Field-based stable isotope probing reveals the identities of benzoic acid-metabolizing microorganisms and their in situ growth in agricultural soil

Benzoic acid facilitates ANF in monocot crops by recruiting nitrogen-fixing Paraburkholderia

Associative nitrogen fixation contributes large portion of N input to agro-ecosystems through monocot-diazotrophic associations. However, the contribution of associative nitrogen fixation is usually neglected in modern agriculture, and the underlying mechanisms of association between monocot and diazotrophs remain elusive. Here, we demonstrated that monocot crops employ mucilage and associated benzoic acid to specially enrich diazotrophic partners in response to nitrogen deficiency, which could be used for enhancing associative nitrogen fixation in monocot crops. To be specific, mucilage and benzoic acid induced in sugarcane roots by nitrogen deficiency mediated enrichment of nitrogen-fixing Paraburkholderia through specific recruitment whereas other bacteria were simultaneously repelled. Further studies suggest maize employs a similar strategy in promoting associations with diazotrophs. In addition, our results also suggest that benzoic acid application significantly increases copy numbers of the nifH gene in soils and enhances associative nitrogen fixation in maize using 15N enrichment assay. Taken together, these results reveal a mechanism regulating the association between monocot crops and nitrogen-fixing bacteria, and, thereby point towards ways to harness these beneficial microbes in efforts to increase nitrogen efficiency in monocot crops through pathways regulated by a specific signaling molecule.

Evaluation of Phenolic Root Exudates as Stimulants of Saptrophic Fungi in the Rhizosphere

The rhizosphere microbial community of crop plants in intensively managed arable soils is strongly dominated by bacteria, especially in the initial stages of plant development. In order to establish more diverse and balanced rhizosphere microbiomes, as seen for wild plants, crop variety selection could be based on their ability to promote growth of saprotrophic fungi in the rhizosphere. We hypothesized that this can be achieved by increasing the exudation of phenolic acids, as generally higher fungal abundance is observed in environments with phenolic-rich inputs, such as exudates of older plants and litter leachates. To test this, a rhizosphere simulation microcosm was designed to establish gradual diffusion of root exudate metabolites from sterile sand into arable soil. With this system, we tested the fungus-stimulating effect of eight phenolic acids alone or in combination with primary root metabolites. Ergosterol-based fungal biomass measurements revealed that most phenolic acids did not increase fungal abundance in the arable soil layer. These results were supported by comparison of fungal biomass in the rhizosphere of wild type Arabidopsis thaliana plants and mutants with altered phenolic acid metabolism. Salicylic acid was the only phenolic acid that stimulated a higher fungal biomass in the arable soil layer of microcosms, but only when combined with a background of primary root metabolites. However, such effect on rhizosphere fungi was not confirmed for a salicylic acid-impaired A. thaliana mutant. For three phenolic acid treatments (chlorogenic acid, salicylic acid, vanillic acid) fungal and bacterial community compositions were analyzed using amplicon sequencing. Despite having little effect on fungal biomass, phenolic acids combined with primary metabolites promoted a higher relative abundance of soil-borne fungi with the ability to invade plant roots (Fusarium, Trichoderma and Fusicolla spp.) in the simulated rhizosphere. Bacterial community composition was also affected by these phenolic acids. Although this study indicates that phenolic acids do not increase fungal biomass in the rhizosphere, we highlight a potential role of phenolic acids as attractants for root-colonizing fungi.

