Utilization of Lignocellulosic Biomass via Novel Sustainable Process

Catabolism of β-5 linked aromatics by Novosphingobium aromaticivorans

Aromatic compounds are an important source of commodity chemicals traditionally produced from fossil fuels. Aromatics derived from plant lignin can potentially be converted into commodity chemicals through depolymerization followed by microbial funneling of monomers and low molecular weight oligomers. This study investigates the catabolism of the β-5 linked aromatic dimer dehydrodiconiferyl alcohol (DC-A) by the bacterium Novosphingobium aromaticivorans. We used genome-wide screens to identify candidate genes involved in DC-A catabolism. Subsequent in vivo and in vitro analyses of these candidate genes elucidated a catabolic pathway composed of four required gene products and several partially redundant dehydrogenases that convert DC-A to aromatic monomers that can be funneled into the central aromatic metabolic pathway of N. aromaticivorans. Specifically, a newly identified γ-formaldehyde lyase, PcfL, opens the phenylcoumaran ring to form a stilbene and formaldehyde. A lignostilbene dioxygenase, LsdD, then cleaves the stilbene to generate the aromatic monomers vanillin and 5-formylferulate (5-FF). We also showed that the aldehyde dehydrogenase FerD oxidizes 5-FF before it is decarboxylated by LigW, yielding ferulic acid. We found that some enzymes involved in the β-5 catabolism pathway can act on multiple substrates and that some steps in the pathway can be mediated by multiple enzymes, providing new insights into the robust flexibility of aromatic catabolism in N. aromaticivorans. A comparative genomic analysis predicted that the newly discovered β-5 aromatic catabolic pathway is common within the order Sphingomonadales.

IMPORTANCE

In the transition to a circular bioeconomy, the plant polymer lignin holds promise as a renewable source of industrially important aromatic chemicals. However, since lignin contains aromatic subunits joined by various chemical linkages, producing single chemical products from this polymer can be challenging. One strategy to overcome this challenge is using microbes to funnel a mixture of lignin-derived aromatics into target chemical products. This approach requires strategies to cleave the major inter-unit linkages of lignin to release monomers for funneling into valuable products. In this study, we report newly discovered aspects of a pathway by which the Novosphingobium aromaticivorans DSM12444 catabolizes aromatics joined by the second most common inter-unit linkage in lignin, the β-5 linkage. This work advances our knowledge of aromatic catabolic pathways, laying the groundwork for future metabolic engineering of this and other microbes for optimized conversion of lignin into products.

Production of carotenoids from aromatics and pretreated lignocellulosic biomass by Novosphingobium aromaticivorans

IMPORTANCE

There is economic and environmental interest in generating commodity chemicals from renewable resources, such as lignocellulosic biomass, that can substitute for chemicals derived from fossil fuels. The bacterium Novosphingobium aromaticivorans is a promising microbial platform for producing commodity chemicals from lignocellulosic biomass because it can produce these from compounds in pretreated lignocellulosic biomass, which many industrial microbial catalysts cannot metabolize. Here, we show that N. aromaticivorans can be engineered to produce several valuable carotenoids. We also show that engineered N. aromaticivorans strains can produce these lipophilic chemicals concurrently with the extracellular commodity chemical 2-pyrone-4,6-dicarboxylic acid when grown in a complex liquor obtained from alkaline pretreated lignocellulosic biomass. Concurrent microbial production of valuable intra- and extracellular products can increase the economic value generated from the conversion of lignocellulosic biomass-derived compounds into commodity chemicals and facilitate the separation of water- and membrane-soluble products.

