Sphingobium lignivorans sp. nov., isolated from river sediment downstream of a paper mill

Genomic-based taxonomic classification of the order Sphingomonadales

The order Sphingomonadales strains are globally distributed in various biomes and are renowned for their biodegradable and biosynthesis capabilities. At present, it consists of 4 families and 49 genera making it the third largest order within the class Alphaproteobacteria. However, their taxonomy remains complex, especially due to polyphyly in the family Sphingomonadaceae. In this study, we collected 429 Sphingomonadales type strain genomes, reconstructed robust phylogenomic relationships, and proposed delineation thresholds at the genus and family levels based on average amino acid identities (AAI) and evolutionary distances (ED). Based on the maximum-likelihood and Bayesian phylogenomic trees reconstructed by two molecular sets determined by orthologous sequence identity and the Genome Taxonomy Database, the consensus degree values were all higher than 90%, revealing that those phylogenomic trees had similar topological structures. By confirming monophyletic taxa and determining stable nodes, we reclassified the order Sphingomonadales into thirteen families including nine novel ones. AAI calculations indicated that the average intra-family AAI values ranged from 0.62 to 0.84, while inter-family ones were 0.51 to 0.60. ED summaries demonstrated that the average and median intra-family ED values were 0.16 to 0.57, and inter-family ones ranged from 0.50 to 1.22. Comparisons of AAI and ED values calculated by using genomic and phylogenetic analyses supported that those 13 families were significantly separated with p values < 2.2×10-16. Thus, it was speculated that the AAI and ED thresholds for distinguishing different families were <0.6 and >0.5, respectively. Additionally, we reclassified 163 species into new genera with their phylogenetic topologies, according to the previous genus AAI and ED boundaries of 0.7 and 0.4. Our study is the first genomic-based study of the order Sphingomonadales and will promote further insights into the evolution of this order.

Bacterial community profiling and identification of bacteria with lignin-degrading potential in different gut segments of African palm weevil larvae (Rhynchophorus phoenicis)

The microbiota within the guts of insects plays beneficial roles for their hosts, such as facilitating digestion and extracting energy from their diet. The African palm weevil (APW) lives within and feeds on the high lignin-containing trunk of palm trees; therefore, their guts could harbour a large community of lignin-degrading microbes. In this study, we aimed to explore the bacterial community within the gut of the APW larvae, specifically with respect to the potential for lignin degradation in various gut segments as a first step to determining the viability of mining bacterial lignin-degrading enzymes for the bioconversion of lignocellulosic biomass to biofuels and biomaterials. Bacterial metagenomic DNA was extracted from the foregut, midgut, and hindgut of larvae of the APW, and the V3-V4 hypervariable region of the 16S rRNA gene was sequenced using the Illumina MiSeq platform. The generated data were analysed and taxonomically classified to identify the different bacterial phylotypes within the gut community cumulatively and per gut segment. We then determined the presence, diversity, and abundance of bacteria associated with lignin degradation within each larval gut compartment as a basis for suggesting the gut segment(s) where lignin degradation occurs the most. All sequences were classified and belonged to the bacterial kingdom. Firmicutes (54.3%) and Proteobacteria (42.5%) were the most dominant phyla within the gut, followed distantly by Bacteroidota (1.7%) and Actinobacteriota (1.4%). Enterococcus, Levilactobacillus, Lactococcus, Shimwellia, Megasphaera, Klebsiella, Pectinatus, Salmonella, Lelliotia, and Enterobacter constituted the most abundant genera found across all gut segments. The foregut and midgut had many similar genera, whilst the hindgut appeared unique. Overall, 29.5% of total gut bacteria comprising 21 genera were lignin degraders found predominantly in the Firmicutes and Proteobacteria phyla (56.8 and 39.5%, respectively), then moderately in Actinobacteriota (2.5%) and Bacteroidota (1.1%). The most abundant ligninolytic genera were Levilactobacillus (46.4%), Klebsiella (22.9%), Enterobacter (10.7%), Lactiplantibacillus (5.9%), Citrobacter (2.2%), Corynebacterium (1.8%), Paucilactobacillus (1.8%), Serratia (1.5%), Bacteroides (1.1%), and Leucobacter (1.0%) found in different amounts in different gut compartments. The foregut had the most diverse and highest abundance of lignin-degrading phylotypes, and we present reasons that point to the foregut as the main location for the depolymerization of lignin in the APW larval gut.

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.

