Recent advances in lignin valorization with bacterial cultures: microorganisms, metabolic pathways, and bio-products

Exploring industrial lignocellulosic waste: Sources, types, and potential as high-value molecules

Lignocellulosic biomass has a promising role in a circular bioeconomy and may be used to produce valuable molecules for green chemistry. Lignocellulosic biomass, such as food waste, agricultural waste, wood, paper or cardboard, corresponded to 15.7% of all waste produced in Europe in 2020, and has a high potential as a secondary raw material for industrial processes. This review first presents industrial lignocellulosic waste sources, in terms of their composition, quantities and types of lignocellulosic residues. Secondly, the possible high added-value chemicals obtained from transformation of lignocellulosic waste are detailed, as well as their potential for applications in the food industry, biomedical, energy or chemistry sectors, including as sources of polyphenols, enzymes, bioplastic precursors or biofuels. In a third part, various available transformation treatments, such as physical treatments with ultrasound or heat, chemical treatments with acids or bases, and biological treatments with enzymes or microorganisms, are presented. The last part discusses the perspectives of the use of lignocellulosic waste and the fact that decreasing the cost of transformation is one of the major issues for improving the use of lignocellulosic biomass in a circular economy and green chemistry approach, since it is currently often more expensive than petroleum-based counterparts.

Comparative analyses of gut microbiota reveal ammonia detoxification and nitrogen assimilation in Cyprinus carpio var. specularis

The complex niche of fish gut is often characterized by the associated microorganisms that have implications in fish gut-health nexus. Although efforts to distinguish the microbial communities have highlighted their disparate structure along the gut length, remarkably little information is available about their distinct structural and functional profiles in different gut compartments in different fish species. Here, we performed comparative taxonomic and predictive functional analyses of the foregut and hindgut microbiota in an omnivorous freshwater fish species, Cyprinus carpio var. specularis, commonly known as mirror carp. Our analyses showed that the hindgut microbiota could be distinguished from foregut based on the abundance of ammonia-oxidizing, denitrifying, and nitrogen-fixing commensals of families such as Rhodospirillaceae, Oxalobacteraceae, Nitrosomonadaceae, and Nitrospiraceae. Functionally, unique metabolic pathways such as degradation of lignin, 2-nitrobenzoate, vanillin, vanillate, and toluene predicted within hindgut also hinted at the ability of hindgut microbiota for assimilation of nitrogen and detoxification of ammonia. The study highlights a major role of hindgut microbiota in assimilating nitrogen, which remains to be one of the limiting nutrients within the gut of mirror carp.

A comprehensive review on biological funnel mechanism in lignin valorization: Pathways and enzyme dynamics

Lignin, a significant byproduct of the paper and pulp industry, is attracting interest due to its potential utilization in biomaterial-based sectors and biofuel production. Investigating biological methods for converting lignin into valuable products is crucial for effective utilization and has recently gained growing attention. Several microorganisms effectively decomposed low molecular weight lignins, transforming them into intermediate compounds via upper and lower metabolic pathways. This review focuses on assessing bacterial metabolic pathways involved in the breakdown of lignin into aromatic compounds and their subsequent utilization by different bacteria through various metabolic pathways. Understanding these pathways is essential for developing efficient synthetic metabolic systems to valorize lignin and obtain valuable industrial aromatic chemicals. The concept of "biological funneling," which involves examining key enzymes, their interactions, and the complex metabolic pathways associated with lignin conversion, is crucial in lignin valorization. By manipulating lignin metabolic pathways and utilizing biological routes, many aromatic compounds can be synthesized within cellular factories. Although there is insufficient evidence regarding the complete metabolism of polyaromatic hydrocarbons by particular microorganisms, understanding lignin-degrading enzymes, regulatory mechanisms, and interactions among various enzyme systems is essential for optimizing lignin valorization. This review highlights recent advancements in lignin valorization, bio-funneling, multi-omics, and analytical characterization approaches for aromatic utilization. It provides up-to-date information and insights into the latest research findings and technological innovations. The review offers valuable insights into the future potential of biological routes for lignin valorization.

Resolving the metabolism of monolignols and other lignin-related aromatic compounds in Xanthomonas citri

Lignin, a major plant cell wall component, has an important role in plant-defense mechanisms against pathogens and is a promising renewable carbon source to produce bio-based chemicals. However, our understanding of microbial metabolism is incomplete regarding certain lignin-related compounds like p-coumaryl and sinapyl alcohols. Here, we reveal peripheral pathways for the catabolism of the three main lignin precursors (p-coumaryl, coniferyl, and sinapyl alcohols) in the plant pathogen Xanthomonas citri. Our study demonstrates all the necessary enzymatic steps for funneling these monolignols into the tricarboxylic acid cycle, concurrently uncovering aryl aldehyde reductases that likely protect the pathogen from aldehydes toxicity. It also shows that lignin-related aromatic compounds activate transcriptional responses related to chemotaxis and flagellar-dependent motility, which might play an important role during plant infection. Together our findings provide foundational knowledge to support biotechnological advances for both plant diseases treatments and conversion of lignin-derived compounds into bio-based chemicals.

Insights on kraft lignin degradation in an anaerobic environment

Lignin is an aromatic macromolecule and one of the main constituents of lignocellulosic materials. Kraft lignin is generated as a residual by-product of the lignocellulosic biomass industrial process, and it might be used as a feedstock to generate low molecular weight aromatic compounds. In this study, we seek to understand and explore the potential of ruminal bacteria in the degradation of kraft lignin. We established two consortia, KLY and KL, which demonstrated significant lignin-degrading capabilities. Both consortia reached maximum growth after two days, with KLY showing a higher growth and decolorization rate. Additionally, SEM analysis revealed morphological changes in the residual lignin from both consortia, indicating significant degradation. This was further supported by FTIR spectra, which showed new bands corresponding to the C-H vibrations of guaiacyl and syringyl units, suggesting structural transformations of the lignin. Taxonomic analysis showed enrichment of the microbial community with members of the Dickeya genus. Seven metabolic pathways related to lignin metabolism were predicted for the established consortia. Both consortia were capable of consuming aromatic compounds such as 4-hydroxybenzoic acid, syringaldehyde, acetovanillone, and syringic acid, highlighting their capacity to convert aromatic compounds into commercially valuable molecules presenting antifungal activity and used as food preservatives as 4-hydroxyphenylacetic, 3-phenylacetic, and phenylacetic acids. Therefore, the microbial consortia shown in the present work are models for understanding the process of lignin degradation and consumption in bacterial anaerobic communities and developing biological processes to add value to industrial processes based on lignocellulosic biomass as feedstock.

