Analysis of 16S rRNA and mxaF genes revealing insights into Methylobacterium niche-specific plant association

The genus Methylobacterium comprises pink-pigmented facultative methylotrophic (PPFM) bacteria, known to be an important plant-associated bacterial group. Species of this group, described as plant-nodulating, have the dual capacity of producing cytokinin and enzymes, such as pectinase and cellulase, involved in systemic resistance induction and nitrogen fixation under specific plant environmental conditions. The aim hereby was to evaluate the phylogenetic distribution of Methylobacterium spp. isolates from different host plants. Thus, a comparative analysis between sequences from structural (16S rRNA) and functional mxaF (which codifies for a subunit of the enzyme methanol dehydrogenase) ubiquitous genes, was undertaken. Notably, some Methylobacterium spp. isolates are generalists through colonizing more than one host plant, whereas others are exclusively found in certain specific plant-species. Congruency between phylogeny and specific host inhabitance was higher in the mxaF gene than in the 16S rRNA, a possible indication of function-based selection in this niche. Therefore, in a first stage, plant colonization by Methylobacterium spp. could represent generalist behavior, possibly related to microbial competition and adaptation to a plant environment. Otherwise, niche-specific colonization is apparently impelled by the host plant

Unravelling the interplay of ecological processes structuring the bacterial rare biosphere

Most ecological communities harbor many rare species (i.e., the rare biosphere), however, relatively little is known about how distinct ecological processes structure their existence. Here, we used spatiotemporal data on soil bacterial communities along a natural ecosystem gradient to model the relative influences of assembly processes structuring the rare and common biospheres. We found a greater influence of homogeneous selection (i.e., imposed by spatiotemporally constant variables) mediating the assembly of the rare biosphere, whereas the common biosphere was mostly governed by variable selection (i.e., imposed by spatial and/or temporal fluctuating variables). By partitioning the different types of rarity, we found homogeneous selection to explain the prevalence of permanently rare taxa, thus suggesting their persistence at low abundances to be restrained by physiological traits. Conversely, the dynamics of conditionally rare taxa were mostly structured by variable selection, which aligns with the ability of these taxa to switch between rarity and commonness as responses to environmental spatiotemporal variations. Taken together, our study contributes to the establishment of a link between conceptual and empirical developments in the ecology of the soil microbial rare biosphere. Besides, this study provides a framework to better understand, model, and predict the existence and dynamics of microbial rare biospheres across divergent systems and scales

Comparing the influence of assembly processes governing bacterial community succession based on DNA and RNA data

Quantifying which assembly processes structure microbiomes can assist prediction, manipulation, and engineering of community outcomes. However, the relative importance of these processes might depend on whether DNA or RNA are used, as they differ in stability. We hypothesized that. RNA-inferred community responses to (a)biotic fluctuations are faster than those inferred by DNA; the relative influence of variable selection is stronger in RNA-inferred communities (environmental factors are spatiotemporally heterogeneous), whereas homogeneous selection largely influences DNA-inferred communities (environmental filters are constant). To test these hypotheses, we characterized soil bacterial communities by sequencing both 16S rRNA amplicons from the extracted DNA and RNA transcripts across distinct stages of soil primary succession and quantified the relative influence of each assembly process using ecological null model analysis. Our results revealed that variations in α-diversity and temporal turnover were higher in RNA- than in DNA-inferred communities across successional stages, albeit there was a similar community composition; in line with our hypotheses, the assembly of RNA-inferred community was more closely associated with environmental variability (variable selection) than using the standard DNA-based approach, which was largely influenced by homogeneous selection. This study illustrates the need for benchmarking approaches to properly elucidate how community assembly processes structure microbial communities

Soil microbial diversity affects the plant-root colonization by arbuscular mycorrhizal fungi

Terrestrial plants establish symbiosis with arbuscular mycorrhizal fungi (AMF) to exchange water and nutrients. However, the extent to which soil biodiversity influences such association remains still unclear. Here, we manipulated the soil microbial diversity using a "dilution-to-extinction" approach in a controlled pot microcosm system and quantified the root length colonization of maize plants by the AMF Rhizophagus clarus. The experiment was performed by manipulating the soil microbiome within a native and foreign soil having distinct physicochemical properties. Overall, our data revealed significant positive correlations between the soil microbial diversity and AMF colonization. Most importantly, this finding opposes the diversity-invasibility hypothesis and highlights for a potential overall helper effect of the soil biodiversity on plant-AMF symbiosis

