Combination of Aspergillus niger MJ1 with Pseudomonas stutzeri DSM4166 or mutant Pseudomonas fluorescens CHA0-nif improved crop quality, soil properties, and microbial communities in barrier soil

Soil salinization and acidification seriously damage soil health and restricts the sustainable development of planting. Excessive application of chemical fertilizer and other reasons will lead to soil acidification and salinization. This study focus on acid and salinized soil, investigated the effect of phosphate-solubilizing bacteria, Aspergillus niger MJ1 combined with nitrogen-fixing bacteria Pseudomonas stutzeri DSM4166 or mutant Pseudomonas fluorescens CHA0-nif on crop quality, soil physicochemical properties, and microbial communities. A total of 5 treatments were set: regular fertilization (T1), regular fertilization with MJ1 and DSM4166 (T2), regular fertilization with MJ1 and CHA0-nif (T3), 30%-reducing fertilization with MJ1 and DSM4166 (T4), and 30%-reducing fertilization with MJ1 and CHA0-nif (T5). It was found that the soil properties (OM, HN, TN, AP, AK, and SS) and crop quality of cucumber (yield production, protein, and vitamin C) and lettuce (yield production, vitamin C, nitrate, soluble protein, and crude fiber) showed a significant response to the inoculated strains. The combination of MJ1 with DSM4166 or CHA0-nif influenced the diversity and richness of bacterial community in the lettuce-grown soil. The organismal system-, cellular process-, and metabolism-correlated bacteria and saprophytic fungi were enriched, which were speculated to mediate the response to inoculated strains. pH, OM, HN, and TN were identified to be the major factors correlated with the soil microbial community. The inoculation of MJ1 with DSM4166 and CHA0-nif could meet the requirement of lettuce and cucumber growth after reducing fertilization in acid and salinized soil, which provides a novel candidate for the eco-friendly technique to meet the carbon-neutral topic.

Genomic Features and Pervasive Negative Selection in Rhodanobacter Strains Isolated from Nitrate and Heavy Metal Contaminated Aquifer

Rhodanobacter species dominate in the Oak Ridge Reservation (ORR) subsurface environments contaminated with acids, nitrate, metal radionuclides, and other heavy metals. To uncover the genomic features underlying adaptations to these mixed-waste environments and to guide genetic tool development, we sequenced the whole genomes of eight Rhodanobacter strains isolated from the ORR site. The genome sizes ranged from 3.9 to 4.2 Mb harboring 3,695 to 4,035 protein-coding genes and GC contents approximately 67%. Seven strains were classified as R. denitrificans and one strain, FW510-R12, as R. thiooxydans based on full length 16S rRNA sequences. According to gene annotation, the top two Cluster of Orthologous Groups (COGs) with high pan-genome expansion rates (Pan/Core gene ratio) were “replication, recombination and repair” and “defense mechanisms.” The denitrifying genes had high DNA homologies except the predicted protein structure variances in NosZ. In contrast, heavy metal resistance genes were diverse with between 7 to 34% of them were located in genomic islands, and these results suggested origins from horizontal gene transfer. Analysis of the methylation patterns in four strains revealed the unique 5mC methylation motifs. Most orthologs (78%) had ratios of nonsynonymous to synonymous substitutions (dN/dS) less than one when compared to the type strain 2APBS1, suggesting the prevalence of negative selection. Overall, the results provide evidence for the important roles of horizontal gene transfer and negative selection in genomic adaptation at the contaminated field site. The complex restriction-modification system genes and the unique methylation motifs in Rhodanobacter strains suggest the potential recalcitrance to genetic manipulation.