Optimization of biogas production from straw wastes by different pretreatments: Progress, challenges, and prospects

Lignocellulosic biomass (LCB) presents a promising feedstock for carbon management due to enormous potential for achieving carbon neutrality and delivering substantial environmental and economic benefit. Bioenergy derived from LCB accounts for about 10.3 % of the global total energy supply. The generation of bioenergy through anaerobic digestion (AD) in combination with carbon capture and storage, particularly for methane production, provides a cost-effective solution to mitigate greenhouse gas emissions, while concurrently facilitating bioenergy production and the recovery of high-value products during LCB conversion. However, the inherent recalcitrant polymer crystal structure of lignocellulose impedes the accessibility of anaerobic bacteria, necessitating lignocellulosic residue pretreatment before AD or microbial chain elongation. This paper seeks to explore recent advances in pretreatment methods for LCB biogas production, including pulsed electric field (PEF), electron beam irradiation (EBI), freezing-thawing pretreatment, microaerobic pretreatment, and nanomaterials-based pretreatment, and provide a comprehensive overview of the performance, benefits, and drawbacks of the traditional and improved treatment methods. In particular, physical-chemical pretreatment emerges as a flexible and effective option for methane production from straw wastes. The burgeoning field of nanomaterials has provoked progress in the development of artificial enzyme mimetics and enzyme immobilization techniques, compensating for the intrinsic defect of natural enzyme. However, various complex factors, such as economic effectiveness, environmental impact, and operational feasibility, influence the implementation of LCB pretreatment processes. Techno-economic analysis (TEA), life cycle assessment (LCA), and artificial intelligence technologies provide efficient means for evaluating and selecting pretreatment methods. This paper addresses current issues and development priorities for the achievement of the appropriate and sustainable utilization of LCB in light of evolving economic and environmentally friendly social development demands, thereby providing theoretical basis and technical guidance for improving LCB biogas production of AD systems.

Deriving high value products from depolymerized lignin oil, aided by (bio)catalytic funneling strategies

Lignin holds tremendous and versatile possibilities to produce value-added chemicals and high performing polymeric materials. Over the years, different cutting-edge lignin depolymerization methodologies have been developed, mainly focusing on achieving excellent yields of mono-phenolic products, some even approaching the theoretical maximum. However, due to lignin's inherent heterogeneity and recalcitrance, its depolymerization leads to relatively complex product streams, also containing dimers, and higher molecular weight fragments in substantial quantities. The subsequent chemo-catalytic valorization of these higher molecular weight streams, containing difficult-to-break, mainly C-C covalent bonds, is tremendously challenging, and has consequently received much less attention. In this minireview, we present an overview of recent advances on the development of sustainable biorefinery strategies aimed at the production of well-defined chemicals and polymeric materials, the prime focus being on depolymerized lignin oils, containing high molecular weight fractions. The key central unit operation to achieve this is (bio)catalytic funneling, which holds great potential to overcome separation and purification challenges.

High-level production of 2-pyrone-4,6-dicarboxylic acid from vanillic acid as a lignin-related aromatic compound by metabolically engineered fermentation to realize industrial valorization processes of lignin

2-Pyrone-4,6-dicarboxylic acid (PDC) is a valuable building block molecule produced from lignin-derived aromatic compounds by biological funneling. This study aimed to design a fermentation process for producing PDC from vanillic acid, which could be applied at an industrial production. Metabolomic analysis revealed that a high primary metabolic activity within cells was required to improve the production efficiency. Moreover, a medium with ammonium salts and no alkali metals was advantageous because it suppressed the formation of PDC-metal complexes. Resulting optimized process yielded the highest PDC titer and productivity ever reported: 99.9 g/L and 1.69 g/L/h, respectively. Per batch, 190 g of PDC was produced per liter of initial culture media, and the final liquid volume was 1.9 L. This study demonstrates the design of fermentation processes for the advanced industrial utilization of lignin by biological funneling.

