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As the most abundant renewable aromatic resource, lignin is an ideal precursor for the production of aromatic fine chemicals. Biotransformation of lignin into 4-vinylphenol (4VP) derivatives holds the promise to contribute to a lignin-based economy. However, the limited bioavailability of lignin and the absence of an effective biotransformation route for lignin to 4VPs have hindered the valorization of lignin and the production efficiency of 4VPs. This review aims to explore an atom-economic biotransformation route for lignin to 4VPs by summarizing the state-of-the-art technologies involved in the entire process, from upstream to downstream. Effective depolymerization technologies have enabled the generation of favorable lignin-derived aromatic precursors for the biosynthesis of 4VPs. Additionally, these technologies also yielded oligomers or monomers that can serve as carbon sources for the cell growth of host strains. Furthermore, the exploration of key enzymes and the construction of effective microbial cell factories have facilitated the assimilation of lignin derivatives and resulted in improved yields and titers of 4VPs. Emerging technologies have further enhanced the biotransformation of lignin by regulating metabolism efficiency, improving the stability and activity of key enzymes, and mitigating inhibitory effects. Overall, by harnessing the inherent aromatic value of lignin, the biotransformation of lignin-derived aromatic precursors into 4VPs offers a promising and sustainable route for 4VPs biosynthesis and lignin valorization. This approach aligns with the atom-economic concept and holds great potential. [ABSTRACT FROM AUTHOR]

As a natural photoluminescent (PL) polymer, lignin has enormous potential in the fabrication of sustainable and environmentally friendly luminescent materials for various applications such as bio-imaging and chemical sensing. However, lignin exhibits weak PL emission due to its limited conjugated structures, which has largely restricted its further applications in PL techniques. In the present study, a one-step ethylenediamine (EDA) pretreatment strategy was designed to directly fractionate lignin from biomass and endowed with electron-rich N atoms or electron-donating subunits. The fractionated lignin showed an increase in its PL intensity by 6.6 times in an aqueous solution. The results show that the introduction of electron-donating N-containing groups (–NH2, –NH–, –C＝N–, and –CONH–) into lignin could red-shift the wavelength through decreasing energy gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and increase the efficiency of clusteroluminescence (CL) simultaneously. This aminated lignin solution shows two-stage concentration-dependent PL, excitation-dependent luminescence effect, multiple emission peaks, and red-shift fluorescence emission, which is solid evidence for the presence of clusterization-triggered emission (CTE). Aggregation of N-containing groups contributes to the electron (n → π*) transition and inter-and/or intra-molecular electron delocalization. It also strengthens the through-space interaction between N and O, stabilizes the structure from non-radiative rotation, and prevents self-quenching. Additionally, due to the presence of numerous basic groups, the fluorescence behaviors of aminated lignin have good pH responsiveness. This amination and amidation approach provides a novel route for broadening the scope of lignin utilization in the field of organic fluorescence materials and helps gain further mechanistic insight into CL. [ABSTRACT FROM AUTHOR]

Lignin, the largest renewable aromatic resource, is a promising alternative feedstock for the sustainable production of various chemicals, fuels, and materials. Despite this potential, lignin is characterized by heterogeneous and macromolecular structures that must be addressed. In this review, we present biological lignin conversion routes (BLCRs) that offer opportunities for overcoming these challenges, making lignin valorization feasible. Funneling heterogeneous aromatics via a 'biological funnel' offers a high-specificity bioconversion route for aromatic platform chemicals. The inherent aromaticity of lignin drives atom-economic functionalization routes toward aromatic natural product generation. By harnessing the ligninolytic capacities of specific microbial systems, powerful aromatic ring-opening routes can be developed to generate various value-added products. Thus, BLCRs hold the promise to make lignin valorization feasible and enable a lignocellulose-based bioeconomy. Biological lignin conversion routes (BLCRs) overcome the heterogeneous structures of lignin by harnessing the inherent capacity of ligninolytic microbes, opening the way to value-added products. Lignin depolymerization provides bioavailable aromatic derivatives suitable for downstream bioconversion. Atom-economic conversion routes are backbones in taping into the inherent aromaticity value of lignin to promote the microbial synthesis of valuable products. The scientific and technical evolution of synthetic biology enable the construction of microbial cell factories to improve lignin bioconversion. Lignin bioconversion would enable the economic viability of biorefineries and contribute to a sustainable lignocellulose-based bioeconomy. [ABSTRACT FROM AUTHOR]