Unraveling interspecies cross-feeding during anaerobic lignin degradation for bioenergy applications

Lignin, a major component of plant biomass, remains underutilized for renewable biofuels due to its complex and heterogeneous structure. Although investigations into depolymerizing lignin using fungi are well-established, studies of microbial pathways that enable anaerobic lignin breakdown linked with methanogenesis are limited. Through an enrichment cultivation approach with inoculation of freshwater sediment, we enriched a microbial community capable of producing methane during anaerobic lignin degradation. We reconstructed the near-complete population genomes of key lignin degraders and methanogens using metagenome-assembled genomes finally selected in this study (MAGs; 92 bacterial and 4 archaeal MAGs affiliated into 45 and 2 taxonomic groups, respectively). This study provides genetic evidence of microbial interdependence in conversion of lignin to methane in a syntrophic community. Metagenomic analysis revealed metabolic linkages, with lignin-hydrolyzing and/or fermentative bacteria such as the genera Alkalibaculum and Propionispora transforming lignin breakdown products into compounds such as acetate to feed methanogens (two archaeal MAGs classified into the genus Methanosarcina or UBA6 of the family Methanomassiliicoccaceae). Understanding the synergistic relationships between microbes that convert lignin could inform strategies for producing renewable bioenergy and treating aromatic-contaminated environments through anaerobic biodegradation processes. Overall, this study offers fundamental insights into complex community-level anaerobic lignin metabolism, highlighting hitherto unknown players, interactions, and pathways in this biotechnologically valuable process.

Metabolic complexity drives divergence in microbial communities

Microbial communities are shaped by environmental metabolites, but the principles that govern whether different communities will converge or diverge in any given condition remain unknown, posing fundamental questions about the feasibility of microbiome engineering. Here we studied the longitudinal assembly dynamics of a set of natural microbial communities grown in laboratory conditions of increasing metabolic complexity. We found that different microbial communities tend to become similar to each other when grown in metabolically simple conditions, but they diverge in composition as the metabolic complexity of the environment increases, a phenomenon we refer to as the divergence-complexity effect. A comparative analysis of these communities revealed that this divergence is driven by community diversity and by the assortment of specialist taxa capable of degrading complex metabolites. An ecological model of community dynamics indicates that the hierarchical structure of metabolism itself, where complex molecules are enzymatically degraded into progressively simpler ones that then participate in cross-feeding between community members, is necessary and sufficient to recapitulate our experimental observations. In addition to helping understand the role of the environment in community assembly, the divergence-complexity effect can provide insight into which environments support multiple community states, enabling the search for desired ecosystem functions towards microbiome engineering applications.

A Complementary Multitechnique Approach to Assess the Bias in Molecular Weight Determination of Lignin by Derivatization-Free Gel Permeation Chromatography

The growing interest in lignin valorization in the past decades calls for analytical techniques for lignin characterization, ranging from wet chemistry techniques to highly sophisticated chromatographic and spectroscopic methods. One of the key parameters to consider is the molecular weight profile of lignin, which is routinely determined by size-exclusion chromatography; however, this is by no means straightforward and is prone to being hampered by considerable errors. Our study expands the fundamental understanding of the bias-inducing mechanisms in gel permeation chromatography (GPC), the magnitude of error originating from using polystyrene standards for mass calibration, and an evaluation of the effects of the solvent and type of lignin on the observed bias. The developed partial least-squares (PLS) regression model for lignin-related monomers revealed that lignin is prone to association mainly via hydrogen bonding. This hypothesis was supported by functional group-based analysis of the bias as well as pulse field gradient (pfg) diffusion NMR spectroscopy of model compounds in THF-d8. Furthermore, although the lack of standards hindered drawing conclusions based on functionalities, direct infusion electrospray ionization mass spectrometry indicated that the relative bias decreases considerably for higher molecular weight species. The results from pfg-diffusion NMR spectroscopy on whole lignin samples were comparable when the same solvents were used in both experiments; in addition, the comparison between results obtained by pfg-diffusion NMR in different solvents gives some additional insights into the aggregation.

Evaluating lignin degradation under limited oxygen conditions by bacterial isolates from forest soil

Lignin, a heterogeneous aromatic polymer present in plant biomass, is intertwined with cellulose and hemicellulose fibrils, posing challenges to its effective utilization due to its phenolic nature and recalcitrance to degradation. In this study, three lignin utilizing bacteria, Klebsiella sp. LEA1, Pseudomonas sp. LEA2, and Burkholderia sp. LEA3, were isolated from deciduous forest soil samples in Nan province, Thailand. These isolates were capable of growing on alkali lignin and various lignin-associated monomers at 40 °C under microaerobic conditions. The presence of Cu2+ significantly enhanced guaiacol oxidation in Klebsiella sp. LEA1 and Pseudomonas sp. LEA2. Lignin-related monomers and intermediates such as 2,6-dimethoxyphenol, 4-vinyl guaiacol, 4-hydroxybenzoic acid, benzoic acid, catechol, and succinic acid were detected mostly during the late stage of incubation of Klebsiella sp. LEA1 and Pseudomonas sp. LEA2 in lignin minimal salt media via GC-MS analysis. The intermediates identified from Klebsiella sp. LEA1 degradation suggested that conversion and utilization occurred through the β-ketoadipate (ortho-cleavage) pathway under limited oxygen conditions. The ability of these bacteria to thrive on alkaline lignin and produce various lignin-related intermediates under limited oxygen conditions suggests their potential utility in oxygen-limited processes and the production of renewable chemicals from plant biomass.

Carbon-wise utilization of lignin-related compounds by synergistically employing anaerobic and aerobic bacteria

BACKGROUND

Lignin is a highly abundant but strongly underutilized natural resource that could serve as a sustainable feedstock for producing chemicals by microbial cell factories. Because of the heterogeneous nature of the lignin feedstocks, the biological upgrading of lignin relying on the metabolic routes of aerobic bacteria is currently considered as the most promising approach. However, the limited substrate range and the inefficient catabolism of the production hosts hinder the upgrading of lignin-related aromatics. Particularly, the aerobic O-demethylation of the methoxyl groups in aromatic substrates is energy-limited, inhibits growth, and results in carbon loss in the form of CO2.

RESULTS

In this study, we present a novel approach for carbon-wise utilization of lignin-related aromatics by the integration of anaerobic and aerobic metabolisms. In practice, we employed an acetogenic bacterium Acetobacterium woodii for anaerobic O-demethylation of aromatic compounds, which distinctively differs from the aerobic O-demethylation; in the process, the carbon from the methoxyl groups is fixed together with CO2 to form acetate, while the aromatic ring remains unchanged. These accessible end-metabolites were then utilized by an aerobic bacterium Acinetobacter baylyi ADP1. By utilizing this cocultivation approach, we demonstrated an upgrading of guaiacol, an abundant but inaccessible substrate to most microbes, into a plastic precursor muconate, with a nearly equimolar yields (0.9 mol/mol in a small-scale cultivation and 1.0 mol/mol in a one-pot bioreactor cultivation). The process required only a minor genetic engineering, namely a single gene knock-out. Noticeably, by employing a metabolic integration of the two bacteria, it was possible to produce biomass and muconate by utilizing only CO2 and guaiacol as carbon sources.

