

Dry co-digestion of sewage sludge and rice straw under mesophilic and thermophilic anaerobic conditions

Dry anaerobic digestion of sewage sludge can recover biogas as energy; however, its low C/N ratio limits it as a single substrate in the anaerobic digestion. Rice straw is an abundant agricultural residue in China, which is rich in carbon and can be used as carbon source. In the present study, the performance of dry co-digestion of sewage sludge and rice straw was investigated under mesophilic (35 °C) and thermophilic (55 °C) conditions. The operational factors impacting dry co-digestion of sewage sludge and rice straw such as C/N ratio, moisture content, and initial pH were explored under mesophilic conditions. The results show that low C/N ratios resulted in a higher biogas production rate, but a lower specific biogas yield; low moisture content of 65 % resulted in the instability of the digestion system and a low specific biogas yield. Initial pH ranging 7.0-9.0 did not affect the performance of the anaerobic digestion. The C/N ratio of 26-29:1, moisture content of 70-80 %, and pH 7.0-9.0 resulted in good performance in the dry mesophilic co-digestion of sewage sludge and rice straw. As compared with mesophilic digestion, thermophilic co-digestion of sewage sludge and rice straw significantly enhanced the degradation efficiency of the substrates and the specific biogas yield (p < 0.05) at the conditions of C/N ratio 26:1, moisture content 80 %, and natural initial pH. Although high concentrations of ammonia-nitrogen (NH4-N, 1500 mg/kg wet weight) were formed during thermophilic digestion, there was no obvious inhibition occurred. The results indicated that rice straw can be used as carbon source for the dry co-digestion of sewage sludge under mesophilic and thermophilic conditions.

Comparison of hematite-facilitated anaerobic digestion of acetate and beef extract

The effect of hematite with different specific surface areas (SSA) on anaerobic methanogenic process was investigated in this study using two different carbon sources (acetate or beef extract). Comparative analyses showed that methane generation rate, microbial growth and the starting-up duration were all significantly enhanced by hematite. The reduction rate of high SSA hematite was 2.5-3.5 times greater than that of low SSA hematite. This work indicated that low SSA hematite was superior to high SSA hematite in facilitating methanogenesis which was partially attributed to the lower affinity for the iron-reduction bacteria.

Transcriptomic analysis of lignocellulosic biomass degradation by the anaerobic fungal isolate Orpinomyces sp. strain C1A

BACKGROUND

Anaerobic fungi reside in the rumen and alimentary tract of herbivores where they play an important role in the digestion of ingested plant biomass. The anaerobic fungal isolate Orpinomyces sp. strain C1A is an efficient biomass degrader, capable of simultaneous saccharification and fermentation of the cellulosic and hemicellulosic fractions in multiple types of lignocellulosic biomass. To understand the mechanistic and regulatory basis of biomass deconstruction in anaerobic fungi, we analyzed the transcriptomic profiles of C1A when grown on four different types of lignocellulosic biomass (alfalfa, energy cane, corn stover, and sorghum) versus a soluble sugar monomer (glucose).

RESULTS

A total of 468.2 million reads (70.2 Gb) were generated and assembled into 27,506 distinct transcripts. CAZyme transcripts identified included 385, 246, and 44 transcripts belonging to 44, 13, and 8 different glycoside hydrolases (GH), carbohydrate esterases, and polysaccharide lyases families, respectively. Examination of CAZyme transcriptional patterns indicates that strain C1A constitutively transcribes a high baseline level of CAZyme transcripts on glucose. Although growth on lignocellulosic biomass substrates was associated with a significant increase in transcriptional levels in few GH families, including the highly transcribed GH1 β-glucosidase, GH6 cellobiohydrolase, and GH9 endoglucanase, the transcriptional levels of the majority of CAZyme families and transcripts were not significantly altered in glucose-grown versus lignocellulosic biomass-grown cultures. Further, strain C1A co-transcribes multiple functionally redundant enzymes for cellulose and hemicellulose saccharification that are mechanistically and structurally distinct. Analysis of fungal dockerin domain-containing transcripts strongly suggests that anaerobic fungal cellulosomes represent distinct catalytic units capable of independently attacking and converting intact plant fibers to sugar monomers.