IMPORTANCE Despite the dominance of Rhodanobacter species in the subsurface of the contaminated Oak Ridge Reservation (ORR) site, very little is known about the mechanisms underlying their adaptions to the various stressors present at ORR. Recently, multiple Rhodanobacter strains have been isolated from the ORR groundwater samples from several wells with varying geochemical properties. Using Illumina, PacBio, and Oxford Nanopore sequencing platforms, we obtained the whole genome sequences of eight Rhodanobacter strains. Comparison of the whole genomes demonstrated the genetic diversity, and analysis of the long nanopore reads revealed the heterogeneity of methylation patterns in strains isolated from the same well. Although all strains contained a complete set of denitrifying genes, the predicted tertiary structures of NosZ differed. The sequence comparison results demonstrate the important roles of horizontal gene transfer and negative selection in adaptation. In addition, these strains may be recalcitrant to genetic manipulation due to the complex restriction-modification systems and methylations.

Sandarakinorhabdus rubra sp. nov., and Sandarakinorhabdus oryzae sp. nov., isolated from oxidized rice paddy soil

Three Gram-stain-negative, motile or non-motile, rod-shaped, facultatively aerobic strains, designated MO-4^T, NP-34 and NM-18^T, were isolated from oxidized rice paddy soil in Chungbuk, Republic of Korea. Colonies were circular and convex with entire margins, red in colour on R2A after 3 days at 30 °C. The three strains grew at pH 5.0-10.0 (optimum, pH 8.0), at 15-45 °C (optimum, 30 °C) and at salinities of 0-1.5 % (w/v) NaCl (optimum, 0.4 % NaCl). The results of phylogenetic analyses based on 16S rRNA gene sequences indicated that the three isolates represent members of the genus Sandarakinorhabdus and strains MO-4^T and NP-34 were most closely related to Sandarakinorhabdus cyanobacteriorum TH057^T (97.7 %) and Sandarakinorhabdus limnophila DSM 17366^T (97.1 %). NM-18^T showed highest 16S rRNA gene sequence similarities to Sandarakinorhabdus limnophila DSM 17366^T (98.7 %) and Sandarakinorhabdus cyanobacteriorum TH057^T (96.7 %). Genomic similarities between strains MO-4^T and NM-18^T and the two type strains of species of the genus Sandarakinorhabdus based on average nucleotide identity and digital DNA-DNA hybridization values were lower than the species delineation thresholds. The major fatty acids were iso-C_18 : 1 ω7c and summed feature 3. The DNA G+C contents of strains MO-4^T and NM-18^T, obtained from genome sequencing data, were 67.6 and 66.6 mol%, respectively. On the basis of these genotypic and phenotypic characteristics, the three strains are assigned to two novel species of the genus Sandarakinorhabdus, for which the names Sandarakinorhabdus rubra sp. nov. (type strain MO-4^T =KACC 21378=NBRC 114106) and Sandarakinorhabdus oryzae sp. nov. (type strain NM-18^T=KACC 21379=NBRC 113957) are proposed.

Seabird and pinniped shape soil bacterial communities of their settlements in Cape Shirreff, Antarctica

Seabirds and pinnipeds play an important role in biogeochemical cycling by transferring nutrients from aquatic to terrestrial environments. Indeed, soils rich in animal depositions have generally high organic carbon, nitrogen and phosphorus contents. Several studies have assessed bacterial diversity in Antarctic soils influenced by marine animals; however most have been conducted in areas with significant human impact. Thus, we chose Cape Shirreff, Livingston Island, an Antarctic Specially Protected Area designated mainly to protect the diversity of marine vertebrate fauna, and selected sampling sites with different types of animals coexisting in a relatively small space, and where human presence and impact are negligible. Using 16S rRNA gene analyses through massive sequencing, we assessed the influence of animal concentrations, via their modification of edaphic characteristics, on soil bacterial diversity and composition. The nutrient composition of soils impacted by Antarctic fur seals and kelp gulls was more similar to that of control soils (i.e. soils without visible presence of plants or animals), which may be due to the more active behaviour of these marine animals compared to other species. Conversely, the soils from concentrations of southern elephant seals and penguins showed greater differences in soil nutrients compared to the control. In agreement with this, the bacterial communities of the soils associated with these animals were most different from those of the control soils, with the soils of penguin colonies also possessing the lowest bacterial diversity. However, all the soils influenced by the presence of marine animals were dominated by bacteria belonging to Gammaproteobacteria, particularly those of the genus Rhodanobacter. Therefore, we conclude that the modification of soil nutrient composition by marine vertebrates promotes specific groups of bacteria, which could play an important role in the recycling of nutrients in terrestrial Antarctic ecosystems.