Diversity and functional prediction of microbial communities involved in the first aerobic bioreactor of coking wastewater treatment system

The pre-aerobic process of coking wastewater treatment has strong capacity of decarbonization and detoxification, which contribute to the subsequent dinitrogen of non-carbon source/heterotrophic denitrification. The COD removal rate can reach > 90% in the first aerobic bioreactor of the novel O/H/O coking wastewater treatment system during long-term operation. The physico-chemical characteristics of influent and effluent coking wastewater in the first aerobic bioreactor were analyzed to examine how they correlated with bacterial communities. The diversity of the activated sludge microbial community was investigated using a culture-independent molecular approach. The microbial community functional profiling and detailed pathways were predicted from the 16S rRNA gene-sequencing data by the PICRUSt software and the KEGG database. High-throughput MiSeq sequencing results revealed a distinct microbial composition in the activated sludge of the first aerobic bioreactor of the O/H/O system. Proteobacteria, Bacteroidetes, and Chlorobi were the decarbonization and detoxification dominant phyla with the relative abundance of 84.07 ± 5.45, 10.89 ± 6.31, and 2.96 ± 1.12%, respectively. Thiobacillus, Rhodoplanes, Lysobacter, and Leucobacter were the potential major genera involved in the crucial functional pathways related to the degradation of phenols, cyanide, benzoate, and naphthalene. These results indicated that the comprehensive understanding of the structure and function diversity of the microbial community in the bioreactor will be conducive to the optimal coking wastewater treatment.

Insights to plant-microbe interactions provide opportunities to improve resistance breeding against root diseases in grain legumes

Root and foot diseases severely impede grain legume cultivation worldwide. Breeding lines with resistance against individual pathogens exist, but these resistances are often overcome by the interaction of multiple pathogens in field situations. Novel tools allow to decipher plant-microbiome interactions in unprecedented detail and provide insights into resistance mechanisms that consider both simultaneous attacks of various pathogens and the interplay with beneficial microbes. Although it has become clear that plant-associated microbes play a key role in plant health, a systematic picture of how and to what extent plants can shape their own detrimental or beneficial microbiome remains to be drawn. There is increasing evidence for the existence of genetic variation in the regulation of plant-microbe interactions that can be exploited by plant breeders. We propose to consider the entire plant holobiont in resistance breeding strategies in order to unravel hidden parts of complex defence mechanisms. This review summarizes (a) the current knowledge of resistance against soil-borne pathogens in grain legumes, (b) evidence for genetic variation for rhizosphere-related traits, (c) the role of root exudation in microbe-mediated disease resistance and elaborates (d) how these traits can be incorporated in resistance breeding programmes.

Phylogenomic Study of Burkholderia glathei-like Organisms, Proposal of 13 Novel Burkholderia Species and Emended Descriptions of Burkholderia sordidicola, Burkholderia zhejiangensis, and Burkholderia grimmiae

Partial gyrB gene sequence analysis of 17 isolates from human and environmental sources revealed 13 clusters of strains and identified them as Burkholderia glathei clade (BGC) bacteria. The taxonomic status of these clusters was examined by whole-genome sequence analysis, determination of the G+C content, whole-cell fatty acid analysis and biochemical characterization. The whole-genome sequence-based phylogeny was assessed using the Genome Blast Distance Phylogeny (GBDP) method and an extended multilocus sequence analysis (MLSA) approach. The results demonstrated that these 17 BGC isolates represented 13 novel Burkholderia species that could be distinguished by both genotypic and phenotypic characteristics. BGC strains exhibited a broad metabolic versatility and developed beneficial, symbiotic, and pathogenic interactions with different hosts. Our data also confirmed that there is no phylogenetic subdivision in the genus Burkholderia that distinguishes beneficial from pathogenic strains. We therefore propose to formally classify the 13 novel BGC Burkholderia species as Burkholderia arvi sp. nov. (type strain LMG 29317^T = CCUG 68412^T), Burkholderia hypogeia sp. nov. (type strain LMG 29322^T = CCUG 68407^T), Burkholderia ptereochthonis sp. nov. (type strain LMG 29326^T = CCUG 68403^T), Burkholderia glebae sp. nov. (type strain LMG 29325^T = CCUG 68404^T), Burkholderia pedi sp. nov. (type strain LMG 29323^T = CCUG 68406^T), Burkholderia arationis sp. nov. (type strain LMG 29324^T = CCUG 68405^T), Burkholderia fortuita sp. nov. (type strain LMG 29320^T = CCUG 68409^T), Burkholderia temeraria sp. nov. (type strain LMG 29319^T = CCUG 68410^T), Burkholderia calidae sp. nov. (type strain LMG 29321^T = CCUG 68408^T), Burkholderia concitans sp. nov. (type strain LMG 29315^T = CCUG 68414^T), Burkholderia turbans sp. nov. (type strain LMG 29316^T = CCUG 68413^T), Burkholderia catudaia sp. nov. (type strain LMG 29318^T = CCUG 68411^T) and Burkholderia peredens sp. nov. (type strain LMG 29314^T = CCUG 68415^T). Furthermore, we present emended descriptions of the species Burkholderia sordidicola, Burkholderia zhejiangensis and Burkholderia grimmiae. The GenBank/EMBL/DDBJ accession numbers for the 16S rRNA and gyrB gene sequences determined in this study are LT158612-LT158624 and LT158625-LT158641, respectively.

