Thermophilic microbial deconstruction and conversion of natural and transgenic lignocellulose

Lignin: An Adaptable Biodegradable Polymer Used in Different Formulation Processes

INTRODUCTION

LIG is a biopolymer found in vascular plant cell walls that is created by networks of hydroxylated and methoxylated phenylpropane that are randomly crosslinked. Plant cell walls contain LIG, a biopolymer with significant potential for usage in modern industrial and pharmaceutical applications. It is a renewable raw resource. The plant is mechanically protected by this substance, which may increase its durability. Because it has antibacterial and antioxidant qualities, LIG also shields plants from biological and chemical challenges from the outside world. Researchers have done a great deal of work to create new materials and substances based on LIG. Numerous applications, including those involving antibacterial agents, antioxidant additives, UV protection agents, hydrogel-forming molecules, nanoparticles, and solid dosage forms, have been made with this biopolymer.

METHODS

For this review, a consistent literature screening using the Pubmed database from 2019-2024 has been performed.

RESULTS

The results showed that there is an increase in interest in lignin as an adaptable biomolecule. The most recent studies are focused on the biosynthesis and antimicrobial properties of lignin-derived molecules. Also, the use of lignin in conjunction with nanostructures is actively explored.

CONCLUSIONS

Overall, lignin is a versatile molecule with multiple uses in industry and medical science.

Beyond low lignin: Identifying the primary barrier to plant biomass conversion by fermentative bacteria

Renewable alternatives for nonelectrifiable fossil-derived chemicals are needed and plant matter, the most abundant biomass on Earth, provide an ideal feedstock. However, the heterogeneous polymeric composition of lignocellulose makes conversion difficult. Lignin presents a formidable barrier to fermentation of nonpretreated biomass. Extensive chemical and enzymatic treatments can liberate fermentable carbohydrates from plant biomass, but microbial routes offer many advantages, including concomitant conversion to industrial chemicals. Here, testing of lignin content of nonpretreated biomass using the cellulolytic thermophilic bacterium, Anaerocellum bescii, revealed that the primary microbial degradation barrier relates to methoxy substitutions in lignin. This contrasts with optimal lignin composition for chemical pretreatment that favors high S/G ratio and low H lignin. Genetically modified poplar trees with diverse lignin compositions confirm these findings. In addition, poplar trees with low methoxy content achieve industrially relevant levels of microbial solubilization without any pretreatments and with no impact on tree fitness in greenhouse.

Integrating multi-platform assembly to recover MAGs from hot spring biofilms: insights into microbial diversity, biofilm formation, and carbohydrate degradation

BACKGROUND

Hot spring biofilms provide a window into the survival strategies of microbial communities in extreme environments and offer potential for biotechnological applications. This study focused on green and brown biofilms thriving on submerged plant litter within the Sungai Klah hot spring in Malaysia, characterised by temperatures of 58-74 °C. Using Illumina shotgun metagenomics and Nanopore ligation sequencing, we investigated the microbial diversity and functional potential of metagenome-assembled genomes (MAGs) with specific focus on biofilm formation, heat stress response, and carbohydrate catabolism.

RESULTS

Leveraging the power of both Illumina short-reads and Nanopore long-reads, we employed an Illumina-Nanopore hybrid assembly approach to construct MAGs with enhanced quality. The dereplication process, facilitated by the dRep tool, validated the efficiency of the hybrid assembly, yielding MAGs that reflected the intricate microbial diversity of these extreme ecosystems. The comprehensive analysis of these MAGs uncovered intriguing insights into the survival strategies of thermophilic taxa in the hot spring biofilms. Moreover, we examined the plant litter degradation potential within the biofilms, shedding light on the participation of diverse microbial taxa in the breakdown of starch, cellulose, and hemicellulose. We highlight that Chloroflexota and Armatimonadota MAGs exhibited a wide array of glycosyl hydrolases targeting various carbohydrate substrates, underscoring their metabolic versatility in utilisation of carbohydrates at elevated temperatures.

CONCLUSIONS

This study advances understanding of microbial ecology on plant litter under elevated temperature by revealing the functional adaptation of MAGs from hot spring biofilms. In addition, our findings highlight potential for biotechnology application through identification of thermophilic lignocellulose-degrading enzymes. By demonstrating the efficiency of hybrid assembly utilising Illumina-Nanopore reads, we highlight the value of combining multiple sequencing methods for a more thorough exploration of complex microbial communities.