Evolution and engineering of pathways for aromatic O-demethylation in Pseudomonas putida KT2440

Biological conversion of lignin from biomass offers a promising strategy for sustainable production of fuels and chemicals. However, aromatic compounds derived from lignin commonly contain methoxy groups, and O-demethylation of these substrates is often a rate-limiting reaction that influences catabolic efficiency. Several enzyme families catalyze aromatic O-demethylation, but they are rarely compared in vivo to determine an optimal biocatalytic strategy. Here, two pathways for aromatic O-demethylation were compared in Pseudomonas putida KT2440. The native Rieske non-heme iron monooxygenase (VanAB) and, separately, a heterologous tetrahydrofolate-dependent demethylase (LigM) were constitutively expressed in P. putida, and the strains were optimized via adaptive laboratory evolution (ALE) with vanillate as a model substrate. All evolved strains displayed improved growth phenotypes, with the evolved strains harboring the native VanAB pathway exhibiting growth rates ∼1.8x faster than those harboring the heterologous LigM pathway. Enzyme kinetics and transcriptomics studies investigated the contribution of selected mutations toward enhanced utilization of vanillate. The VanAB-overexpressing strains contained the most impactful mutations, including those in VanB, the reductase for vanillate O-demethylase, PP_3494, a global regulator of vanillate catabolism, and fghA, involved in formaldehyde detoxification. These three mutations were combined into a single strain, which exhibited approximately 5x faster vanillate consumption than the wild-type strain in the first 8 h of cultivation. Overall, this study illuminates the details of vanillate catabolism in the context of two distinct enzymatic mechanisms, yielding a platform strain for efficient O-demethylation of lignin-related aromatic compounds to value-added products.

Sphingobium cyanobacteriorum sp. nov., isolated from fresh water

A novel Gram-stain-negative, yellow-pigmented, short rod-shaped bacterial strain, HBC34T, was isolated from a freshwater sample collected from Daechung Reservoir, Republic of Korea. The results of 16S rRNA gene sequence analysis indicated that HBC34T was affiliated with the genus Sphingobium and shared the highest sequence similarity to the type strains of Sphingobium vermicomposti (98.01 %), Sphingobium psychrophilum (97.87 %) and Sphingobium rhizovicinum (97.59 %). The average nucleotide identity (ANI) and digital DNA-DNA hybridisation (dDDH) values between HBC34T and species of the genus Sphingobium with validly published names were below 84.01 and 28.1 %, respectively. These values were lower than the accepted species-delineation thresholds, supporting its recognition as representing a novel species of the genus Sphingobium. The major fatty acids (>10 % of the total fatty acids) were identified as summed feature 8 (C18 : 1ω7c and/or C18 : 1ω6c) and summed feature 3 (C16 : 1ω7c and/or C16 : 1ω6c). The main polar lipids were phosphatidylglycerol, phosphatidylcholine, phosphatidylethanolamine, two phospholipids and two unidentified polar lipids. The respiratory quinone was Q-10. The genomic DNA G+C content of HBC34T was 64.04 %. The polyphasic evidence supports the classification of HBC34T as the type strain of a novel species of the genus Sphingobium, for which the name Sphingobium cyanobacteriorum sp. nov is proposed. The type strain is HBC34T (= KCTC 8002T= LMG 33140T).

Catabolic System of 5-Formylferulic Acid, a Downstream Metabolite of a β-5-Type Lignin-Derived Dimer, in Sphingobium lignivorans SYK-6

Sphingobium lignivorans SYK-6 can assimilate various lignin-derived aromatic compounds, including a β-5-type (phenylcoumaran-type) dimer, dehydrodiconiferyl alcohol (DCA). SYK-6 converts DCA to a stilbene-type intermediate via multiple reaction steps and then to vanillin and 5-formylferulic acid (FFA). Here, we first elucidated the catabolic pathway of FFA, which is the only unknown pathway in DCA catabolism. Then, we identified and characterized the enzyme-encoding genes responsible for this pathway. Analysis of the metabolites revealed that FFA was converted to 5-carboxyferulic acid (CFA) through oxidation of the formyl group, followed by conversion to ferulic acid by decarboxylation. A comprehensive analysis of the aldehyde dehydrogenase genes in SYK-6 indicated that NAD+-dependent FerD (SLG_12800) is crucial for the conversion of FFA to CFA. LigW and LigW2, which are 5-carboxyvanillic acid decarboxylases involved in the catabolism of a 5,5-type dimer, were found to be involved in the conversion of CFA to ferulic acid, and LigW2 played a significant role. The ligW2 gene forms an operon with ferD, and their transcription was induced during growth in DCA.