Bacterial diversity of herbal rhizospheric soils in Ordos desert steppes under different degradation gradients.

Objectives: This study explored the effects of different degradation gradients on bacterial diversity in the rhizospheric soils of herb plants. Methods: The alpha diversity, species composition and correlations of bacterial communities in the rhizospheric soils of herb plants were studied using metagenomics 16SrDNA gene high-throughput sequencing. Results: The diversity of bacterial communities in the rhizospheric soils of herb plants differed during the degradation of desert steppes. An analysis of bacterial community alpha diversity indices showed the bacterial diversity and species evenness of rhizospheric soils were best in moderately degraded desert steppes. Among all samples, a total of 43 phyla, 133 classes, 261 orders, 421 families, 802 genera and 1,129 species were detected. At the phylum level, the predominant bacterial phyla were: Actinobacteria, Proteobacteria, Acidobacteria, Gemmatimonadetes, Chloroflexi, Planctomycetes and Bacteroidetes. At the genus level, the predominant bacterial genera were: RB41, Sphingomonas, WD2101_soil_group_unclassified, Pseudomonas and Actinomyces. The relative abundance of unknown genera was very large, which deserves further research. At the phylum and genus levels, the species abundance levels under slight and moderate degradation were significantly higher than those under extreme degradation. Correlation network diagrams showed there were many nodes in both slightly deteriorated and moderately deteriorated soils, and the node proportions were large and mostly positively correlated. These results indicate the bacterial communities in rhizospheric soils under slight or moderate deterioration are relatively stable. The rhizospheric soil microbes of desert steppes can form a stable network structure, allowing them to adequately respond to environmental conditions. Conclusions: The bacterial communities in the rhizospheric soils of herb plants differ between different degradation gradients. The species number, abundance and diversity of bacterial communities in rhizospheric soils are not directly correlated with degree of degradation. The abundance, species diversity and species abundance of bacterial communities in the rhizospheric soils of moderately degraded desert steppes are the highest and most stable. The soil bacterial diversity is lowest in severely degraded desert steppes.

Rhizosphere impacts bacterial community structure in the tea (Camellia sinensis (L.) O. Kuntze.) estates of Darjeeling, India.

India contributes 28% of the world's tea production, and the Darjeeling tea of India is a world-famous tea variety known for its unique quality, flavour and aroma. This study analyzed the spatial distribution of bacterial communities in the tea rhizosphere of six different tea estates at different altitudes. The organic carbon, total nitrogen and available phosphate were higher in the rhizosphere soils than the bulk soils, irrespective of the sites. Alpha and beta diversities were significantly (p < 0.05) higher in the bulk soil than in the rhizosphere. Among the identified phyla, the predominant ones were Proteobacteria, Actinobacteria and Acidobacteria. At the genus level, only four out of 23 predominant genera (>1% relative abundance) could be classified, viz., Candidatus Solibacter (5.36 ± 0.36%), Rhodoplanes (4.87 ± 0.3%), Candidatus Koribacter (2.3 ± 0.67%), Prevotella (1.49 ± 0.26%). The rhizosphere effect was prominent from the significant depletion of more ASVs (n = 39) compared to enrichment (n = 11). The functional genes also exhibit a similar trend with the enrichment of N2 fixation genes, disease suppression and Acetoin synthesis. Our study reports that the rhizobiome of tea is highly selective by reducing the alpha and beta diversity while enriching the significant functional genes.

Ecological memory of recurrent drought modifies soil processes via changes in soil microbial community.

Climate change is altering the frequency and severity of drought events. Recent evidence indicates that drought may produce legacy effects on soil microbial communities. However, it is unclear whether precedent drought events lead to ecological memory formation, i.e., the capacity of past events to influence current ecosystem response trajectories. Here, we utilize a long-term field experiment in a mountain grassland in central Austria with an experimental layout comparing 10 years of recurrent drought events to a single drought event and ambient conditions. We show that recurrent droughts increase the dissimilarity of microbial communities compared to control and single drought events, and enhance soil multifunctionality during drought (calculated via measurements of potential enzymatic activities, soil nutrients, microbial biomass stoichiometry and belowground net primary productivity). Our results indicate that soil microbial community composition changes in concert with its functioning, with consequences for soil processes. The formation of ecological memory in soil under recurrent drought may enhance the resilience of ecosystem functioning against future drought events.