As the most abundant natural aromatic resource, lignin valorization will contribute to a feasible biobased economy. Recently, biological lignin valorization has been advocated since ligninolytic microbes possess proficient funneling pathways of lignin to valuable products. In the present study, the potential to convert an actual lignin stream into polyhydroxyalkanoates (PHAs) had been evaluated using ligninolytic genome-reduced Pseudomonas putida. The results showed that the genome-reduced P. putida can grow well on an actual lignin stream to successfully yield a high PHA content and titer. The designed fermentation strategy almost eliminated the substrate effects of lignin on PHA accumulation. Employing a fed-batch strategy produced the comparable PHA contents and titers of 0.35 g/g dried cells and 1.4 g/L, respectively. The molecular mechanism analysis unveiled that P. putida consumed more small and hydrophilic lignin molecules to stimulate cell growth and PHA accumulation. Overall, the genome-reduced P. putida exhibited a superior capacity of lignin bioconversion and promote PHA accumulation, providing a promising route for sustainable lignin valorization. [ABSTRACT FROM AUTHOR]

Biological lignin valorization represents an emerging green approach to upgrade lignin for sustainable and economic biorefineries. However, lignin generally exhibits poor water solubility and inhomogeneous distribution in an aqueous medium, significantly limiting its bioconversion efficiency. Herein, we develop a novel alkali sterilization strategy to effectively enhance the dispersion and fermentation performance of lignin substrates. The colloidal particle size and molecular structure variations of lignin during the sterilization were thoroughly investigated to reveal the mechanisms of enhanced fermentation performance. Results showed that alkali sterilization achieved a completely aseptic effect when mixing lignin medium at an initial pH of 12.7 for 24 h. Dynamic light scattering (DLS) analysis demonstrated that the hydrodynamic volume of colloidal lignin particles decreased by 96.3% by alkali sterilization compared with the conventional thermal sterilization. Moreover, lignin characterizations by nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC) suggested that alkali sterilization modified the lignin molecular structure by generating 50% more hydrophilic carboxyl groups, reducing the weight-average molecular weight (Mw) by 23.0%, and narrowing the molar-mass dispersity (ĐM) by 23.8%. The generation of lignin substrates with more uniform distribution and lower molecular weight improved Rhodococcus opacus PD630 cell growth and metabolism. Microbial cell amount, lignin degradation, and lipid production in alkali sterilized medium increased by 309%, 30.3%, and 48.3%, respectively, compared to those in thermally sterilized medium. These results clearly demonstrated that alkali sterilization dramatically improved the lignin bioconversion performance. This work presents a facile and effective sterilization strategy to overcome inhomogeneous lignin distribution in aqueous fermentation media, showing great potentials as a platform technique for promoting biological lignin valorization. [ABSTRACT FROM AUTHOR]

Biological lignin valorization has emerged as a major solution for sustainable and cost-effective biorefineries. However, current biorefineries yield lignin with inadequate fractionation for bioconversion, yet substantial changes of these biorefinery designs to focus on lignin could jeopardize carbohydrate efficiency and increase capital costs. We resolve the dilemma by designing 'plug-in processes of lignin' with the integration of leading pretreatment technologies. Substantial improvement of lignin bioconversion and synergistic enhancement of carbohydrate processing are achieved by solubilizing lignin via lowering molecular weight and increasing hydrophilic groups, addressing the dilemma of lignin- or carbohydrate-first scenarios. The plug-in processes of lignin could enable minimum polyhydroxyalkanoate selling price at as low as $6.18/kg. The results highlight the potential to achieve commercial production of polyhydroxyalkanoates as a co-product of cellulosic ethanol. Here, we show that the plug-in processes of lignin could transform biorefinery design toward sustainability by promoting carbon efficiency and optimizing the total capital cost. The current biorefineries yield lignin with inadequate fractionation for bioconversion, yet substantial changes of these biorefinery designs could jeopardize carbohydrate efficiency and increase capital costs. Here the authors resolve the dilemma by designing 'plug-in processes of lignin' to enable economic waste valorization. [ABSTRACT FROM AUTHOR]

Biological lignin valorization represents a promising approach contributing to sustainable and economic biorefineries. The low level of valuable lignin‐derived products remains a major challenge hindering the implementation of microbial lignin conversion. Lignin's properties play a significant role in determining the efficiency of lignin bioconversion. To date, despite significant progress in the development of biomass pretreatment, lignin fractionation, and fermentation over the last few decades, little efforts have gone into identifying the ideal lignin substrates for an efficient microbial metabolism. In this Minireview, emerging and state‐of‐the‐art strategies for biomass pretreatment and lignin fractionation are summarized to elaborate their roles in modifying lignin structure for bioconversion. Fermentation strategies aimed at enhancing lignin depolymerization for microbial utilization are systematically reviewed as well. With an improved understanding of the ideal lignin structure elucidated by comprehensive metabolic pathways and/or big data analysis, modifying lignin chemistry could be more directional and effective. Ultimately, together with the progress of fermentation process optimization, biological lignin valorization will become more competitive in biorefineries. [ABSTRACT FROM AUTHOR]