Metataxonomic profiling and prediction of functional behaviour of wheat straw degrading microbial consortia

Background: Mixed microbial cultures, in which bacteria and fungi interact, have been proposed as an efficient way to deconstruct plant waste. The characterization of specific microbial consortia could be the starting point for novel biotechnological applications related to the efficient conversion of lignocellulose to cello-oligosaccharides, plastics and/or biofuels. Here, the diversity, composition and predicted functional profiles of novel bacterial-fungal consortia are reported, on the basis of replicated aerobic wheat straw enrichment cultures. Results: In order to set up biodegradative microcosms, microbial communities were retrieved from a forest soil and introduced into a mineral salt medium containing 1% of (un) treated wheat straw. Following each incubation step, sequential transfers were carried out using 1 to 1,000 dilutions. The microbial source next to three sequential batch cultures (transfers 1, 3 and 10) were analyzed by bacterial 16S rRNA gene and fungal ITS1 pyrosequencing. Faith's phylogenetic diversity values became progressively smaller from the inoculum to the sequential batch cultures. Moreover, increases in the relative abundances of Enterobacteriales, Pseudomonadales, Flavobacteriales and Sphingobacteriales were noted along the enrichment process. Operational taxonomic units affiliated with Acinetobacter johnsonii, Pseudomonas putida and Sphingobacterium faecium were abundant and the underlying strains were successfully isolated. Interestingly, Klebsiella variicola (OTU1062) was found to dominate in both consortia, whereas K. variicola-affiliated strains retrieved from untreated wheat straw consortia showed endoglucanase/xylanase activities. Among the fungal players with high biotechnological relevance, we recovered members of the genera Penicillium, Acremonium, Coniochaeta and Trichosporon. Remarkably, the presence of peroxidases, alpha-L-fucosidases, beta-xylosidases, beta-mannases and beta-glucosidases, involved in lignocellulose degradation, was indicated by predictive bacterial metagenome reconstruction. Reassuringly, tests for specific (hemi) cellulolytic enzymatic activities, performed on the consortial secretomes, confirmed the presence of such gene functions. Conclusion: In an in-depth characterization of two wheat straw degrading microbial consortia, we revealed the enrichment and selection of specific bacterial and fungal taxa that were presumably involved in (hemi) cellulose degradation. Interestingly, the microbial community composition was strongly influenced by the wheat straw pretreatment. Finally, the functional bacterial-metagenome prediction and the evaluation of enzymatic activities (at the consortial secretomes) revealed the presence and enrichment of proteins involved in the deconstruction of plant biomass

Transcriptional responses of the bacterium <i>Burkholderia terrae</i> BS001 to the fungal host <i>Lyophyllum</i> sp strain Karsten under soil-mimicking conditions

In this study, the mycosphere isolate Burkholderia terrae BS001 was confronted with the soil fungus Lyophyllum sp. strain Karsten on soil extract agar plates in order to examine its transcriptional responses over time. At the initial stages of the experiment (T1-day 3; T2-day 5), contact between both partner organisms was absent, whereas in the final stage (T3-day 8), the two populations made intimate physical contact. Overall, a strong modulation of the strain BS001 gene expression patterns was found. First, the stationary-phase sigma factor RpoS, and numerous genes under its control, were strongly expressed as a response to the soil extract agar, and this extended over the whole temporal regime. In the system, B. terrae BS001 apparently perceived the presence of the fungal hyphae already at the early experimental stages (T1, T2), by strongly upregulating a suite of chemotaxis and flagellar motility genes. With respect to specific metabolism and energy generation, a picture of differential involvement in different metabolic routes was obtained. Initial (T1, T2) up- or downregulation of ethanolamine and mandelate uptake and utilization pathways was substituted by a strong investment, in the presence of the fungus, in the expression of putative metabolic gene clusters (T3). Specifically at T3, five clustered genes that are potentially involved in energy generation coupled to an oxidative stress response, and two genes encoding short-chain dehydrogenases/oxidoreductases (SDR), were highly upregulated. In contrast, the dnaE2 gene (related to general stress response; encoding error-prone DNA polymerase) was transcriptionally downregulated at this stage. This study revealed that B. terrae BS001, from a stress-induced state, resulting from the soil extract agar milieu, responds positively to fungal hyphae that encroach upon it, in a temporally dynamic manner. The response is characterized by phases in which the modulation of (1) chemotaxis, (2) metabolic activity, and (3) oxidative stress responses are key mechanisms

Towards meaningful scales in ecosystem microbiome research

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