CONCLUSIONS

By the novel approach, we were able to overcome the issues related to aerobic O-demethylation of methoxylated aromatic substrates and demonstrated carbon-wise conversion of lignin-related aromatics to products with yields unattainable by aerobic processes. This study highlights the power of synergistic integration of distinctive metabolic features of bacteria, thus unlocking new opportunities for harnessing microbial cocultures in upgrading challenging feedstocks.

Cloning, expression and application of a novel laccase derived from water buffalo ruminal lignin-degrading bacteria

Water buffalo is the only mammal found to degrade lignin so far, and laccase plays an indispensable role in the degradation of lignin. In this study, multiple laccase genes were amplified based on the water buffalo rumen derived lignin-degrading bacteria Bacillus cereus and Ochrobactrum pseudintermedium. Subsequently, the corresponding recombinant plasmids were transformed into E. coli expression system BL21 (DE3) for induced expression by Isopropyl-β-D-thiogalactopyranoside (IPTG). After preliminary screening, protein purification and enzyme activity assays, Lac3833 with soluble expression and high enzyme activity was selected to test its characteristics, especially the ability of lignin degradation. The results showed that the optimum reaction temperature of Lac3833 was 40 °C for different substrates. The relative activity of Lac3833 reached the highest at pH 4.5 and pH 5.5 when the substrates were ABTS or 2,6-DMP and guaiacol, respectively. Additionally, Lac3833 could maintain high enzyme activity in different temperatures, pH and solutions containing Na+, K+, Mg2+, Ca2+ and Mn2+. Importantly, compared to negative treatment, recombinant laccase Lac3833 treatment showed that it had a significant function in degrading lignin. In conclusion, this is a pioneering study to produce recombinant laccase with lignin-degrading ability by bacteria from water buffalo rumen, which will provide new insights for the exploitation of more lignin-degrading enzymes.

Genomic and transcriptomic analyses provide new insights into the allelochemical degradation preference of a novel Acinetobacter strain

A novel n-alkane- and phenolic acid-degrading Acinetobacter strain (designated C16S1T) was isolated from rhizosphere soil. The strain was identified as a novel species named Acinetobacter suaedae sp. nov. using a polyphasic taxonomic approach. Strain C16S1T showed preferential degradation of three compounds: p-hydroxybenzoate (PHBA) > ferulic acid (FA) > n-hexadecane. In a medium containing two or three of these allelochemicals, coexisting n-hexadecane and PHBA accelerated each other's degradation and that of FA. FA typically hindered the degradation of n-hexadecane but accelerated PHBA degradation. The upregulated expression of n-hexadecane- and PHBA-degrading genes induced, by their related substrates, was mutually enhanced by coexisting PHBA or n-hexadecane; in contrast, expression of both gene types was reduced by FA. Coexisting PHBA or n-hexadecane enhanced the upregulation of FA-degrading genes induced by FA. The expressions of degrading genes affected by coexisting chemicals coincided with the observed degradation efficiencies. Iron shortage limited the degradation efficiency of all three compounds and changed the degradation preference of Acinetobacter. The present study demonstrated that the biodegradability of the chemicals, the effects of coexisting chemicals on the expression of degrading genes and the strain's growth, the shortage of essential elements, and the toxicity of the chemicals were the four major factors affecting the removal rates of the coexisting allelochemicals.

Catalytic carbon-carbon bond cleavage in lignin via manganese-zirconium-mediated autoxidation

Efforts to produce aromatic monomers through catalytic lignin depolymerization have historically focused on aryl-ether bond cleavage. A large fraction of aromatic monomers in lignin, however, are linked by various carbon-carbon (C-C) bonds that are more challenging to cleave and limit the yields of aromatic monomers from lignin depolymerization. Here, we report a catalytic autoxidation method to cleave C-C bonds in lignin-derived dimers and oligomers from pine and poplar. The method uses manganese and zirconium salts as catalysts in acetic acid and produces aromatic carboxylic acids as primary products. The mixtures of the oxygenated monomers are efficiently converted to cis,cis-muconic acid in an engineered strain of Pseudomonas putida KT2440 that conducts aromatic O-demethylation reactions at the 4-position. This work demonstrates that autoxidation of lignin with Mn and Zr offers a catalytic strategy to increase the yield of valuable aromatic monomers from lignin.

From Waste to Value: Recent Insights into Producing Vanillin from Lignin

Vanillin, one of the most widely used and appreciated flavoring agents worldwide, is the main constituent of vanilla bean extract, obtained from the seed pods of various members belonging to the Orchidaceae family. Due to the great demand in the food confectionery industry, as well as in the perfume industry, medicine, and more, the majority of vanillin used today is produced synthetically, and only less than one percent of the world's vanilla flavoring market comes directly from the traditional natural sources. The increasing global demand for vanillin requires alternative and overall sustainable new production methods, and the recovery from biobased polymers, like lignin, is an environmentally friendly alternative to chemical synthesis. The present review provides firstly an overview of the different types of vanillin, followed by a description of the main differences between natural and synthetic vanillin, their preparation, the market of interest, and the authentication issues and the related analytical techniques. Then, the review explores the real potentialities of lignin for vanillin production, presenting firstly the well-assessed classical methods and moving towards the most recent promising approaches through chemical, biotechnological and photocatalytic methodologies, together with the challenges and the principal issues associated with each technique.

Bacterial transformation of lignin: key enzymes and high-value products

Lignin, a natural organic polymer that is recyclable and inexpensive, serves as one of the most abundant green resources in nature. With the increasing consumption of fossil fuels and the deterioration of the environment, the development and utilization of renewable resources have attracted considerable attention. Therefore, the effective and comprehensive utilization of lignin has become an important global research topic, with the goal of environmental protection and economic development. This review focused on the bacteria and enzymes that can bio-transform lignin, focusing on the main ways that lignin can be utilized to produce high-value chemical products. Bacillus has demonstrated the most prominent effect on lignin degradation, with 89% lignin degradation by Bacillus cereus. Furthermore, several bacterial enzymes were discussed that can act on lignin, with the main enzymes consisting of dye-decolorizing peroxidases and laccase. Finally, low-molecular-weight lignin compounds were converted into value-added products through specific reaction pathways. These bacteria and enzymes may become potential candidates for efficient lignin degradation in the future, providing a method for lignin high-value conversion. In addition, the bacterial metabolic pathways convert lignin-derived aromatics into intermediates through the "biological funnel", achieving the biosynthesis of value-added products. The utilization of this "biological funnel" of aromatic compounds may address the heterogeneous issue of the aromatic products obtained via lignin depolymerization. This may also simplify the separation of downstream target products and provide avenues for the commercial application of lignin conversion into high-value products.