CONCLUSIONS

Collectively, these results demonstrate that strain C1A achieves fast, effective biomass degradation by the simultaneous employment of a wide array of constitutively-transcribed cellulosome-bound and free enzymes with considerable functional overlap. We argue that the utilization of this indiscriminate strategy could be justified by the evolutionary history of anaerobic fungi, as well as their functional role within their natural habitat in the herbivorous gut.

Biodegradation and speciation of roxarsone in an anaerobic granular sludge system and its impacts

Roxarsone (3-nitro-4-hydroxy benzene arsenic acid) is an organoarsenic feed additive and has been widely used in the poultry industry to prevent coccidiosis and improve feed efficiency. The presence of roxarsone and its degradation products results in the instability of the anaerobic methanogenic process. This study investigated the degradation and speciation of roxarsone in an anaerobic granular sludge (AGS) system and the impacts of roxarsone and its degradation products on the structure of AGS. Roxarsone inhibited methane production, and the added roxarsone was rapidly degraded into 3-amino-4-hydroxyphenylarsonic acid (HAPA). After 240 days of incubation, the distribution of arsenic differed between the aqueous solution and the AGS in the assays of 20 and 350mg/L roxarsone. Species analysis indicated that HAPA was completely degraded in all of the assays with roxarsone addition after 240 days of incubation. Species distribution was affected by the phases and the initial concentration of roxarsone added. The concentration of As(III) was higher than that of As(V) in both the aqueous solution and the AGS in all assays with roxarsone addition. The toxicity of roxarsone and its degradation products resulted in changes in the structure and the microorganism species in the AGS.

Direct three-dimensional characterization and multiscale visualization of wheat straw deconstruction by white rot fungus

Microbial degradation of lignocellulose for resource and energy recovery has received increasing interest. Despite its obvious importance, the mechanism behind the biodegradation, especially the changes of morphological structure and surface characteristics, has not been fully understood. Here, we used three-dimensional (3D) characterization and multiscale visualization methods, in combination with chemical compositional analyses, to elucidate the degradation process of wheat straw by a white rot fungus, Phanerochaete chrysosporium. It was found that the fungal attack initiated from stomata. Lignin of the straw decayed in both size and quantity, and heterogeneity in the biodegradation was observed. After treatment with the fungus, the straw surface turned from hydrophobic to hydrophilic, and the adhesion of the straw surface increased in the fungal degradation. The morphology of the straw outer layer became heterogeneous and loose with the formation of many holes with various sizes. The wasp-tunnels-like structure of the collenchyma and parenchyma of the straw as well as the fungal hyphae interspersed inside the straw structure were clearly visualized in the 3D reconstruction structure. This work offers a new insight into the mechanism of lignocellulose biodegradation and demonstrates that multiscale visualization methods could be a useful tool to explore such complex processes.

Advancing lignocellulose bioconversion through direct assessment of enzyme action on insoluble substrates

Microbial utilization of lignocellulose from plant cell walls is integral to carbon cycling on Earth. Correspondingly, secreted enzymes that initiate lignocellulose depolymerization serve a crucial step in the bioconversion of lignocellulosic biomass to fuels and chemicals. Genome and metagenome sequencing efforts that span the past decade reveal the diversity of enzymes that have evolved to transform lignocellulose from wood, herbaceous plants and grasses. Nevertheless, there are relatively few examples where 'omic' technologies have identified novel enzyme activities or combinations thereof that dramatically improve the economics of lignocellulose bioprocessing and utilization. A likely factor contributing to the discrepancy between sequence-based enzyme discovery and enzyme application is the common practice to screen enzyme candidates based on activity measurements using soluble model compounds. In this context, the development and application of imaging, physicochemical, and spectromicroscopic techniques that allow direct assessment of enzyme action on relevant lignocellulosic substrates is reviewed.

Anaerobic fermentation of biogas liquid pretreated maize straw by rumen microorganisms in vitro

This study intended to investigate the effect of pretreatment of maize straw with biogas liquid on followed fermentation by rumen microorganisms in vitro. The multiple effects including treated time, temperature and dosage of biogas liquid in pretreatment on the followed fermentation performance were analyzed by orthogonal array. The optimum conditions of pretreatment were 9days, 25°C and 50% (v/w) dosage of biogas liquid, which were indicated by the corresponding crystallinity index, dry matter digestibility (DMD) and acetate limiting-step concentration were 57.5%, 73.76% and 1756mg/L, respectively. The ordering sequence of the influential factors for pretreatment was treated time > temperature > dosage of biogas liquid. The results of fermentation showed that the maize straw pretreated by biogas liquid was an efficient and economic pretreatment method of maize straw.