Multifaced application of AFEX-pretreated biomass in producing second-generation biofuels, ruminant animal feed, and value-added bioproducts

Lignocellulosic biomass holds a crucial position in the prospective bio-based economy, serving as a sustainable and renewable source for a variety of bio-based products. These products play a vital role in displacing fossil fuels and contributing to environmental well-being. However, the inherent recalcitrance of biomass poses a significant obstacle to the efficient access of sugar polymers. Consequently, the bioconversion of lignocellulosic biomass into fermentable sugars remains a prominent challenge in biorefinery processes to produce biofuels and biochemicals. In addressing these challenges, extensive efforts have been dedicated to mitigating biomass recalcitrance through diverse pretreatment methods. One noteworthy process is Ammonia Fiber Expansion (AFEX) pretreatment, characterized by its dry-to-dry nature and minimal water usage. The volatile ammonia, acting as a catalyst in the process, is recyclable. AFEX contributes to cleaning biomass ester linkages and facilitating the opening of cell wall structures, enhancing enzyme accessibility and leading to a fivefold increase in sugar conversion compared to untreated biomass. Over the last decade, AFEX has demonstrated substantial success in augmenting the efficiency of biomass conversion processes. This success has unlocked the potential for sustainable and economically viable biorefineries. This paper offers a comprehensive review of studies focusing on the utilization of AFEX-pretreated biomass in the production of second-generation biofuels, ruminant feed, and additional value-added bioproducts like enzymes, lipids, proteins, and mushrooms. It delves into the details of the AFEX pretreatment process at both laboratory and pilot scales, elucidates the mechanism of action, and underscores the role of AFEX in the biorefinery for developing biofuels and bioproducts, and nutritious ruminant animal feed production. While highlighting the strides made, the paper also addresses current challenges in the commercialization of AFEX pretreatment within biorefineries. Furthermore, it outlines critical considerations that must be addressed to overcome these challenges, ensuring the continued progress and widespread adoption of AFEX in advancing sustainable and economically viable bio-based industries.

Sugar transport in thermophiles: Bridging lignocellulose deconstruction and bioconversion

Biomass degrading thermophiles play an indispensable role in building lignocellulose-based supply chains. They operate at high temperatures to improve process efficiencies and minimize mesophilic contamination, can overcome lignocellulose recalcitrance through their native carbohydrate-active enzyme (CAZyme) inventory, and can utilize a wide range of sugar substrates. However, sugar transport in thermophiles is poorly understood and investigated, as compared to enzymatic lignocellulose deconstruction and metabolic conversion of sugars to value-added chemicals. Here, we review the general modes of sugar transport in thermophilic bacteria and archaea, covering the structural, molecular, and biophysical basis of their high-affinity sugar uptake. We also discuss recent genetic studies on sugar transporter function. With this understanding of sugar transport, we discuss strategies for how sugar transport can be engineered in thermophiles, with the potential to enhance the conversion of lignocellulosic biomass into renewable products.

ONE-SENTENCE SUMMARY

Sugar transport is the understudied link between extracellular biomass deconstruction and intracellular sugar metabolism in thermophilic lignocellulose bioprocessing.

Complete Genome Sequences of Two Thermophilic Indigenous Bacteria Isolated from Wheat Straw, Thermoclostridium stercorarium subsp. Strain RKWS1 and Thermoanaerobacter sp. Strain RKWS2

Reported here are complete genome sequences for two anaerobic, thermophilic bacteria isolated from wheat straw, i.e., the (hemi)cellulolytic Thermoclostridium stercorarium subspecies strain RKWS1 (3,029,933 bp) and the hemicellulolytic Thermoanaerobacter species strain RKWS2 (2,827,640 bp). Discovery of indigenous thermophiles in plant biomass suggests that high-temperature microorganisms are more ubiquitous than previously thought.

Facultative anaerobic conversion of lignocellulose biomass to new bioemulsifier by thermophilic Geobacillus thermodenitrificans NG80-2

Heavy oil has hindered crude oil exploitation and pollution remediation due to its high density and viscosity. Bioemulsifiers efficiently facilitate the formation and stabilization of oil-in-water emulsions in low concentrations thus eliminating the above bottleneck. Despite their potential benefits, various obstacles had still impeded the practical applications of bioemulsifiers, including high purification costs and poor adaptability to extreme environments such as high temperature and oxygen deficiency. Herein, thermophilic facultative anaerobic Geobacillus thermodenitrificans NG80-2 was proved capable of emulsifying heavy oils and reducing their viscosity. An exocelluar bioemulsifier could be produced by NG80-2 using low-cost lignocellulose components as carbon sources even under anaerobic condition. The purified bioemulsifier was proved to be polysaccharide-protein complexes, and both components contributed to its emulsifying capability. In addition, it displayed excellent stress tolerance over wide ranges of temperatures, salinities, and pHs. Meanwhile, the bioemulsifier significantly improved oil recovery and degradation efficiency. An eps gene cluster for polysaccharide biosynthesis and genes for the covalently bonded proteins was further certificated. Therefore, the bioemulsifier produced by G. thermodenitrificans NG80-2 has immense potential for applications in bioremediation and EOR, and its biosynthesis pathway revealed here provides a theoretical basis for increasing bioemulsifier output.