Synergistic Processing of Biphenyl and Benzoate: Carbon Flow Through the Bacterial Community in Polychlorinated-Biphenyl-Contaminated Soil

Aerobic mineralization of PCBs, which are toxic and persistent organic pollutants, involves the upper (biphenyl, BP) and lower (benzoate, BZ) degradation pathways. The activity of different members of the soil microbial community in performing one or both pathways, and their synergistic interactions during PCB biodegradation, are not well understood. This study investigates BP and BZ biodegradation and subsequent carbon flow through the microbial community in PCB-contaminated soil. DNA stable isotope probing (SIP) was used to identify the bacterial guilds involved in utilizing ¹³C-biphenyl (unchlorinated analogue of PCBs) and/or ¹³C-benzoate (product/intermediate of BP degradation and analogue of chlorobenzoates). By performing SIP with two substrates in parallel, we reveal microbes performing the upper (BP) and/or lower (BZ) degradation pathways, and heterotrophic bacteria involved indirectly in processing carbon derived from these substrates (i.e. through crossfeeding). Substrate mineralization rates and shifts in relative abundance of labeled taxa suggest that BP and BZ biotransformations were performed by microorganisms with different growth strategies: BZ-associated bacteria were fast growing, potentially copiotrophic organisms, while microbes that transform BP were oligotrophic, slower growing, organisms. Our findings provide novel insight into the functional interactions of soil bacteria active in processing biphenyl and related aromatic compounds in soil, revealing how carbon flows through a bacterial community.

Microbial community dynamics and stability during an ammonia-induced shift to syntrophic acetate oxidation

Anaerobic digesters rely on the diversity and distribution of parallel metabolic pathways mediated by complex syntrophic microbial communities to maintain robust and optimal performance. Using mesophilic swine waste digesters, we experimented with increased ammonia loading to induce a shift from aceticlastic methanogenesis to an alternative acetate-consuming pathway of syntrophic acetate oxidation. In comparison with control digesters, we observed shifts in bacterial 16S rRNA gene content and in functional gene repertoires over the course of the digesters' 3-year operating period. During the first year, under identical startup conditions, all bioreactors mirrored each other closely in terms of bacterial phylotype content, phylogenetic structure, and evenness. When we perturbed the digesters by increasing the ammonia concentration or temperature, the distribution of bacterial phylotypes became more uneven, followed by a return to more even communities once syntrophic acetate oxidation had allowed the experimental bioreactors to regain stable operation. The emergence of syntrophic acetate oxidation coincided with a partial shift from aceticlastic to hydrogenotrophic methanogens. Our 16S rRNA gene analysis also revealed that acetate-fed enrichment experiments resulted in communities that did not represent the bioreactor community. Analysis of shotgun sequencing of community DNA suggests that syntrophic acetate oxidation was carried out by a heterogeneous community rather than by a specific keystone population with representatives of enriched cultures with this metabolic capacity.