Rhodanobacter ginsengiterrae sp. nov., an antagonistic bacterium against root rot fungal pathogen Fusarium solani, isolated from ginseng rhizospheric soil

A novel bacterium, designated DCY112^T, was isolated from the rhizospheric soil of a ginseng-cultivated field in Gochang-gun, Republic of Korea. Based on 16S rRNA gene sequence analysis, this isolate was assigned to the genus Rhodanobacter and is closely related to Rhodanobacter soli DCY45^T (98.0%) and R. umsongensis GR24-2^T (98.0%). Strain DCY112^T is Gram-negative, catalase- and oxidase-positive, aerobic, non-motile, rod-shaped, and produces yellow-pigmented colonies on R2A medium. Q-8 was the predominant respiratory quinone. The major cellular fatty acids were iso-C_15:0, iso-C_17:0, and summed feature 9 (iso-C_17:1 ω9c and/or 10-methyl-C_16:0). The major polar lipids were phosphatidylglycerol (PG), phosphatidylethanolamine (PE), an unknown amino lipid (AL1), and an unidentified polar lipid (L3). The genomic DNA G + C content was 65.2 mol%. DNA-DNA homology values between strain DCY112^T and related strains were lower than 55%. The low DNA relatedness data in combination with phenotypic and genotypic tests indicated that strain DCY112^T could not be assigned to a recognized species. Strain DCY112^T showed antagonistic activity against the fungal pathogen Fusarium solani (KACC 44891^T), which causes ginseng root rot. The results of this study support that strain DCY112^T is a novel species belonging to the genus Rhodanobacter, for which the name Rhodanobacter ginsengiterrae is proposed. The type strain is DCY112^T (= KCTC 62018^T = JCM 32167^T).

Rhodanobacter hydrolyticus sp. nov., a novel DNA- and tyrosine-hydrolysing gammaproteobacterium isolated from forest soil

A bacterial isolate, designated G-5-5^T, was isolated from forest soil at Kyonggi University. Strain G-5-5^T was acid-tolerant and alkali-tolerant. Cells were strictly aerobic, Gram-stain-negative, catalase- and oxidase-positive, non-motile, non-spore-forming, rod-shaped, and yellow-coloured. Strain G-5-5^T hydrolysed DNA and tyrosine; assimilated d-glucose, maltose, N-acetyl-glucosamine and l-fucose; and tolerated only 0.5 % NaCl (w/v). Phylogenetic analysis based on its 16S rRNA gene sequence revealed that strain G-5-5^T formed a lineage within the family Rhodanobacteraceae and that it grouped with but was distinct from various members of the genus Rhodanobacter. The closest member was Rhodanobacter umsongensis GR24-2^T (97.8 % sequence similarity). The sole respiratory quinone was Q-8. The major polar lipids of strain G-5-5^T were phosphatidylethanolamine, phosphatidyl-N-methylethanolamine, phosphatidylglycerol and diphosphatidylglycerol. The major cellular fatty acids were summed feature 9 (iso-C17 : 1ω9c and/or C16 : 0 10-methyl), iso-C15 : 0, iso-C17 : 0, iso-C16 : 0 and anteiso-C15 : 0. The DNA G+C content of strain G-5-5^T was 64.1 mol%. DNA-DNA hybridization relatedness between strain G-5-5^T and other close members of the genus Rhodanobacter ranged from 19 % to 45 %. On the basis of the polyphasic characterization and phylogenetic analyses, strain G-5-5^T represents a novel species of the genus Rhodanobacter, for which the name Rhodanobacter hydrolyticus sp. nov. is proposed. The type strain is G-5-5^T (=KEMB 9005-533^T=KACC 19113^T=NBRC 112685^T).