Integrated process development for grass biomass utilization through enzymatic saccharification and upgrading hydroxycinnamic acids via microbial funneling

In grass biomass, hydroxycinnamic acids (HCAs) play crucial roles in the crosslinking of lignin and polysaccharides and can be easily extracted by mild alkaline pretreatment, albeit heterogeneously. Here, HCAs were extracted from bamboo and rice straw as model grass biomass with different HCAs composition, and microbial funneling was then conducted to produce 2-pyrone-4,6-dicarboxylic acid (PDC) and (4S)-3-carboxymuconolactone (4S-3CML), promising building blocks for bio-based polymers, respectively. Pseudomonas putida PpY1100 engineered for efficient microbial funneling completely converted HCAs to PDC and 4S-3CML with high titers of 3.9-9.3 g/L and molar yields of 92-99%, respectively. The enzymatic saccharification efficiencies of lignocellulose after HCAs extraction were 29.5% in bamboo and 73.8% in rice straw, which are 8.9 and 6.8 times higher than in alkaline-untreated media, respectively. These results provide a green-like process for total valorization of grass biomass through enzymatic saccharification integrated with upgrading heterogeneous HCAs to a valuable single chemical via microbial funneling.

Green and Low-Cost Natural Lignocellulosic Biomass-Based Carbon Fibers—Processing, Properties, and Applications in Sports Equipment: A Review

At present, high-performance carbon fibers (CFs) are mainly produced from petroleum-based materials. However, the high costs and environmental problems of the production process prompted the development of new precursors from natural biopolymers. This review focuses on the latest research on the conversion of natural lignocellulosic biomass into precursor fibers and CFs. The influence of the properties, advantages, separation, and extraction of lignin and cellulose (the most abundant natural biopolymers), as well as the spinning process on the final CF performance are detailed. Recent strategies to further improve the quality of such CFs are discussed. The importance and application of CFs in sports equipment manufacturing are briefly summarized. While the large-scale production of CFs from natural lignocellulosic biomass and their applications in sports equipment have not yet been realized, CFs still provide a promising market prospect as green and low-cost materials. Further research is needed to ensure the market entry of lignocellulosic biomass-based CFs.

Intensified wheat husk conversion employing energy-efficient hybrid electromagnetic radiations for production of fermentable sugar: process optimization and life cycle assessment

This article reports an energy-efficient green pathway for the sustainable conversion of an abundant agro-residue viz. wheat husk (WH) into fermentable sugar (FS). The intensification effects of tungsten-halogen (TH) (150 W) and ultraviolet (UV) (100 W) irradiations on the pretreatment and subsequent hydrolysis of WH have been experimented with and optimized by Taguchi Orthogonal Design Array (TODA). In this study, two commercial catalysts, viz. Amberlyst-15 (A15) and nano-anataseTiO₂ (NAT) have been used in varying concentrations for the WH conversion process in a novel TH-UV radiated rotating reactor (THUVRR). At optimized peracetic acid pretreatment conditions [90 °C reaction temperature; 1: 2.5 w/w of WH: H₂O₂; 1: 5 w/w of WH: CH₃COOH (1 M); 2h of reaction time] maximum 20.2 wt. % FS yield and 15 wt. % isolated lignin (purity 97.6 %) were obtained. Subsequently, the pretreated WH (PWH) was hydrolyzed at optimized conditions [(70⁰C reaction temperature; 7.5wt. % catalyst concentration (1:1 w/w A15: NAT); 1: 30 w/w of WH: water; 30 min reaction time)] in THUVRR to render maximum yield of FS (36.9g/ L) (67.4 wt. %), which was significantly greater than that obtained (20.2g/ L) (38.42 wt. %) employing a conventional thermal reactor (CTR). Besides, the energy consumption was 70% more in CTR (500 W) in comparison with THUVRR (150 W); thus, demonstrating markedly superior energy-efficiency vis-à-vis appreciable improvement in FS yield in THUVRR over CTR. Overall sustainability of the process analyzed by LCA proved the approach to be energy-saving and environmentally benign and is expected to be applicable to similar lignocellulosic agro-wastes.