Nutrients cause consolidation of soil carbon flux to small proportion of bacterial community.

Nutrient amendment diminished bacterial functional diversity, consolidating carbon flow through fewer bacterial taxa. Here, we show strong differences in the bacterial taxa responsible for respiration from four ecosystems, indicating the potential for taxon-specific control over soil carbon cycling. Trends in functional diversity, defined as the richness of bacteria contributing to carbon flux and their equitability of carbon use, paralleled trends in taxonomic diversity although functional diversity was lower overall. Among genera common to all ecosystems, Bradyrhizobium, the Acidobacteria genus RB41, and Streptomyces together composed 45-57% of carbon flow through bacterial productivity and respiration. Bacteria that utilized the most carbon amendment (glucose) were also those that utilized the most native soil carbon, suggesting that the behavior of key soil taxa may influence carbon balance. Mapping carbon flow through different microbial taxa as demonstrated here is crucial in developing taxon-sensitive soil carbon models that may reduce the uncertainty in climate change projections.

Legume-microbiome interactions unlock mineral nutrients in regrowing tropical forests.

Legume trees form an abundant and functionally important component of tropical forests worldwide with N2-fixing symbioses linked to enhanced growth and recruitment in early secondary succession. However, it remains unclear how N2-fixers meet the high demands for inorganic nutrients imposed by rapid biomass accumulation on nutrient-poor tropical soils. Here, we show that N2-fixing trees in secondary Neotropical forests triggered twofold higher in situ weathering of fresh primary silicates compared to non-N2-fixing trees and induced locally enhanced nutrient cycling by the soil microbiome community. Shotgun metagenomic data from weathered minerals support the role of enhanced nitrogen and carbon cycling in increasing acidity and weathering. Metagenomic and marker gene analyses further revealed increased microbial potential beneath N2-fixers for anaerobic iron reduction, a process regulating the pool of phosphorus bound to iron-bearing soil minerals. We find that the Fe(III)-reducing gene pool in soil is dominated by acidophilic Acidobacteria, including a highly abundant genus of previously undescribed bacteria, Candidatus Acidoferrum, genus novus. The resulting dependence of the Fe-cycling gene pool to pH determines the high iron-reducing potential encoded in the metagenome of the more acidic soils of N2-fixers and their nonfixing neighbors. We infer that by promoting the activities of a specialized local microbiome through changes in soil pH and C:N ratios, N2-fixing trees can influence the wider biogeochemical functioning of tropical forest ecosystems in a manner that enhances their ability to assimilate and store atmospheric carbon.

Unraveling the xylanolytic potential of Acidobacteria bacterium AB60 from Cerrado soils.

The presence of genes for glycosyl hydrolases in many Acidobacteria genomes indicates an important role in the degradation of plant cell wall material. Acidobacteria bacterium AB60 was obtained from Cerrado oligotrophic soil in Brazil, where this phylum is abundant. The 16S rRNA gene analyses showed that AB60 was closely related to the genera Occallatibacter and Telmatobacter. However, AB60 grew on xylan as carbon source, which was not observed in Occallatibacter species; but growth was not detected on medium containing carboxymethyl cellulose, as observed in Telmatobacter. Nevertheless, the genome analysis of AB60 revealed genes for the enzymes involved in cellulose as well as xylan degradation. In addition to enzymes involved in xylan degradation, α-l-rhamnosidase was detected in the cultures of AB60. Functional screening of a small-insert genomic library did not identify any clones capable of carboxymethyl cellulose degradation, but open reading frames coding α-l-arabinofuranosidase and α-l-rhamnosidase were present in clones showing xylan degradation halos. Both enzymes act on the lateral chains of heteropolymers such as pectin and some hemicelluloses. These results indicate that the hydrolysis of α-linked sugars may offer a metabolic niche for slow-growing Acidobacteria, allowing them to co-exist with other plant-degrading microbes that hydrolyze ß-linked sugars from cellulose or hemicellulose backbones.