Efficient polymalic acid production from corn straw hydrolysate by detoxification of phenolic inhibitors

Inhibitory compounds generated from lignocellulose pretreatment would inhibit Poly (malic acid) (PMA) production by Aureobasidium pullulans, but the tolerance mechanism of A. pullulans to lignocellulosic inhibitor is poorly understood. In this study, the cellular response of A. pullulans to lignocellulosic inhibitor stress was studied. Among the three groups of inhibitors (furans, weak acids and phenolic aldehydes), phenolic aldehyde was the dominant inhibitor for PMA production. Phenolic aldehyde was mainly converted into phenolic alcohol by A. pullulans, and phenolic alcohol also exhibited severe inhibition on PMA production. Furthermore, the effect of detoxification methods on inhibitor-removal and PMA fermentation was investigated, both CaCO3 and overliming presented poor detoxification effect, whereas resin H103 could remove both furan derivatives and phenolic compounds efficiently, thereby producing 26.27 g/L of PMA with a yield of 0.30 g/g in batch fermentation. This study will be beneficial for the development of PMA production from lignocellulosic biomass.

Unleashing synergistic potential of microbially enhanced anaerobic co-digestion of petroleum refinery biosludge and yard waste: Impact of nutrient balance and microbial diversity

Petroleum refinery sludge, an egregious solid residue generated from the wastewater treatment plants poses an environmental hazard owing to its intricate hydrocarbon composition, necessitating competent treatment for secure disposal. The study proposes a green solution through anaerobic co-digestion of nitrogen-rich petroleum refinery sludge (PS) with carbon-rich yard waste (YW), balancing the nutrients and moisture content for efficient microbial proliferation. Using Central Composite Design-Response Surface Methodology, 1 L batch experiments were conducted with varying carbon/nitrogen (C/N) ratios and pH to achieve maximum biogas yield within 50 days of co-digestion. However, the sluggish biogas recovery (40%) indicated a slow rate-limiting hydrolysis, necessitating pretreatment. Feedstock incubation with Bacillus subtilis IH1 strain, isolated from the microbially-enriched PS, at 108 colony forming units (CFU) per mL for 5 days maximized the soluble chemical oxygen demand and volatile fatty acids by 2.2 and 1.4 folds respectively compared to untreated feedstock. Scale-up Bacillus subtilis aided co-digestion studies further augmented biogas by 76% against untreated monodigestion of PS with significant total petroleum hydrocarbons, emulsions, and lignocellulosic degradation. Further identification of major organic pollutants in the batch digestate revealed significant degradation of the toxic organic hydrocarbon pollutants apotheosizing the efficacy of the synergistic sustainable technique for the management of PS. ENVIRONMENTAL IMPLICATION: The effluent treatment plants (ETPs) of petroleum refining industries generate sludge which is a complex mixture of petroleum hydrocarbons, oil-water (O/W) emulsions and heavy metals. These petroleum hydrocarbon constituents can be linear/cyclic alkanes, polyaromatics, resins and asphaltenes, whose intricate composition is reportedly carcinogenic, cytogenic and mutagenic, classifying it as hazardous waste. Biological treatment of these sludge through anaerobic digestion leads to utilization of petroleum hydrocarbons with subsequent energy recovery. Co-digestion of these sludge with competent co-substrates leads to nutrient balance, diverse microbial proliferation and toxicant dilution. Microbially aided co-digestion further augments methane rendering a digestate with utmost pollutant degradation.

Valorization of lignin-derived compounds into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by engineered Halomonas sp. Y3

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) is a biopolyester with great potential, but its high production cost via the propionate-dependent pathway has hindered its development. Herein, we engineer Halomonas sp. Y3 to achieve efficient conversion of various LDCs into PHBV without propionate supplement. Initially, we successfully achieve PHBV production without propionate supplement by overexpressing threonine synthesis. The resulting biopolyester exhibits a 3 HV proportion of up to 7.89 mol%, comparable to commercial PHBV (8 mol%) available from Sigma Aldrich (403105). To further enhance PHBV production, we rationally design the reconstruction of aromatic compound catabolism. The engineered strain Y3_18 efficiently assimilates all LDCs containing syringyl (S), guaiacyl (G), and p-hydroxyphenyl-type (H) units. From 1 g/L of S-, G-, and H-type LDCs, Y3_18 produces PHBV at levels of 449 mg/L, 488 mg/L, and 716 mg/L, respectively, with yields of 44.9 % (g/g), 48.8 % (g/g), and 71.6 % (g/g). Moreover, to improve PHBV yield from lignin, we integrate laccase-secretion and PHBV production modules. This integration leads to the accumulation of 425.84 mg/L of PHBV with a yield of 21.29 % (g/g) and a 3 HV proportion of 6.38 mol%. By harnessing the capabilities of Halomonas sp. Y3, we demonstrate an efficient and sustainable approach for PHBV production from a variety of LDCs.

Exoproteomic Study and Transcriptional Responses of Laccase and Ligninolytic Peroxidase Genes of White-Rot Fungus Trametes hirsuta LE-BIN 072 Grown in the Presence of Monolignol-Related Phenolic Compounds

Being an abundant renewable source of aromatic compounds, lignin is an important component of future bio-based economy. Currently, biotechnological processing of lignin through low molecular weight compounds is one of the conceptually promising ways for its valorization. To obtain lignin fragments suitable for further inclusion into microbial metabolism, it is proposed to use a ligninolytic system of white-rot fungi, which mainly comprises laccases and peroxidases. However, laccase and peroxidase genes are almost always represented by many non-allelic copies that form multigene families within the genome of white-rot fungi, and the contributions of exact family members to the overall process of lignin degradation has not yet been determined. In this article, the response of the Trametes hirsuta LE-BIN 072 ligninolytic system to the presence of various monolignol-related phenolic compounds (veratryl alcohol, p-coumaric acid, vanillic acid, and syringic acid) in culture media was monitored at the level of gene transcription and protein secretion. By showing which isozymes contribute to the overall functioning of the ligninolytic system of the T. hirsuta LE-BIN 072, the data obtained in this study will greatly contribute to the possible application of this fungus and its ligninolytic enzymes in lignin depolymerization processes.

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.

Unravelling the hidden power of esterases for biomanufacturing of short-chain esters

Microbial production of esters has recently garnered wide attention, but the current production metrics are low. Evidently, the ester precursors (organic acids and alcohols) can be accumulated at higher titers by microbes like Escherichia coli. Hence, we hypothesized that their 'direct esterification' using esterases will be efficient. We engineered esterases from various microorganisms into E. coli, along with overexpression of ethanol and lactate pathway genes. High cell density fermentation exhibited the strains possessing esterase-A (SSL76) and carbohydrate esterase (SSL74) as the potent candidates. Fed-batch fermentation at pH 7 resulted in 80 mg/L of ethyl acetate and 10 mg/L of ethyl lactate accumulation by SSL76. At pH 6, the total ester titer improved by 2.5-fold, with SSL76 producing 225 mg/L of ethyl acetate, and 18.2 mg/L of ethyl lactate, the highest reported titer in E. coli. To our knowledge, this is the first successful demonstration of short-chain ester production by engineering 'esterases' in E. coli.