Stable isotope probing in the metagenomics era: a bridge towards improved bioremediation

Microbial biodegradation and biotransformation reactions are essential to most bioremediation processes, yet the specific organisms, genes, and mechanisms involved are often not well understood. Stable isotope probing (SIP) enables researchers to directly link microbial metabolic capability to phylogenetic and metagenomic information within a community context by tracking isotopically labeled substances into phylogenetically and functionally informative biomarkers. SIP is thus applicable as a tool for the identification of active members of the microbial community and associated genes integral to the community functional potential, such as biodegradative processes. The rapid evolution of SIP over the last decade and integration with metagenomics provide researchers with a much deeper insight into potential biodegradative genes, processes, and applications, thereby enabling an improved mechanistic understanding that can facilitate advances in the field of bioremediation.

Efficient and direct identification of fructose fermenting and non-fermenting bacteria from calf gut microbiota using stable isotope probing and modified T-RFLP

The aim of this study was the development of an efficient method to identify the prebiotics-assimilating-bacteria in gut microbiota using DNA-stable isotope probing (DNA-SIP) technology. For efficient probing of microbiota with stable isotopes, a small-scale repeated batch culture using a low-carbon-source-containing medium was developed. Fecal samples from cattle were inoculated and [U-(13)C]-fructose was applied to the culture after 24 h stabilization. Organic acid production, pH value of the period and the total diversity of microorganisms of the culture were successfully maintained during the chasing period. DNA samples were extracted from the culture and were subjected to isopycnic centrifugation and fractionation in order to separate fructose fermenters from non-fermenters. T-RFLP (Terminal Restriction Fragment Length Polymorphism) and the modified T-RFLP of each fraction suggested that Streptococcus bovis was the most dominant fructose fermenter in this culture. In addition, we improved the modified T-RFLP method and successfully identified Lactobacillus vitulinus and Megasphaella eldenii as minor fructose-fermenters and several species of Clostridium cluster IV as non-fermenters. From these results we concluded that the methods shown here provide a means for assessing the importance of individual prebiotics on gut microbiota.

Identification of bacteria utilizing biphenyl, benzoate, and naphthalene in long-term contaminated soil

Bacteria were identified associated with biodegradation of aromatic pollutants biphenyl, benzoate, and naphthalene in a long-term polychlorinated biphenyl- and polyaromatic hydrocarbon-contaminated soil. In order to avoid biases of culture-based approaches, stable isotope probing was applied in combination with sequence analysis of 16 S rRNA gene pyrotags amplified from (13)C-enriched DNA fractions. Special attention was paid to pyrosequencing data analysis in order to eliminate the errors caused by either generation of amplicons (random errors caused by DNA polymerase, formation of chimeric sequences) or sequencing itself. Therefore, sample DNA was amplified, sequenced, and analyzed along with the DNA of a mock community constructed out of 8 bacterial strains. This warranted that appropriate tools and parameters were chosen for sequence data processing. (13)C-labeled metagenomes isolated after the incubation of soil samples with all three studied aromatics were largely dominated by Proteobacteria, namely sequences clustering with the genera Rhodanobacter Burkholderia, Pandoraea, Dyella as well as some Rudaea- and Skermanella-related ones. Pseudomonads were mostly labeled by (13)C from naphthalene and benzoate. The results of this study show that many biphenyl/benzoate-assimilating bacteria derive carbon also from naphthalene, pointing out broader biodegradation abilities of some soil microbiota. The results also demonstrate that, in addition to traditionally isolated genera of degradative bacteria, yet-to-be cultured bacteria are important players in bioremediation. Overall, the study contributes to our understanding of biodegradation processes in contaminated soil. At the same time our results show the importance of sequencing and analyzing a mock community in order to more correctly process and analyze sequence data.