Change of rhizospheric bacterial community of the ancient wild tea along elevational gradients in Ailao mountain, China.

The rhizospheric microbial community is one of the major environmental factors affecting the distribution and fitness of plants. Ancient wild tea plants are rare genetic resource distributed in Southwest China. In this study, we investigated that rhizospheric bacterial communities of ancient wild tea plants along the elevational gradients (2050, 2200, 2350 and 2500 m) in QianJiaZhai Reserve of Ailao Mountains. According to the Illumina MiSeq sequencing of 16 S rRNA gene amplicons, Proteobacteria, Acidobacteria and Actinobacteria were the dominant phyla with the relative abundance 43.12%, 21.61% and 14.84%, respectively. The Variibacter was the most dominant genus in rhizosphere of ancient wild tea plant. Phylogenetic null modeling analysis suggested that rhizospheric bacterial communities of ancient wild tea plants were more phylogenetically clustered than expected by chance. The bacterial community at 2050 m was unique with the highest alpha diversity, tend to cluster the nearest taxon and simple co-occurrence network structure. The unique bacterial community was correlated to multiple soil factors, and the content soil ammonium nitrogen (NH4+-N) was the key factor affecting the diversity and distribution of bacterial community along the elevational gradients. This study provided the necessary basic information for the protection of ancient tea trees and cultivation of tea plants.

Water-soluble phosphorus contributes significantly to shaping the community structure of rhizospheric bacteria in rocky desertification areas.

Microorganisms play important roles in soil improvement. Therefore, clarifying the contribution of environmental factors in shaping the microbial community structure is beneficial to improve soil fertility in karst rocky desertification areas. Here, the bacterial community structures of eight rhizospheric soil samples collected from perennial fruit plantations were analysed using an Illumina HiSeq2500 platform. The diversity and abundance of bacteria in rocky desertification areas were significantly lower than those in non-rocky desertification areas, while the bacterial community structure was not significantly different between root surface and non-root surface soils in the same rhizospheric soil samples. Proteobacteria predominated in rocky desertification areas, while Actinobacteria predominated in non-rocky desertification areas. Correlation analysis revealed that water-soluble phosphorus content (r2 = 0.8258), latitude (r2 = 0.7556), altitude (r2 = 0.7501), and the age of fruit trees (r2 = 0.7321) were positively correlated with the bacterial community structure, while longitude, pH, and total phosphorus content did not significantly influence the soil bacterial community structure. As water-soluble phosphorus content is derived from insoluble phosphorus minerals, supplementing phosphorus-solubilising bacteria to soils in rocky desertification areas is a feasible strategy for accelerating the dissolution of insoluble phosphorus minerals and improving agricultural production and environment ecology.

Effect of aridity and dune type on rhizosphere soil bacterial communities of Caragana microphylla in desert regions of northern China.

Understanding the response of soil properties and bacterial communities in rhizosphere soil to aridity and dune types is fundamental to desertification control. This study investigated soil properties and bacterial communities of both rhizosphere and bulk soils of Caragana microphylla from four sites with different aridity indices, and one site with three different types of dunes. All sites were located in the desert regions of northern China. The results indicated that compared with the bulk soil, the soil nutrient content of rhizosphere, especially the content of total phosphorus, was generally significantly improved in different desertification environments. The bacterial richness and diversity were also higher than those of bulk soil, especially in arid regions and fixed dunes. Firmicutes, Actinobacteria, Proteobacteria, and Acidobacteria were the most dominant phyla in all samples. The regression analyses showed that at different sites, soil total organic C, total N, Na+, and total P played key roles in determining the bacterial community structure while total organic carbon, electronic conductivity, pH and total phosphorus were the dominant factors at the different dunes. The results further revealed that the dominant phyla strongly affected by environmental factors at different sites were Acidobacteria, Gemmatimonadetes, and Actinobacteria among which, Acidobacteria and Gemmatimonadetes were negatively correlated with Na+ content. At different types of dunes, Actinobacteria, Planctomycetes, and Gemmatimonadetes were particularly affected by environmental factors. The increased abundance of Actinobacteria in the rhizosphere soil was mainly caused by the decreased soil pH.

