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.

Description of Sphingobium psychrophilum sp. nov., a cold-adapted bacterium isolated from Arctic soil

A yellow-coloured, Gram-stain-negative, non-sporulating, psychrotolerant and motile bacterium, designated AR-3-1^T, was isolated from the Arctic soil of Cambridge Bay, Nunavut, Canada. Strain AR-3-1^T could grow at 4-32 °C and pH 5.0- 11.0. Phylogenetic analysis based on its 16S rRNA gene sequence indicated that strain AR-3-1^T formed a lineage within the family Sphingomonadaceae and clustered as a member of the genus Sphingobium. The closest members within this genus were Sphingobium cupriresistens CU4^T (98.1 % sequence similarity), Sphingobium vermicomposti VC-230^T (97.6 %) and Sphingobium lactosutens DS20^T (97.5 %). The only respiratory quinone was the ubiquinone Q-10. Spermidine was the predominant polyamine. The principal cellular fatty acids were summed feature 8 (C_18 : 1  ω7c and/or C_18 : 1  ω6c), summed feature 3 (iso-C_{15  : 0} 2-OH and/or C_{16  : 1} ω7c), C_16 : 0 and C_14 : 0 2-OH. The major polar lipids were phosphatidylethanolamine, phosphatidylmonomethylethanolamine, phosphatidyldimethylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, sphingoglycolipid and phosphoglycolipid. The DNA G+C content was 63.1 %. The average nucleotide identity and in silico DNA-DNA hybridization relatedness values between strain AR-3-1^T and its most closely related genus members were ≤89.6 and 39.6 %, respectively. The genome was 5 162 327 bp long, with 83 scaffolds and 4824 protein-coding genes. The genome showed six putative biosynthetic gene clusters responsible for various secondary metabolites. Based on this polyphasic study, strain AR-3-1^T represents a novel species within the genus Sphingobium, for which the name Sphingobium psychrophilum sp. nov. is proposed. The type strain is AR-3-1^T (=KACC 21613^T=NBRC 114604^T).

Sphingobium paulinellae sp. nov. and Sphingobium algicola sp. nov., isolated from a freshwater green alga Paulinella chromatophora

Two Gram-stain-negative, aerobic, catalase- and oxidase-positive, yellow-pigmented bacteria, designated strains Pch-B^T and Pch-E^T, were isolated from a green alga Paulinella chromatophora. Both strains were motile short rods with a flagellum and optimally grew at pH 6.0-7.0, 25-30 °C and 0-1 % NaCl. They contained ubiquinone-10 as the major quinone, spermidine as the major polyamine, C16 : 0, C14 : 0 2-OH and summed features 3 (comprising C16 : 1ω7c and/or C16 : 1ω6c) and 8 (comprising C18 : 1ω7c and/or C18 : 1ω6c) as the major fatty acids and diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, phosphatidyl-N-methylethanolamine, two sphingoglycolipids and phosphatidylcholine as the major polar lipids. DNA G+C contents of strains Pch-B^T and Pch-E^T were 61.4 and 63.9 mol%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strains Pch-B^T and Pch-E^T formed a phylogenic lineage with Sphingobium limneticum 301^T. Strains Pch-B^T and Pch-E^T had a 99.86 % 16S rRNA gene sequence similarity and their DNA-DNA hybridization (DDH) relatedness value was 40.6 %. Strains Pch-B^T and Pch-E^T were most closely related to S. limneticum 301^T with 99.93 and 99.76 % 16S rRNA gene sequence similarities, respectively and the DDH values between strains Pch-B^T and Pch-E^T and the type strain of S. limneticum were 43.3 and 32.1 %, respectively. In conclusion, strains Pch-B^T and Pch-E^T represent novel species of the genus Sphingobium, for which the names Sphingobiumpaulinellae sp. nov. and Sphingobium algicola sp. nov. are proposed, respectively. The type strains of S. paulinellae and S. algicola are Pch-B^T (=KACC 19283^T=JCM 32054^T) and Pch-E^T (=KACC 19284^T=JCM 32055^T), respectively.

Description of Sphingomonas mohensis sp. nov., Isolated from Sediment

An aerobic, gram-reaction-negative, rod-shaped bacterium, designated strain Z6(T), was isolated from sediment collected at Mohe Basin, China. And its taxonomic position was investigated by applying a polyphasic approach. Growth occurs at 10-45 °C (optimum, 30 °C), at pH 6.0-11.0 (optimum, pH 7.0) and in the presence of 0-3.0 % (w/v) NaCl (optimum, 0 %). The polar lipid profile of strain Z6(T) revealed the presence of diphosphatidylglycerol, phosphatidylethanolamine, sphingoglycolipid and two unidentified phospholipids, and the major quinone was Q-10. The major fatty acids were C18:1 ω7c and/or C18:1 ω6c (summed feature 8) and C16:1 ω6c and/or C16:1 ω7c (summed feature 3). The predominant polyamine was homospermidine. The DNA G + C content of strain Z6(T) is 65.2 mol%. On the basis of the polyphasic evidence presented, strain Z6(T) represents a novel species of the genus Sphingomonas, for which the name Sphingomonas mohensis sp. nov. is proposed. The type strain is Z6(T) (=CGMCC 1.12891(T) = JCM 19983(T)).