Sustainable biosynthetic pathways to value-added bioproducts from hydroxycinnamic acids

The biorefinery concept, in which biomass is utilized for the production of fuels and chemicals, emerges as an eco-friendly, cost-effective, and renewable alternative to petrochemical-based production. The hydroxycinnamic acid fraction of lignocellulosic biomass represents an untapped source of aromatic molecules that can be converted to numerous high-value products with industrial applications, including in the flavor and fragrance sector and pharmaceuticals. This review describes several biochemical pathways useful in the development of a biorefinery concept based on the biocatalytic conversion of the hydroxycinnamic acids ferulic, caffeic, and p-coumaric acid into high-value molecules. KEY POINTS: • The phenylpropanoids bioconversion pathways in the context of biorefineries • Description of pathways from hydroxycinnamic acids to high-value compounds • Metabolic engineering and synthetic biology advance hydroxycinnamic acid-based biorefineries.

Structural insights, biocatalytic characteristics, and application prospects of lignin-modifying enzymes for sustainable biotechnology-A review

Lignin modifying enzymes (LMEs) have gained widespread recognition in depolymerization of lignin polymers by oxidative cleavage. LMEs are a robust class of biocatalysts that include lignin peroxidase (LiP), manganese peroxidase (MnP), versatile peroxidase (VP), laccase (LAC), and dye-decolorizing peroxidase (DyP). Members of the LMEs family act on phenolic, non-phenolic substrates and have been widely researched for valorization of lignin, oxidative cleavage of xenobiotics and phenolics. LMEs implementation in the biotechnological and industrial sectors has sparked significant attention, although its potential future applications remain underexploited. To understand the mechanism of LMEs in sustainable pollution mitigation, several studies have been undertaken to assess the feasibility of LMEs in correlating to diverse pollutants for binding and intermolecular interactions at the molecular level. However, further investigation is required to fully comprehend the underlying mechanism. In this review we presented the key structural and functional features of LMEs, including the computational aspects, as well as the advanced applications in biotechnology and industrial research. Furthermore, concluding remarks and a look ahead, the use of LMEs coupled with computational frameworks, built upon artificial intelligence (AI) and machine learning (ML), has been emphasized as a recent milestone in environmental research.

Adaptive laboratory evolution boost Yarrowia lipolytica tolerance to vanillic acid

Microbial tolerance to lignocellulose-derived inhibitors, such as aromatic acids, is critical for the economical production of biofuels and biochemicals. Here, adaptive laboratory evolution was applied to improve the tolerance of Yarrowia lipolytica to a representative aromatic acid inhibitor vanillic acid. The transcriptome profiling of evolved strain suggested that the tolerance could be related to the up-regulation of RNA processing and multidrug transporting pathways. Further analysis by reverse engineering confirmed that the amplification of YALI0_F13475g coding for transcriptional coactivator and YALI0_E25201g coding for multidrug transporter conferred tolerance not only to vanillic acid but also towards ferulic acid, p-coumaric acid, p-hydroxybenzoic acid and syringic acid. These findings suggested that regulation of RNA processing and multidrug transporting pathways may be important for enhanced aromatic acid tolerance in Y. lipolytica. This study provides valuable genetic information for robust strain construction for lignocellulosic biorefinery.

Localization of Pyranose 2-Oxidase from Kitasatospora aureofaciens: A Step Closer to Elucidate a Biological Role

Lignin degradation in fungal systems is well characterized. Recently, a potential for lignin depolymerization and modification employing similar enzymatic activities by bacteria is increasingly recognized. The presence of genes annotated as peroxidases in Actinobacteria genomes suggests that these bacteria should contain auxiliary enzymes such as flavin-dependent carbohydrate oxidoreductases. The only auxiliary activity subfamily with significantly similar representatives in bacteria is pyranose oxidase (POx). A biological role of providing H₂O₂ for peroxidase activation and reduction of radical degradation products suggests an extracellular localization, which has not been established. Analysis of the genomic locus of POX from Kitasatospora aureofaciens (KaPOx), which is similar to fungal POx, revealed a start codon upstream of the originally annotated one, and the additional sequence was considered a putative Tat-signal peptide by computational analysis. We expressed KaPOx including this additional upstream sequence as well as fusion constructs consisting of the additional sequence, the KaPOx mature domain and the fluorescent protein mRFP1 in Streptomyces lividans. The putative signal peptide facilitated secretion of KaPOx and the fusion protein, suggesting a natural extracellular localization and supporting a potential role in providing H₂O₂ and reducing radical compounds derived from lignin degradation.

Single-Standard Quantification Strategy for Lignin Dimers by Supercritical Fluid Chromatography with Charged Aerosol Detection

The increased interest in utilizing lignin as a feedstock to produce various aromatic compounds requires advanced chemical analysis methods to provide qualitative and quantitative characterization of lignin samples along different technology streamlines. However, due to the lack of commercially available chemical standards, routine quantification of industrially relevant lignin oligomers in complex lignin samples remains a challenge. This study presents a novel method for universal quantification of lignin dimers based on supercritical fluid chromatography with charged aerosol detection (CAD). A series of lignin-derived dimeric compounds that have been reported from reductive catalytic fractionation (RCF) were synthesized and used as standards. The applicability of using linear regression instead of quadratic calibration curves was evaluated over a concentration range of 15-125 mg/L, demonstrating that the former calibration method is as appropriate as the latter. The response factors of lignin dimeric compounds were compared to assess the uniformity of the CAD signal, revealing that the CAD response for the tested lignin dimers did not differ substantially. It was also found that the response factors were not dependent on the number of methoxy groups or linkage motifs, ultimately enabling the use of only one calibrant for these compounds. The importance of chromatographic peak resolution in CAD was stressed, and the use of a digital peak sharpening technique was adopted and applied to address this challenge. The developed method was verified and used for the quantification of lignin dimers in an oil obtained by a RCF of birch sawdust.

Optimized pulse-feeding fed-batch fermentation for enhanced lignin to polyhydroxyalkanoate transformation

With an anticipated growth of Bio-Circular-Green economy, the amount of lignin generated as by-product from biorefineries is increasing. Hence, lignin valorising strategies become a very interesting option to improve economic viability of the biorefineries. This study revealed the development of bioprocesses for upgrading lignin into bioplastic. Specifically, a novel strain of Pseudomonas fulva has been applied for microbial bioconversion of organosolv lignin to fermentative polyhydroxyalkanoate (PHA) production. Fed-batch fermentation of lignin-to-PHA with pulse-feeding approach was optimized using Design of Experiment. Effects of C:N ratio and feeding profiles on PHA accumulation in P. fulva were investigated to determine optimal operation. Under optimized fed-batch, the PHA concentration of 195.2 ± 6.6 mg/L could be reached and the PHA content was more than 2 folds enhancement compared to batch process. Type of PHA produced was also characterized for chemical composition via GC-MS analysis. The established lignin to PHA conversion could provide platform for developing integrated lignin bioprocessing to promote cost-effective biorefineries.