Microbial taxonomic, nitrogen cycling and phosphorus recycling community composition during long-term organic greenhouse farming.

Understanding the interplay between the farming system and soil microbiomes could aid the design of a sustainable and efficient farming system. A comparative greenhouse experiment consisting of organic (ORG), integrated (INT) and conventional (CON) farming systems was established in northern China in 2002. The effects of 12 years of organic farming on soil microbiomes were explored by metagenomic and 16S rRNA gene amplicon sequencing analyses. Long-term ORG shifted the community composition of dominant phyla, especially Acidobacteria, increased the relative abundance of Ignavibacteria and Acidobacteria Gp6 and decreased the relative abundance of Nitrosomonas, Bacillus and Paenibacillus. Metagenomic analysis further revealed that relative abundance of ammonia oxidizing microorganisms (Bacteria and Archaea) and anaerobic ammonium oxidation bacteria decreased during ORG. Conversely, the relative abundance of bacteria-carrying periplasmic nitrate reductases (napA) was slightly higher for ORG. Long-term organic farming also caused significant alterations to the community composition of functional groups associated with ammonia oxidation, denitrification and phosphorus recycling. In summary, this study provides key insights into the composition of soil microbiomes and long-term organic farming under greenhouse conditions.

Microbial community changes during a toxic cyanobacterial bloom in an alkaline Hungarian lake.

The Carpathian Basin is a lowland plain located mainly in Hungary. Due to the nature of the bedrock, alluvial deposits, and a bowl shape, many lakes and ponds of the area are characterized by high alkalinity. In this study, we characterized temporal changes in eukaryal and bacterial community dynamics with high throughput sequencing and relate the changes to environmental conditions in Lake Velence located in Fejér county, Hungary. The sampled Lake Velence microbial populations (algal and bacterial) were analyzed to identify potential correlations with other community members and environmental parameters at six timepoints over 6 weeks in the Spring of 2012. Correlations between community members suggest a positive relationship between certain algal and bacterial populations (e.g. Chlamydomondaceae with Actinobacteria and Acidobacteria), while other correlations allude to changes in these relationships over time. During the study, high nitrogen availability may have favored non-nitrogen fixing cyanobacteria, such as the toxin-producing Microcystis aeruginosa, and the eutrophic effect may have been exacerbated by high phosphorus availability as well as the high calcium and magnesium content of the Carpathian Basin bedrock, potentially fostering exopolymer production and cell aggregation. Cyanobacterial bloom formation could have a negative environmental impact on other community members and potentially affect overall water quality as well as recreational activities. To our knowledge, this is the first prediction for relationships between photoautotrophic eukaryotes and bacteria from an alkaline, Hungarian lake.

Peatland Acidobacteria with a dissimilatory sulfur metabolism.

Sulfur-cycling microorganisms impact organic matter decomposition in wetlands and consequently greenhouse gas emissions from these globally relevant environments. However, their identities and physiological properties are largely unknown. By applying a functional metagenomics approach to an acidic peatland, we recovered draft genomes of seven novel Acidobacteria species with the potential for dissimilatory sulfite (dsrAB, dsrC, dsrD, dsrN, dsrT, dsrMKJOP) or sulfate respiration (sat, aprBA, qmoABC plus dsr genes). Surprisingly, the genomes also encoded DsrL, which so far was only found in sulfur-oxidizing microorganisms. Metatranscriptome analysis demonstrated expression of acidobacterial sulfur-metabolism genes in native peat soil and their upregulation in diverse anoxic microcosms. This indicated an active sulfate respiration pathway, which, however, might also operate in reverse for dissimilatory sulfur oxidation or disproportionation as proposed for the sulfur-oxidizing Desulfurivibrio alkaliphilus. Acidobacteria that only harbored genes for sulfite reduction additionally encoded enzymes that liberate sulfite from organosulfonates, which suggested organic sulfur compounds as complementary energy sources. Further metabolic potentials included polysaccharide hydrolysis and sugar utilization, aerobic respiration, several fermentative capabilities, and hydrogen oxidation. Our findings extend both, the known physiological and genetic properties of Acidobacteria and the known taxonomic diversity of microorganisms with a DsrAB-based sulfur metabolism, and highlight new fundamental niches for facultative anaerobic Acidobacteria in wetlands based on exploitation of inorganic and organic sulfur molecules for energy conservation.