In-planta production of the biodegradable polyester precursor 2-pyrone-4,6-dicarboxylic acid (PDC): Stacking reduced biomass recalcitrance with value-added co-product

2-Pyrone-4,6-dicarboxylic acid (PDC), a chemically stable intermediate that naturally occurs during microbial degradation of lignin by bacteria, represents a promising building block for diverse biomaterials and polyesters such as biodegradable plastics. The lack of a chemical synthesis method has hindered large-scale utilization of PDC and metabolic engineering approaches for its biosynthesis have recently emerged. In this study, we demonstrate a strategy for the production of PDC via manipulation of the shikimate pathway using plants as green factories. In tobacco leaves, we first showed that transient expression of bacterial feedback-resistant 3-deoxy-D-arabinoheptulosonate 7-phosphate synthase (AroG) and 3-dehydroshikimate dehydratase (QsuB) produced high titers of protocatechuate (PCA), which was in turn efficiently converted into PDC upon co-expression of PCA 4,5-dioxygenase (PmdAB) and 4-carboxy-2-hydroxymuconate-6-semialdehyde dehydrogenase (PmdC) derived from Comamonas testosteroni. We validated that stable expression of AroG in Arabidopsis in a genetic background containing the QsuB gene enhanced PCA content in plant biomass, presumably via an increase of the carbon flux through the shikimate pathway. Further, introducing AroG and the PDC biosynthetic genes (PmdA, PmdB, and PmdC) into the Arabidopsis QsuB background, or introducing the five genes (AroG, QsuB, PmdA, PmdB, and PmdC) stacked on a single construct into wild-type plants, resulted in PDC titers of ~1% and ~3% dry weight in plant biomass, respectively. Consistent with previous studies of plants expressing QsuB, all PDC producing lines showed strong reduction in lignin content in stems. This low lignin trait was accompanied with improvements of biomass saccharification efficiency due to reduced cell wall recalcitrance to enzymatic degradation. Importantly, most transgenic lines showed no reduction in biomass yields. Therefore, we conclude that engineering plants with the proposed de-novo PDC pathway provides an avenue to enrich biomass with a value-added co-product while simultaneously improving biomass quality for the supply of fermentable sugars. Implementing this strategy into bioenergy crops has the potential to support existing microbial fermentation approaches that exploit lignocellulosic biomass feedstocks for PDC production.

Redundancy in aromatic O-demethylation and ring opening reactions in Novosphingobium aromaticivorans and their impact in the metabolism of plant derived phenolics

Lignin is a plant heteropolymer composed of phenolic subunits. Because of its heterogeneity and recalcitrance, the development of efficient methods for its valorization still remains an open challenge. One approach to utilize lignin is its chemical deconstruction into mixtures of monomeric phenolic compounds followed by biological funneling into a single product. Novosphingobium aromaticivorans DSM12444 has been previously engineered to produce 2-pyrone-4,6-dicarboxylic acid (PDC) from depolymerized lignin by simultaneously metabolizing multiple aromatics through convergent routes involving the intermediates 3-methoxygallic acid (3-MGA) and protocatechuic acid (PCA). We investigated enzymes predicted to be responsible for O-demethylation and oxidative aromatic ring opening, two critical reactions involved in the metabolism of phenolics compounds by N. aromaticivorans The results showed the involvement of DesA in O-demethylation of syringic and vanillic acids, LigM in O-demethylation of vanillic acid and 3-MGA, and a new O-demethylase, DmtS, in the conversion of 3-MGA into gallic acid (GA). In addition, we found that LigAB was the main aromatic ring opening dioxygenase involved in 3-MGA, PCA, and GA metabolism, and that a previously uncharacterized dioxygenase, LigAB2, had high activity with GA. Our results indicate a metabolic route not previously identified in N. aromaticivorans that involves O-demethylation of 3-MGA to GA. We predict this pathway channels ∼15% of the carbon flow from syringic acid, with the rest following ring opening of 3-MGA. The new knowledge obtained in this study allowed for the creation of an improved engineered strain for the funneling of aromatic compounds that exhibits stoichiometric conversion of syringic acid into PDC.IMPORTANCE For lignocellulosic biorefineries to effectively contribute to reduction of fossil fuel use, they need to become efficient at producing chemicals from all major components of plant biomass. Making products from lignin will require engineering microorganisms to funnel multiple phenolic compounds to the chemicals of interest, and N. aromaticivorans is a promising chassis for this technology. The ability of N. aromaticivorans to efficiently and simultaneously degrade many phenolic compounds may be linked to having functionally redundant aromatic degradation pathways and enzymes with broad substrate specificity. A detailed knowledge of aromatic degradation pathways is thus essential to identify genetic engineering targets to maximize product yields. Furthermore, knowledge of enzyme substrate specificity is critical to redirect flow of carbon to desired pathways. This study described an uncharacterized pathway in N. aromaticivorans and the enzymes that participate in this pathway, allowing the engineering of an improved strain for production of PDC from lignin.