Application of rumen microorganisms for anaerobic bioconversion of lignocellulosic biomass

Rumen in the mammalian animals is a natural cellulose-degrading system and the microorganisms inside have been found to be able to effectively digest lignocellulosic biomass. Furthermore, methane or volatile fatty acids, which could be further converted to other biofuels, are the two major products in such a system. This paper offers an overview of recent development in the application of rumen microorganisms for lignocellulosic biomass conversion. Application of recent molecular tools in the analysis of rumen microbial community, progress in the development of artificial rumen reactors, the latest research results about characterizing rumen-dominated anaerobic digestion process and energy products are summarized. Also, the potential application of such a rumen-dominated process is discussed.

Effect of different steam explosion conditions on methane potential and enzymatic saccharification of birch

Birch (Betula pubescens) was steam exploded at 13 different conditions with temperatures ranging from 170 to 230 °C and residence times ranging from 5 to 15 min. Increasing severity in the pretreatment led to degradation of xylan and formation of pseudo-lignin. The effect of the pretreatments was evaluated by running enzymatic saccharification and anaerobic digestion followed by analysis of sugar and methane yields, respectively. Enzymatically released glucose increased with pretreatment severity up to 220 °C for 10 min and levels of solubilized glucose reached 97% of the theoretical maximum. The highest methane yield (369 mL gVS(-1)) was obtained at a severity factor of 4.5 and this yield was 1.8 times higher than the yield from untreated birch. Enzymatic glucose yields and methane yields were generally correlated. The results indicate that steam-exploded birch can be effectively converted to either glucose or methane.

Effects of pH on the hydrolysis of lignocellulosic wastes and volatile fatty acids accumulation: the contribution of biotic and abiotic factors

In this study, a combination of micro-scale structure and extracellular enzyme activity (EEA) analysis was successfully used to explore the effect of pH (from 6.0 to 12.0) on wetland plant litter (WPL) hydrolysis and VFAs accumulation. During 30days of fermentation, the maximum VFAs production and abiotic release of carbohydrate, combined with the minimum EEAs were observed at pH 12.0, suggesting that abiotic factors were most important for hydrolysis and VFAs accumulation at pH 12.0. As the pH decreased, the factors most important to carbohydrate hydrolysis shifted from abiotic factors to biotic factors with the maximum bio-release of carbohydrate occurring at pH 9.0. Further investigation showed that pH 9.0 could significantly enhance the bio-release of carbohydrate through the increase in the mesoporous surface area, surface cellulose accessibility and cellulase activity. Alkaline fermentation at ambient temperature can be considered as a sustainable technology for VFAs recovery and WPL management.

Anaerobic degradation of microcrystalline cellulose: kinetics and micro-scale structure evolution

The degradation kinetics and micro-scale structure change of microcrystalline cellulose during anaerobic biodegradation were investigated. A modified Logistic model was established to properly describe the kinetics, which showed good fitness and wide applicability for cellulose degradation. A maximum degradation rate of 0.14 g L(-1) h(-1) was achieved after cultivating for 51.5 h. This result was in good agreement with the scanning electron microscope and X-ray diffraction analysis. Channels of 400-500 nm size started to occur on the crystalline surface of cellulose at around the inflexion time. Accordingly, the crystallinity significantly decreased at this point, indicating a degradation of the crystalline structure zones by anaerobic bacteria. This study offers direct morphological evidence and quantitative analysis of the biodegradation process of cellulose, and is beneficial to a better understanding of the cellulose degradation mechanism.

Multiple fluorescence labeling and two dimensional FTIR-13C NMR heterospectral correlation spectroscopy to characterize extracellular polymeric substances in biofilms produced during composting

Knowledge on the structure and function of extracellular polymeric substances (EPS) in biofilms is essential for understanding biodegradation processes. Herein, a novel method based on multiple fluorescence labeling and two-dimensional (2D) FTIR-(13)C NMR heterospectral correlation spectroscopy was developed to gain insight on the composition, architecture, and function of EPS in biofilms during composting. Compared to other environmental biofilms, biofilms in the thermophilic (>55 °C) and cooling (mature) stage of composting have distinct characteristics. The results of multiple fluorescence labeling demonstrated that biofilms were distributed in clusters during the thermophilic stage (day 14), and dead cells were detected. In the mature stage (day 26), the biofilm formed a continuous layer with a thickness of approximately 20-100 μm around the compost, and recolonization of cells at the surface of the compost was easily observed. Through 2D FTIR-(13)C NMR correlation heterospectral spectroscopy, the following trend in the ease of the degradation of organic compounds was observed: heteropolysaccharides > cellulose > amide I in proteins. And proteins and cellulose showed significantly more degradation than heteropolysaccharides. In summary, the combination of multiple fluorescence labeling and 2D correlation spectroscopy is a promising approach for the characterization of EPS in biofilms.