Engineering isoprenoids production in metabolically versatile microbial host Pseudomonas putida

With the increasing need for microbial bioproduction to replace petrochemicals, it is critical to develop a new industrial microbial workhorse that improves the conversion of lignocellulosic carbon to biofuels and bioproducts in an economically feasible manner. Pseudomonas putida KT2440 is a promising microbial host due to its capability to grow on a broad range of carbon sources and its high tolerance to xenobiotics. In this study, we engineered P. putida KT2440 to produce isoprenoids, a vast category of compounds that provide routes to many petrochemical replacements. A heterologous mevalonate (MVA) pathway was engineered to produce potential biofuels isoprenol (C₅) and epi-isozizaene (C₁₅) for the first time in P. putida. We compared the difference between three different isoprenoid pathways in P. putida on isoprenol production and achieved 104 mg/L of isoprenol production in a batch flask experiment through optimization of the strain. As P. putida can natively consume isoprenol, we investigated how to prevent this self-consumption. We discovered that supplementing L-glutamate in the medium can effectively prevent isoprenol consumption in P. putida and metabolomics analysis showed an insufficient energy availability and an imbalanced redox status during isoprenol degradation. We also showed that the engineered P. putida strain can produce isoprenol using aromatic substrates such as p-coumarate as the sole carbon source, and this result demonstrates that P. putida is a valuable microbial chassis for isoprenoids to achieve sustainable biofuel production from lignocellulosic biomass.

Microbial detoxification of lignocellulosic biomass hydrolysates: Biochemical and molecular aspects, challenges, exploits and future perspectives

Valorization of lignocellulosic biomass (LB) has the potential to secure sustainable energy production without impacting food insecurity, whist relieving over reliance on finite fossil fuels. Agro-derived lignocellulosic residues such as wheat straw, switchgrass, rice bran, and miscanthus have gained relevance as feedstocks for the production of biofuels and chemicals. However, the microorganisms employed in fermentative conversion of carbohydrates to fuels and chemicals are unable to efficiently utilize the sugars derived from LB due to co-production of lignocellulose-derived microbial inhibitory compounds (LDMICs) during LB pretreatment. LDMICs impact microbial growth by inhibition of specific enzymes, cause DNA and cell membrane damage, and elicit cellular redox imbalance. Over the past decade, success has been achieved with the removal of LDMICs prior to fermentation. However, LDMICs removal by chemical processes is often accompanied by sugar losses, which negatively impacts the overall production cost. Hence, in situ removal of LDMICs by fermentative organisms during the fermentation process has garnered considerable attention as the "go-to" approach for economical LDMICs detoxification and bio-chemicals production. In situ removal of LDMICs has been pursued by either engineering more robust biocatalysts or isolating novel microbial strains with the inherent capacity to mineralize or detoxify LDMICs to less toxic compounds. While some success has been made along this line, efficient detoxification and robust production of target bio-chemicals in lignocellulosic hydrolysates (LHs) under largely anaerobic fermentative conditions remains a lingering challenge. Consequently, LB remains an underutilized substrate for bio-chemicals production. In this review, the impact of microbial LH detoxification on overall target molecule production is discussed. Further, the biochemical pathways and mechanisms employed for in situ microbial detoxification of furanic LDMICs [e.g., furfural and 5-hydroxymethylfurfural (HMF)] and phenolic LDMICs (e.g., syringaldehyde, p-coumaric acid, 4-hydroxybenzaldehyde, vanillin, and ferulic acid) are discussed. More importantly, metabolic engineering strategies for the development of LDMIC-tolerant and bio-chemicals overproducing strains and processes are highlighted.

Deciphering the transcriptional regulation of the catabolism of lignin-derived aromatics in Rhodococcus opacus PD630

Rhodococcus opacus PD630 has considerable potential as a platform for valorizing lignin due to its innate "biological funneling" pathways. However, the transcriptional regulation of the aromatic catabolic pathways and the mechanisms controlling aromatic catabolic operons in response to different aromatic mixtures are still underexplored. Here, we identified and studied the transcription factors for aromatic degradation using GFP-based sensors and comprehensive deletion analyses. Our results demonstrate that the funneling pathways for phenol, guaiacol, 4-hydroxybenzoate, and vanillate are controlled by transcriptional activators. The two different branches of the β-ketoadipate pathway, however, are controlled by transcriptional repressors. Additionally, promoter activity assays revealed that the substrate hierarchy in R. opacus may be ascribed to the transcriptional cross-regulation of the individual aromatic funneling pathways. These results provide clues to clarify the molecule-level mechanisms underlying the complex regulation of aromatic catabolism, which facilitates the development of R. opacus as a promising chassis for valorizing lignin.

A review on lignin pyrolysis: pyrolytic behavior, mechanism, and relevant upgrading for improving process efficiency

Lignin is a promising alternative to traditional fossil resources for producing biofuels due to its aromaticity and renewability. Pyrolysis is an efficient technology to convert lignin to valuable chemicals, which is beneficial for improving lignin valorization. In this review, pyrolytic behaviors of various lignin were included, as well as the pyrolytic mechanism consisting of initial, primary, and charring stages were also introduced. Several parallel reactions, such as demethoxylation, demethylation, decarboxylation, and decarbonylation of lignin side chains to form light gases, major lignin structure decomposition to generate phenolic compounds, and polymerization of active lignin intermediates to yield char, can be observed through the whole pyrolysis process. Several parameters, such as pyrolytic temperature, time, lignin type, and functional groups (hydroxyl, methoxy), were also investigated to figure out their effects on lignin pyrolysis. On the other hand, zeolite-driven lignin catalytic pyrolysis and lignin co-pyrolysis with other hydrogen-rich co-feedings were also introduced for improving process efficiency to produce more aromatic hydrocarbons (AHs). During the pyrolysis process, phenolic compounds and/or AHs can be produced, showing promising applications in biochemical intermediates and biofuel additives. Finally, some challenges and future perspectives for lignin pyrolysis have been discussed.

Recent Advancements and Challenges in Lignin Valorization: Green Routes towards Sustainable Bioproducts

The aromatic hetero-polymer lignin is industrially processed in the paper/pulp and lignocellulose biorefinery, acting as a major energy source. It has been proven to be a natural resource for useful bioproducts; however, its depolymerization and conversion into high-value-added chemicals is the major challenge due to the complicated structure and heterogeneity. Conversely, the various pre-treatments techniques and valorization strategies offers a potential solution for developing a biomass-based biorefinery. Thus, the current review focus on the new isolation techniques for lignin, various pre-treatment approaches and biocatalytic methods for the synthesis of sustainable value-added products. Meanwhile, the challenges and prospective for the green synthesis of various biomolecules via utilizing the complicated hetero-polymer lignin are also discussed.