Betaproteobacteria dominance and diversity shifts in the bacterial community of a PAH-contaminated soil exposed to phenanthrene

In this study, the PAH-degrading bacteria of a constructed wetland collecting road runoff has been studied through DNA stable isotope probing. Microcosms were spiked with (13)C-phenanthrene at 34 or 337 ppm, and bacterial diversity was monitored over a 14-day period. At 337 ppm, PAH degraders became dominated after 5 days by Betaproteobacteria, including novel Acidovorax, Rhodoferax and Hydrogenophaga members, and unknown bacteria related to Rhodocyclaceae. The prevalence of Betaproteobacteria was further demonstrated by phylum-specific quantitative PCR, and was correlated with a burst of phenanthrene mineralization. Striking shifts in the population of degraders were observed after most of the phenanthrene had been removed. Soil exposed to 34 ppm phenanthrene showed a similar population of degraders, albeit only after 14 days. Results demonstrate that specific Betaproteobacteria are involved in the main response to soil PAH contamination, and illustrate the potential of SIP approaches to investigate PAH biodegradation in soil.

Genomic analysis of the potential for aromatic compounds biodegradation in Burkholderiales

The relevance of the β-proteobacterial Burkholderiales order in the degradation of a vast array of aromatic compounds, including several priority pollutants, has been largely assumed. In this review, the presence and organization of genes encoding oxygenases involved in aromatics biodegradation in 80 Burkholderiales genomes is analysed. This genomic analysis underscores the impressive catabolic potential of this bacterial lineage, comprising nearly all of the central ring-cleavage pathways reported so far in bacteria and most of the peripheral pathways involved in channelling of a broad diversity of aromatic compounds. The more widespread pathways in Burkholderiales include protocatechuate ortho ring-cleavage, catechol ortho ring-cleavage, homogentisate ring-cleavage and phenylacetyl-CoA ring-cleavage pathways found in at least 60% of genomes analysed. In general, a genus-specific pattern of positional ordering of biodegradative genes is observed in the catabolic clusters of these pathways indicating recent events in its evolutionary history. In addition, a significant bias towards secondary chromosomes, now termed chromids, is observed in the distribution of catabolic genes across multipartite genomes, which is consistent with a genus-specific character. Strains isolated from environmental sources such as soil, rhizosphere, sediment or sludge show a higher content of catabolic genes in their genomes compared with strains isolated from human, animal or plant hosts, but no significant difference is found among Alcaligenaceae, Burkholderiaceae and Comamonadaceae families, indicating that habitat is more of a determinant than phylogenetic origin in shaping aromatic catabolic versatility.

Applications of DNA-stable isotope probing in bioremediation studies

DNA-stable isotope probing, a method to identify active microorganisms without the prerequisite of cultivation, has been widely applied in the study of microorganisms involved in the degradation of environmental pollutants. Recent advances and technique considerations in applying DNA-SIP in bioremediation are highlighted. A detailed protocol of a DNA-SIP experiment is provided.

Characterization of a polycyclic aromatic hydrocarbon degradation gene cluster in a phenanthrene-degrading Acidovorax strain

Acidovorax sp. strain NA3 was isolated from polycyclic aromatic hydrocarbon (PAH)-contaminated soil that had been treated in a bioreactor and enriched with phenanthrene. The 16S rRNA gene of the isolate possessed 99.8 to 99.9% similarity to the dominant sequences recovered during a previous stable-isotope probing experiment with [U-(13)C]phenanthrene on the same soil (D. R. Singleton, S. N. Powell, R. Sangaiah, A. Gold, L. M. Ball, and M. D. Aitken, Appl. Environ. Microbiol. 71:1202-1209, 2005). The strain grew on phenanthrene as a sole carbon and energy source and could mineralize (14)C from a number of partially labeled PAHs, including naphthalene, phenanthrene, chrysene, benz[a]anthracene, and benzo[a]pyrene, but not pyrene or fluoranthene. Southern hybridizations of a genomic fosmid library with a fragment of the large subunit of the ring-hydroxylating dioxygenase gene from a naphthalene-degrading Pseudomonas strain detected the presence of PAH degradation genes subsequently determined to be highly similar in both nucleotide sequence and gene organization to an uncharacterized Alcaligenes faecalis gene cluster. The genes were localized to the chromosome of strain NA3. To test for gene induction by selected compounds, RNA was extracted from amended cultures and reverse transcribed, and cDNA associated with the enzymes involved in the first three steps of phenanthrene degradation was quantified by quantitative real-time PCR. Expression of each of the genes was induced most strongly by phenanthene and to a lesser extent by naphthalene, but other tested PAHs and PAH metabolites had negligible effects on gene transcript levels.