Bioconversion of bioresources/wastes (e.g., lignin, chemical pulping byproducts) represents a promising approach for developing a bioeconomy to help address growing energy and materials demands. Rhodococcus , a promising microbial strain, utilizes numerous carbon sources to produce lipids, which are precursors for synthesizing biodiesel and aviation fuels. However, compared to chemical conversion, bioconversion involves living cells, which is a more complex system that needs further understanding and upgrading. Various wastes amenable to bioconversion are reviewed herein to highlight the potential of Rhodococci for producing lipid-derived bioproducts. In light of the abundant availability of these substrates, Rhodococcus' metabolic pathways converting them to lipids are analyzed from a "beginning-to-end" view. Based on an in-depth understanding of microbial metabolic routes, genetic modifications of Rhodococcus by employing emerging tools (e.g., multiplex genome editing, biosensors, and genome-scale metabolic models) are presented for promoting the bioconversion. Co-solvent enhanced lignocellulose fractionation (CELF) strategy facilitates the generation of a lignin-derived aromatic stream suitable for the Rhodococcus' utilization. Novel alkali sterilization (AS) and elimination of thermal sterilization (ETS) approaches can significantly enhance the bioaccessibility of lignin and its derived aromatics in aqueous fermentation media, which promotes lipid titer significantly. In order to achieve value-added utilization of lignin, biodiesel and aviation fuel synthesis from lignin and lipids are further discussed. The possible directions for unleashing the capacity of Rhodococcus through synergistically modifying microbial strains, substrates, and fermentation processes are proposed toward a sustainable biological lignin valorization. [Display omitted] [ABSTRACT FROM AUTHOR]

As the largest renewable aromatic resource, lignin is a promising feedstock for production of value-added products. However, lignin valorization has not been implemented due to the recalcitrant and heterogeneity of lignin. Herein, this work provides a systematic overview of bacterial lignin valorization for producing value-added products from the viewpoint of a cascaded conversion route. The combinatorial depolymerization strategy facilitates the yield of a lignin-derived aromatic stream suitable for the bacterial conversion. Bacterial active transports are curial to improve the uptake of lignin-derived aromatics. Intracellular metabolic pathways of bacteria assimilate heterogenous lignin-derived aromatics through "biological funnel" into central aromatic intermediates. These intermediates can be effectively metabolized in bacteria through aromatic ring cleavage pathways to enable the biosynthesis of various value-added products. The techno-economic analysis highlights that bacterial conversion improves the feasibility of co-production of value-added products from lignin. Therefore, the bacterial cascaded conversion routes hold great promise for upgrading heterogeneous lignin into value-added products and thus contribute to the profitability of lignin valorization. Cascaded conversion routes of bacterial lignin valorization facilitate production of value-added products. [Display omitted] • Cascaded conversion route paves the way for bacterial lignin valorization. • Combinatorial fractionation improves lignin depolymerization and bioconversion. • Bacterial active transports of aromatics are crucial to lignin bioconversion. • Intracellular metabolic pathways assimilate heterogenous lignin-derived aromatics. • Ligninolytic bacteria can convert lignin to value-added products. [ABSTRACT FROM AUTHOR]

Bioconversion of lignin into value-added chemicals could promote lignin valorization and the lignin-based bioeconomy. The heterogeneity nature of lignin and the lack of efficient conversion pathways pose limitations on the efficiency of lignin conversion. In this study, Pseudomonas putida KT2440 was successfully engineered for converting lignin to β -ketoadipic acid, a C6 keto-diacid that functions as building block for high performance polymers. Blocking the degradation pathway of β -ketoadipic acid enabled its biosynthesis from both H- and G-type lignin-derived monomers. The aromatic hydroxylation and O -demethylation was identified as the rate-limiting step for producing β -ketoadipic acid from H- and G- type lignin-derived monomers, respectively. Overexpressing pobA and vanAB successfully vanished the accumulation of 4-hydroxybenzoic acid and vanillic acid within 36 h and 30 h, respectively. The beneficial gene operations were integrated into the chromosome to create the strain HE65. Employing this strain, fed-batch strategies were used to achieve a record β -ketoadipic acid production of 14.80 g/L from mixed lignin-derived aromatics in shake-flask experiments. Remarkably, the strain could also thrive in actual lignin hydrolysates from corn stover, yielding a record β -ketoadipic acid output of 2.48 g/L without the need for separation or detoxification processes. Overall, these results underscore the significant potential of engineering P. putida for converting lignin into β -ketoadipic acid, offering a promising pathway for feasible lignin valorization. [Display omitted] • Pseudomonas putida KT2440 was designed for converting lignin into β-ketoadipic acid. • Overexpression of rate-limiting enzyme eliminated 4-HBA and VA accumulation. • Fed-batch strategy promoted β-ketoadipic acid production from mixed lignin-derived aromatics. • The cell factory yielded a recorded titer of β-ketoadipic acid from lignin hydrolysates. [ABSTRACT FROM AUTHOR]