Characterization and Expression Analysis of Extradiol and Intradiol Dioxygenase of Phenol-Degrading Haloalkaliphilic Bacterial Isolates

Haloalkophilic bacteria have a potential advantage as a bioremediation organism of high oil-polluted and industrial wastewater. In the current study, Haloalkaliphilic isolates were obtained from Hamralake, Wadi EL-Natrun, Egypt. The phenotype script, biochemical characters, and sequence analysis of bacterial-16S rRNA were used to identify the bacterial isolates; Halomonas HA1 and Marinobacter HA2. These strains required high concentrations of NaCl to ensure bacterial growth, especially Halomonas HA1 strain. Notably, both isolates can degrade phenol at optimal pH values, between 8 and 9, with the ability to grow in pH levels up to 11, like what was seen in the Halomonas HA1 strain. Moreover, both isolates represent two different mechanistic pathways for phenol degradation. Halomonas HA1 exploits the 1,2 phenol meta-cleavage pathway, while Marinobacter HA2 uses the 2,3 ortho-cleavage pathway as indicated by universal primers for 1,2 and 2,3 CTD genes. Interestingly, Marinobacter HA2 isolate eliminated the added phenol within an incubation period of 72 h, while the Halomonas HA1 isolate invested 96 h in degrading 84% of the same amount of phenol. Phylogenetic analysis of these 1,2 CTD (catechol dioxygenase) sequences clearly showed an evolutionary relationship between 1,2 dioxygenases of both Halomonadaceae and Pseudomonadaceae. In comparison, 2,3 CTD of Marinobacter HA2 shared the main domains of the closely related species. Furthermore, semi-quantitative RT-PCR analysis proved the constitutive expression pattern of both dioxygenase genes. These findings provide new isolates of Halomonas sp. and Marinobacter sp. that can degrade phenol at high salt and pH conditions via two independent mechanisms.

Spatio-Temporal Modification of Lignin Biosynthesis in Plants: A Promising Strategy for Lignocellulose Improvement and Lignin Valorization

Lignin is essential for plant growth, structural integrity, biotic/abiotic stress resistance, and water transport. Besides, lignin constitutes 10–30% of lignocellulosic biomass and is difficult to utilize for biofuel production. Over the past few decades, extensive research has uncovered numerous metabolic pathways and genes involved in lignin biosynthesis, several of which have been highlighted as the primary targets for genetic manipulation. However, direct manipulation of lignin biosynthesis is often associated with unexpected abnormalities in plant growth and development for unknown causes, thus limiting the usefulness of genetic engineering for biomass production and utilization. Recent advances in understanding the complex regulatory mechanisms of lignin biosynthesis have revealed new avenues for spatial and temporal modification of lignin in lignocellulosic plants that avoid growth abnormalities. This review explores recent work on utilizing specific transcriptional regulators to modify lignin biosynthesis at both tissue and cellular levels, focusing on using specific promoters paired with functional or regulatory genes to precisely control lignin synthesis and achieve biomass production with desired properties. Further advances in designing more appropriate promoters and other regulators will increase our capacity to modulate lignin content and structure in plants, thus setting the stage for high-value utilization of lignin in the future.

Looking into the world's largest elephant population in search of ligninolytic microorganisms for biorefineries: a mini-review

Gastrointestinal tracts (GIT) of herbivores are lignin-rich environments with the potential to find ligninolytic microorganisms. The occurrence of the microorganisms in herbivore GIT is a well-documented mutualistic relationship where the former benefits from the provision of nutrients and the latter benefits from the microorganism-assisted digestion of their recalcitrant lignin diets. Elephants are one of the largest herbivores that rely on the microbial anaerobic fermentation of their bulky recalcitrant low-quality forage lignocellulosic diet given their inability to break down major components of plant cells. Tapping the potential of these mutualistic associations in the biggest population of elephants in the whole world found in Botswana is attractive in the valorisation of the bulky recalcitrant lignin waste stream generated from the pulp and paper, biofuel, and agro-industries. Despite the massive potential as a feedstock for industrial fermentations, few microorganisms have been commercialised. This review focuses on the potential of microbiota from the gastrointestinal tract and excreta of the worlds' largest population of elephants of Botswana as a potential source of extremophilic ligninolytic microorganisms. The review further discusses the recalcitrance of lignin, achievements, limitations, and challenges with its biological depolymerisation. Methods of isolation of microorganisms from elephant dung and their improvement as industrial strains are further highlighted.

Current status, challenges and prospects for lignin valorization by using Rhodococcus sp

Lignin represents the most abundant renewable aromatics in nature, which has complicated and heterogeneous structure. The rapid development of biotransformation technology has brought new opportunities to achieve the complete lignin valorization. Especially, Rhodococcus sp. possesses excellent capabilities to metabolize aromatic hydrocarbons degraded from lignin. Furthermore, it can convert these toxic compounds into high value added bioproducts, such as microbial lipids, polyhydroxyalkanoate and carotenoid et al. Accordingly, this review will discuss the potentials of Rhodococcus sp. as a cell factory for lignin biotransformation, including phenol tolerance, lignin depolymerization and lignin-derived aromatic hydrocarbon metabolism. The detailed metabolic mechanism for lignin biotransformation and bioproducts spectrum of Rhodococcus sp. will be comprehensively discussed. The available molecular tools for the conversion of lignin by Rhodococcus sp. will be reviewed, and the possible direction for lignin biotransformation in the future will also be proposed.

Getting Value from Pulp and Paper Industry Wastes: On the Way to Sustainability and Circular Economy

The pulp and paper industry is recognized as a well-established sector, which throughout its process, generates a vast amount of waste streams with the capacity to be valorized. Typically, these residues are burned for energy purposes, but their use as substrates for biological processes could be a more efficient and sustainable alternative. With this aim, it is essential to identify and characterize each type of waste to determine its biotechnological potential. In this context, this research highlights possible alternatives with lower environmental impact and higher revenues. The bio-based pathway should be a promising alternative for the valorization of pulp and paper industry wastes, in particular for bioproduct production such as bioethanol, polyhydroxyalkanoates (PHA), and biogas. This article focuses on state of the art regarding the identification and characterization of these wastes, their main applied deconstruction technologies and the valorization pathways reported for the production of the abovementioned bioproducts.

Unraveling microbe-mediated degradation of lignin and lignin-derived aromatic fragments in the Pearl River Estuary sediments

Estuaries are one of the most crucial areas for the transformation and burial of terrestrial organic carbon (TerrOC), playing an important role in the global carbon cycle. While the transformation and degradation of TerrOC are mainly driven by microorganisms, the specific taxa and degradation processes involved remain largely unknown in estuaries. We collected surface sediments from 14 stations along the longitudinal section of the Pearl River Estuary (PRE), P. R. China. By combining analytical chemistry, metagenomics, and bioinformatics methods, we analyzed composition, source and degradation pathways of lignin/lignin-derived aromatic fragments and their potential decomposers in these samples. A diversity of bacterial and archaeal taxa, mostly those from Proteobacteria (Deltaproteobacteria, Gammaproteobacteria etc.), including some lineages (e.g., Nitrospria, Polyangia, Tectomicrobia_uc) not previously implicated in lignin degradation, were identified as potential polymeric lignin or its aromatic fragments degraders. The abundance of lignin degradation pathways genes exhibited distinct spatial distribution patterns with the area adjacent to the outlet of Modaomen as a potential degradation hot zone and the Syringyl lignin fragments, 3,4-PDOG, and 4,5-PDOG pathways as the primary potential lignin aromatic fragments degradation processes. Notably, the abundance of ferulic acid metabolic pathway genes exhibited significant correlations with degree of lignin oxidation and demethylation/demethoxylization and vegetation source. Additionally, the abundance of 2,3-PDOG degradation pathways genes also showed a positive significant correlation with degree of lignin oxidation. Our study provides a meaningful insight into the microbial ecology of TerrOC degradation in the estuary.