Well-Defined Lignin Model Films from Colloidal Lignin Particles

The transformation of a molecularly complex and irregularly shaped lignin into a nanoscale spherical architecture is anticipated to play a pivotal role in the promotion of lignin valorization. From the standpoint of using colloidal lignin particles (CLPs) as building blocks for a diverse range of applications, it has become essential to study their interactions with soluble compounds of varied origin. However, the lack of model films with well-defined surface properties similar to those of CLPs has hindered fundamental studies using surface-sensitive techniques. Here, we report well-defined and stable thin films prepared from CLPs and demonstrate their suitability for investigation of surface phenomena. Direct adsorption on substrates coated with a cationic anchoring polymer resulted in uniform distribution of CLPs as shown with atomic force microscopy (AFM). Quartz crystal microbalance with dissipation monitoring (QCM-D) experiments revealed higher adsorbed mass of cationic lignin onto the CLP-coated substrate in comparison to the film prepared from dissolved lignin, suggesting preferential adsorption via the carboxylic acid enriched surfaces of CLPs. QCM-D further enabled detection of small changes such as particle swelling or partial dissolution not detectable via bulk methods such as light scattering. The CLP thin films remained stable until pH 8 and displayed only a low degree of swelling. Increasing the pH to 10 led to some instability, but their spherical geometry remained intact until complete dissolution was observed at pH 12. Particles prepared from aqueous acetone or aqueous tetrahydrofuran solution followed similar trends regarding adsorption, pH stability, and wetting, although the particle size affected the magnitude of adsorption. Overall, our results present a practical way to prepare well-defined CLP thin films that will be useful not only for fundamental studies but also as a platform for testing stability and interactions of lignin nanoparticles with materials of technical and biomedical relevance.

Isolation of a novel platform bacterium for lignin valorization and its application in glucose-free cis,cis-muconate production

Microbial production of cis,cis-muconate (ccMA) from phenolic compounds obtained by chemical depolymerization of lignin is a promising approach to valorize lignin. Because microbial production requires a large amount of carbon and energy source, it is desirable to establish a ccMA-producing strain that utilizes lignin-derived phenols instead of general sources like glucose. We isolated Pseudomonas sp. strain NGC7 that grows well on various phenolic compounds derived from p-hydroxyphenyl, guaiacyl, and syringyl units of lignin. An NGC7 mutant of protocatechuate (PCA) 3,4-dioxygenase and ccMA cycloisomerase genes (NGC703) lost the ability to grow on vanillate and p-hydroxybenzoate but grew normally on syringate. Introduction of a plasmid carrying genes encoding PCA decarboxylase, flavin prenyltransferase, vanillate O-demethylase, and catechol 1,2-dioxygenase into NGC703 enabled production of 3.2 g/L ccMA from vanillate with a yield of 75% while growing on syringate. This strain also produced ccMA from birch lignin-derived phenols. All these results indicate the utility of NGC7 in glucose-free ccMA production.