Fermentative conversion of unpretreated plant biomass: A thermophilic threshold for indigenous microbial growth

Naturally occurring, microbial contaminants were found in plant biomasses from common bioenergy crops and agricultural wastes. Unexpectedly, indigenous thermophilic microbes were abundant, raising the question of whether they impact thermophilic consolidated bioprocessing fermentations that convert biomass directly into useful bioproducts. Candidate microbial platforms for biomass conversion, Acetivibrio thermocellus (basionym Clostridium thermocellum; Topt 60 °C) and Caldicellulosiruptor bescii (Topt 78 °C), each degraded a wide variety of plant biomasses, but only A. thermocellus was significantly affected by the presence of indigenous microbial populations harbored by the biomass. Indigenous microbial growth was eliminated at ≥75 °C, conditions where C. bescii thrives, but where A. thermocellus cannot survive. Therefore, 75 °C is the thermophilic threshold to avoid sterilizing pre-treatments on the biomass that prevents native microbes from competing with engineered microbes and forming undesirable by-products. Thermophiles that naturally grow at and above 75 °C offer specific advantages as platform microorganisms for biomass conversion into fuels and chemicals.

Role of Nrf2 Pathway Activation in Neurological Disorder: A Brief Review

Oxidative stress plays a crucial role in the emergence of numerous neurodegenerative diseases, with protein accumulation and mitochondrial damage, which result in neurological disorders. To minimize oxidative stress, several defensive mechanisms protect nerve cells by releasing antioxidants such as nuclear erythroid factor2 (Nrf2)-Kelch-like ECH-associated protein1 (Keap1) signaling pathway activation has been proved to be a prospective treatment to reduce oxidative stress and neuroinflammation for protection of neurons in a variety of neurological disorders. In this review, we focus beneficial role of Nrf2 in Alzheimer’s and Parkinson’s diseases. Nrf2 is proved to be a master regulator of antioxidants by releasing over 250 cytoprotective genes aimed at oxidative stress and neuroinflammation. In animal studies Nrf2 activation is proved to improve autophagy, mitochondrial biogenesis, and Suppression of inflammatory cytokinin which protects neuronal cells and inhibit progressive neurodegeneration.

Plant biomass fermentation by the extreme thermophile Caldicellulosiruptor bescii for co-production of green hydrogen and acetone: Technoeconomic analysis

A variety of chemical and biological processes have been proposed for conversion of sustainable low-cost feedstocks into industrial products. Here, a biorefinery concept is formulated, modeled, and analyzed in which a naturally (hemi)cellulolytic and extremely thermophilic bacterium, Caldicellulosiruptor bescii, is metabolically engineered to convert the carbohydrate content of lignocellulosic biomasses (i.e., soybean hulls, transgenic poplar) into green hydrogen and acetone. Experimental validation of C. bescii fermentative performance demonstrated 82% carbohydrate solubilization of soybean hulls and 55% for transgenic poplar. A detailed technical design, including equipment specifications, provides the basis for an economic analysis that establishes metabolic engineering targets. This robust industrial process leveraging metabolically engineered C. bescii yields 206 kg acetone and 25 kg H2 per metric ton of soybean hull, or 174 kg acetone and 21 kg H2 per metric ton transgenic poplar. Beyond this specific case, the model demonstrates industrial feasibility and economic advantages of thermophilic fermentation.

Biorefinery Gets Hot: Thermophilic Enzymes and Microorganisms for Second-Generation Bioethanol Production

To mitigate the current global energy and the environmental crisis, biofuels such as bioethanol have progressively gained attention from both scientific and industrial perspectives. However, at present, commercialized bioethanol is mainly derived from edible crops, thus raising serious concerns given its competition with feed production. For this reason, lignocellulosic biomasses (LCBs) have been recognized as important alternatives for bioethanol production. Because LCBs supply is sustainable, abundant, widespread, and cheap, LCBs-derived bioethanol currently represents one of the most viable solutions to meet the global demand for liquid fuel. However, the cost-effective conversion of LCBs into ethanol remains a challenge and its implementation has been hampered by several bottlenecks that must still be tackled. Among other factors related to the challenging and variable nature of LCBs, we highlight: (i) energy-demanding pretreatments, (ii) expensive hydrolytic enzyme blends, and (iii) the need for microorganisms that can ferment mixed sugars. In this regard, thermophiles represent valuable tools to overcome some of these limitations. Thus, the aim of this review is to provide an overview of the state-of-the-art technologies involved, such as the use of thermophilic enzymes and microorganisms in industrial-relevant conditions, and to propose possible means to implement thermophiles into second-generation ethanol biorefineries that are already in operation.