• Tandem fractionation strategies (TAFT) were designed to tune lignin chemistries. • TAFT produced the lignins with unique chemistries to define the structure of PCs. • PCs possessed high surface area, porous structure and abundant functional groups. • PCs exhibited the excellent adsorption capacity of methylene blue and congo red. • TAFT provided new opportunities of lignin valorization in environmental remediation. Valorization of lignin into functional materials has great potential for advancing the environmental sustainability and economic feasibility of biorefineries. However, the chemistries of lignin varied with the fractionation option, which could affect the fabrication of functional materials such as porous carbon (PC). Herein, several tandem fractionation strategies were designed in a biorefinery concept to tune lignin chemistries for fabricating functional PC toward environmental remediation. The results showed tandem fractionation produced soluble lignins with higher purity, less interunit linkage, and lower molecular weight, while yielding lignins in the solid fraction with higher syringyl/guaiacyl ratio, higher molecular weight, and more residual carbohydrates. The PCs prepared from the soluble lignin had higher specific surface area (1129–1389 m2/g), higher total pore volumes (0.63–0.76 cm3/g), and more micropores, exhibiting excellent adsorption performance of methylene blue ranging from 469.2 to 563.5 mg/g. Meanwhile, the PCs prepared from the insoluble lignin had larger pores, lower specific surface area (353–976 m2/g), and abundant oxygen-containing groups, which showed remarkable adsorption performance of congo red ranging from 384.3 to 1058.9 mg/g. Interestingly, tandem fractionation produced alkaline-fermented lignin (AL-FL) with notable characteristics, including more syringyl-type units, uniform structure and high molecular weight, which promoted the formation of graphitic and porous structures. Additionally, AL-FL introduced nitrogen-containing functional groups and enhanced the surface chemistry of the resulting PCs, which demonstrated exceptional adsorption functionality for organic dyes. Overall, these findings provided new opportunities of lignin valorization in a biorefinery, specifically for the production of functional materials aimed at environmental remediation. [ABSTRACT FROM AUTHOR]

[Display omitted] • An easy-recycled ethylenediamine pretreatment (EREP) was developed. • EREP significantly improved the enzymatic hydrolysis of carbohydrates. • EREP introduced Schiff base and amide structures in the lignin molecules. • EDA-lignin showed a pH-responsive behavior and excellent flocculation capacity. • The molecular mechanism was proposed for the lignin modification by EREP. Lignocellulosic biomass (LCB) is the promising feedstock for value-added products, which would contribute to the bioeconomy and sustainable development. The efficient pretreatment is still required in the biorefinery of LCB. To make a simultaneous utilization of carbohydrates and lignin, a novel easy-recycled ethylenediamine (EDA) pretreatment was designed and evaluated in the present study. The results highlighted that this pretreatment yielded 96% glucose and 70% xylose in enzymatic hydrolysis. It simultaneously promoted the depolymerization of lignin into small molecules and functionalized the yielded lignin with Schiff base and amide structures. These animated-lignins showed a pH-responsive behavior and the excellent flocculation capacity by reducing more than 90% turbidity of kaolin suspensions. Therefore, easy-recycled EDA pretreatment hold the promise to simultaneously enhance the enzymatic hydrolysis of carbohydrates and endowed the new functionality of lignin toward downstream valorization, which improved the process feasibility and potentially enable the sustainability of LCB utilization. [ABSTRACT FROM AUTHOR]

• CELF lignin facilitates Rhodococcus opacus PD630 growth and lipid accumulation. • CELF cleaves β -O-4 and β -5 bonds completely. • Increases in carboxylic acid OH group of CELF lignin benefit microbial conversion. • CELF lignin is a promising candidate for producing lipid building blocks. 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. [ABSTRACT FROM AUTHOR]