Dynamic secondary ion mass spectrometry imaging of microbial populations utilizing C-labelled substrates in pure culture and in soil

We demonstrate that dynamic secondary ion mass spectrometry (SIMS)-based ion microscopy can provide a means of measuring (13)C assimilation into individual bacterial cells grown on (13)C-labelled organic compounds in the laboratory and in field soil. We grew pure cultures of Pseudomonas putida NCIB 9816-4 in minimal media with known mixtures of (12)C- and (13)C-glucose and analysed individual cells via SIMS imaging. Individual cells yielded signals of masses 12, 13, 24, 25, 26 and 27 as negative secondary ions indicating the presence of (12)C(-), (13)C(-), (24)((12)C(2))(-), (25)((12)C(13)C)(-), (26)((12)C(14)N)(-) and (27)((13)C(14)N)(-) ions respectively. We verified that ratios of signals taken from the same cells only changed minimally during a approximately 4.5 min period of primary O(2)(+) beam sputtering by the dynamic SIMS instrument in microscope detection mode. There was a clear relationship between mass 27 and mass 26 signals in Pseudomonas putida cells grown in media containing varying proportions of (12)C- to (13)C-glucose: a standard curve was generated to predict (13)C-enrichment in unknown samples. We then used two strains of Pseudomonas putida able to grow on either all or only a part of a mixture of (13)C-labelled and unlabelled carbon sources to verify that differential (13)C signals measured by SIMS were due to (13)C assimilation into cell biomass. Finally, we made three key observations after applying SIMS ion microscopy to soil samples from a field experiment receiving (12)C- or (13)C-phenol: (i) cells enriched in (13)C were heterogeneously distributed among soil populations; (ii) (13)C-labelled cells were detected in soil that was dosed a single time with (13)C-phenol; and (iii) in soil that received 12 doses of (13)C-phenol, 27% of the cells in the total community were more than 90% (13)C-labelled.

Stable isotope probing reveals Trichosporon yeast to be active in situ in soil phenol metabolism

The aim of this study was to extend the results of our previous stable isotope probing (SIP) investigation: we identified a soil fungus involved in phenol biodegradation at an agricultural field site. DNA extracts from our previous study were examined using fungi-specific PCR amplification of the 18S-28S internal transcribed spacer (ITS) region. We prepared an 80-member clone library using PCR-amplified, (13)C-labeled DNA derived from field soil that received 12 daily doses of (13)C-phenol. Restriction-fragment-length-polymorphism screening and DNA sequencing revealed a dominant clone (41% of the clone library), the ITS sequence of which corresponded to that of the fungal genus Trichosporon. We successfully grew and isolated a white, filamentous fungus from site soil samples after plating soil dilutions on mineral salts agar containing 250 p.p.m. phenol. Restreaking on both yeast extract-peptone-galactose and Sabouraud dextrose agar plates led to further purification of the fungus, the morphological characteristics of which matched those of the genus Trichosporon. The ITS sequence of our isolated fungus was identical to that of a clone from our SIP-based library, confirming it to be Trichosporon multisporum. High-performance liquid chromatography and turbidometeric analyses showed that the culture was able to metabolize and grow on 200 p.p.m. phenol in an aqueous mineral salts medium within 24 h at room temperature. Gas chromatography-mass spectrometry analysis of (13)CO(2) respiration from laboratory soil incubations demonstrated accelerated phenol mineralization in treatments inoculated with T. multisporum. These findings show that T. multisporum actively degraded phenol in our field-based, soil experiments.