Effective Biotransformation of Variety of Guaiacyl Lignin Monomers Into Vanillin by Bacillus pumilus

Biotransformation has gained increasing attention due to its being an eco-friendly way for the production of value-added chemicals. The present study aimed to assess the potential of Bacillus pumilus ZB1 on guaiacyl lignin monomers biotransformation for the production of vanillin. Consequently, isoeugenol, eugenol, and vanillyl alcohol could be transformed into vanillin by B. pumilus ZB1. Based on the structural alteration of masson pine and the increase of total phenol content in the supernatant, B. pumilus ZB1 exhibited potential in lignin depolymerization and valorization using masson pine as the substrate. As the precursors of vanillin, 61.1% of isoeugenol and eugenol in pyrolyzed bio-oil derived from masson pine could be transformed into vanillin by B. pumilus ZB1. Four monooxygenases with high specific activity were identified that were involved in the transformation process. Thus, B. pumilus ZB1 could emerge as a candidate in the biosynthesis of vanillin by using wide guaiacyl precursors as the substrates.

Recent Biotechnology Advances in Bio-Conversion of Lignin to Lipids by Bacterial Cultures

The complexity and recalcitrance of the lignin structure is a major barrier to its efficient utilization and commercial production of high-value products. In recent years, the “bio-funneling” transformation ability of microorganisms has provided a significant opportunity for lignin conversion and integrated biorefinery. Based on the chemical structure of lignin, this mini-review introduces the recent advances of lignin depolymerization by bacterial strains and the application of microbial lignin degradation in lipids production. Furthermore, the current challenges, future trends and perspectives for microbe-based lignin conversion to lipids are discussed.

Cosolvent enhanced lignocellulosic fractionation tailoring lignin chemistry and enhancing lignin bioconversion

Cosolvent Enhanced Lignocellulosic Fractionation (CELF) is an emerging solvolysis pretreatment to fractionate lignocellulosic biomass. Herein, the bioconversion performance of CELF lignin was fully evaluated for the first time. Results showed that CELF lignin possessed higher content of carboxylic acid OH, lower molecular weight, and disappeared β-O-4 and β-5 linkages compared to other two technical lignins including a conventional ethanol organosolv lignin (EOL) and a kraft lignin (KL). Rhodococcus opacus PD630 cell count from CELF lignin fermentation reached the highest value of 3.9 × 10⁷ CFU/mL, representing a 62.5% and 77.3% improvement over EOL and KL, respectively. Correspondingly, lipid yield reached 143 mg/L from CELF lignin, which was 36.2% and 26.5% higher than from EOL and KL, respectively. Principal component analysis (PCA) revealed that more carboxylic acid groups and lower molecular weight contributed to the enhanced bioconversion performance of CELF lignin. This study demonstrates that CELF lignin is a promising candidate for bioconversion.

Deciphering the metabolic distribution of vanillin in Rhodococcus opacus during lignin valorization

Vanillin bioconversion is important for the biological lignin valorization. In this study, the obscure vanillin metabolic distribution in Rhodoccous opacus PD630 was deciphered by combining the strategies of intermediate detection, putative gene prediction, and target gene verification. The results suggest that approximately 10% (mol/mol) of consumed vanillin is converted to vanillic acid for further metabolism, and a large amount is converted to dead-end vanillyl alcohol in R. opacus PD630. Subsequently, five vanillin reductases were identified in R. opacus PD630, among which Pd630_LPD03722 product exhibited the greatest activity. With the detected metabolic distributions of vanillin, the conversion of vanillin to muconic acid was facilitated by deleting domestic vanillin reductase genes and introducing vanillin dehydrogenase from Sphingobium sp. SYK-6. Ultimately, the muconic acid yield from vanillin increased to 97.83% (mol/mol) from the initial 10% (mol/mol). Moreover, this study demonstrated the existence of vanillin reductases in Escherichia coli, Bacillus subtilis, and Corynebacterium glutamicum.

Efficient vanillin biosynthesis by recombinant lignin-degrading bacterium Arthrobacter sp. C2 and its environmental profile via life cycle assessment

Vanillin is a natural flavoring agent that is widely used in the bioengineering industry. To enable sustainable development, joint consideration of bacterial performance and negative environmental impacts are critical to vanillin biosynthesis. In this study, a cold shock protein (csp) gene was upregulated for maintaining stable growth in Arthrobacter sp. C2 responding to vanillin and cold stress. Furthermore, the recombinant strain C2 was constructed by simultaneously deleting the xylC gene encoding benzaldehyde dehydrase and overexpressing the pchF gene encoding vanillyl alcohol oxidase and achieved a maximum vanillin productivity of 0.85 mg/g DCW/h with alkaline lignin as the substrate. Finally, this process generated an environmental impact value of 25.05, which was the lowest environmental impact achieved according to life cycle assessment (LCA). Improvement strategies included reducing electricity consumption and replacing chemicals. This study achieved the development of an effective strategy, and future studies should focus on precise vanillin biosynthesis methods for large-scale application.

Next generation applications of lignin derived commodity products, their life cycle, techno-economics and societal analysis

The pulp and biorefining industries produce their waste as lignin, which is one of the most abundant renewable resources. So far, lignin has been remained severely underutilized and generally burnt in a boiler as a low-value fuel. To demonstrate lignin's potential as a value-added product, we will review market opportunities for lignin related applications by utilizing the thermo-chemical/biological depolymerization strategies (with or without catalysts) and their comparative evaluation. The application of lignin and its derived aromatics in various sectors such as cement industry, bitumen modifier, energy materials, agriculture, nanocomposite, biomedical, H2 source, biosensor and bioimaging have been summarized. This comprehensive review article also highlights the technical, economic, environmental, and socio-economic variable that affect the market value of lignin-derived by-products. The review shows the importance of lignin, and its derived products are a platform for future bioeconomy and sustainability.

A review on physico-chemical delignification as a pretreatment of lignocellulosic biomass for enhanced bioconversion

Effective pretreatment of lignocellulosic biomass (LCB) is one of the most important steps in biorefinery, ensuring the quality and commercial viability of the overall bioprocess. Lignin recalcitrance in LCB is a major bottleneck in biological conversion as the polymerization of lignin with hemicellulose hinders enzyme accessibility and further bioconversion to fuels and chemicals. Therefore, there is a need to delignify LCB to ease further bioprocessing. The efficiency of delignification, quality and quantity of the desired products, and generation of inhibitors depend upon the type of pretreatment employed. This review summarizes different single and integrated physicochemical pretreatments for delignification. Additionally, conditions required for effective delignification and the advantages and drawbacks of each method were evaluated. Advances in overcoming the recalcitrance of residual lignin to saccharification and the methods to recover lignin after delignification are also discussed. Efficient lignin recovery and valorization strategies provide an avenue for the sustainable lignocellulose biorefinery.