Efficient bioconversion of rice straw to ethanol with TiO2/UV pretreatment

Rice straw is a lignocellulosic biomass that constitutes a renewable organic substance and alternative source of energy; however, its structure confounds the liberation of monosaccharides. Pretreating rice straw using a TiO(2)/UV system facilitated its hydrolysis with Accellerase 1000(™), suggesting that hydroxyl radicals (OH·) from the TiO(2)/UV system could degrade lignin and carbohydrates. TiO(2)/UV pretreatment was an essential step for conversion of hemicellulose to xylose; optimal conditions for this conversion were a TiO(2) concentration of 0.1% (w/v) and an irradiation time of 2 h with a UV-C lamp at 254 nm. After enzymatic hydrolysis, the sugar yields from rice straw pretreated with these parameters were 59.8 ± 0.7% of the theoretical for glucose (339 ± 13 mg/g rice straw) and 50.3 ± 2.8% for xylose (64 ± 3 mg/g rice straw). The fermentation of enzymatic hydrolysates containing 10.5 g glucose/L and 3.2 g xylose/L with Pichia stipitis produced 3.9 g ethanol/L with a corresponding yield of 0.39 g/g rice straw. The maximum possible ethanol conversion rate is 76.47%. TiO(2)/UV pretreatment can be performed at room temperature and atmospheric pressure and demonstrates potential in large-scale production of fermentable sugars.

Direct electricity recovery from Canna indica by an air-cathode microbial fuel cell inoculated with rumen microorganisms

Aquatic plants are widely used for phytoremediation, and effective disposal methods should be pursued for their utilization and to avoid further environmental pollution problems. This study demonstrated that, using an air-cathode microbial fuel cell (MFC) inoculated with rumen microorganisms, electricity could be directly produced with a maximum power density of 0.405 W/m(3) from Canna indica (canna), a lignocellulosic aquatic plant rich in cellulose, hemicellulose, and lignin, without pretreatment. The mechanisms of the Canna indica degradation in the MFC were elucidated through analyzing the changes of canna structure and intermediates, that is, soluble sugars and volatile fatty acids (VFAs), in the electricity generation process. The results showed that lignin was partially removed and more cellulose became exposed on the sample surface during the electricity generation in the MFC. The electron transfer in this MFC was mainly completed through electron shuttling via self-produced mediators. This work presents an attempt to understand how complex substrates like aquatic plants are decomposed in an MFC during electricity generation. It might, hopefully, provide a promising way to utilize lignocellulosic biomass for energy generation.

Application of AFM in microbiology: a review

Atomic force microscopy (AFM) is a powerful tool for microbiological investigation. This versatile technique cannot only image cellular surfaces at high resolution, but also measure many forms of fundamental interactions over scales ranging from molecules to cells. In this work, we review the recent development of AFM applications in the microbial area. We discuss several approaches for using AFM scanning images to investigate morphological characteristics of microbes and the use of force-distance curves to investigate interaction of microbial samples at the nanometer and cellular levels. Complementary techniques used in combination with AFM for study of microbes are also discussed.

Enhanced enzymatic saccharification of rice straw by microwave pretreatment

In this study, Box-Behnken design and response surface methodology were employed to plan experiments and optimize the microwave pretreatment of rice straw. Experimental results show that microwave intensity (MI), irradiation time (IT) and substrate concentration (SC) were main factors governing the enzymatic saccharification of rice straw. The maximal efficiencies of cellulose, hemicellulose and total saccharification were respectively increased by 30.6%, 43.3% and 30.3% under the optimal conditions of MI 680 W, IT 24 min and SC 75 g/L. The chemical composition analysis of straw further confirmed that microwave pretreatment could disrupt the silicified waxy surface, break down the lignin-hemicellulose complex and partially remove silicon and lignin.