Contrasting bacterial communities in two indigenous Chionochloa (Poaceae) grassland soils in New Zealand.

The cultivation of grasslands can modify both bacterial community structure and impact on nutrient cycling as well as the productivity and diversity of plant communities. In this study, two pristine New Zealand grassland sites dominated by indigenous tall tussocks (Chionochloa pallens or C. teretifolia) were examined to investigate the extent and predictability of variation of the bacterial community. The contribution of free-living bacteria to biological nitrogen fixation is predicted to be ecologically significant in these soils; therefore, the diazotrophic community was also examined. The C. teretifolia site had N-poor and poorly-drained peaty soils, and the C. pallens had N-rich and well-drained fertile soils. These soils also differ in the proportion of organic carbon (C), Olsen phosphorus (P) and soil pH. The nutrient-rich soils showed increased relative abundances of some copiotrophic bacterial taxa (including members of the Proteobacteria, Bacteroidetes and Firmicutes phyla). Other copiotrophs, Actinobacteria and the oliogotrophic Acidobacteria showed increased relative abundance in nutrient-poor soils. Greater diversity based on 16S rRNA gene sequences and the Tax4Fun prediction of enhanced spore formation associated with nutrient-rich soils could indicate increased resilience of the bacterial community. The two sites had distinct diazotrophic communities with higher diversity in C. teretifolia soils that had less available nitrate and ammonium, potentially indicating increased resilience of the diazotroph community at this site. The C. teretifolia soils had more 16S rRNA gene and nifH copies per g soil than the nutrient rich site. However, the proportion of the bacterial community that was diazotrophic was similar in the two soils. We suggest that edaphic and vegetation factors are contributing to major differences in the composition and diversity of total bacterial and diazotrophic communities at these sites. We predict the differences in the communities at the two sites will result in different responses to environmental change.

Hydrocarbon degraders establish at the costs of microbial richness, abundance and keystone taxa after crude oil contamination in permafrost environments.

Oil spills from pipeline ruptures are a major source of terrestrial petroleum pollution in cold regions. However, our knowledge of the bacterial response to crude oil contamination in cold regions remains to be further expanded, especially in terms of community shifts and potential development of hydrocarbon degraders. In this study we investigated changes of microbial diversity, population size and keystone taxa in permafrost soils at four different sites along the China-Russia crude oil pipeline prior to and after perturbation with crude oil. We found that crude oil caused a decrease of cell numbers together with a reduction of the species richness and shifts in the dominant phylotypes, while bacterial community diversity was highly site-specific after exposure to crude oil, reflecting different environmental conditions. Keystone taxa that strongly co-occurred were found to form networks based on trophic interactions, that is co-metabolism regarding degradation of hydrocarbons (in contaminated samples) or syntrophic carbon cycling (in uncontaminated samples). With this study we demonstrate that after severe crude oil contamination a rapid establishment of endemic hydrocarbon degrading communities takes place under favorable temperature conditions. Therefore, both endemism and trophic correlations of bacterial degraders need to be considered in order to develop effective cleanup strategies.

Local Environmental Factors Drive Divergent Grassland Soil Bacterial Communities in the Western Swiss Alps.

Mountain ecosystems are characterized by a diverse range of climatic and topographic conditions over short distances and are known to shelter a high biodiversity. Despite important progress, still little is known on bacterial diversity in mountain areas. Here, we investigated soil bacterial biogeography at more than 100 sampling sites randomly stratified across a 700-km2 area with 2,200-m elevation gradient in the western Swiss Alps. Bacterial grassland communities were highly diverse, with 12,741 total operational taxonomic units (OTUs) across 100 sites and an average of 2,918 OTUs per site. Bacterial community structure was correlated with local climatic, topographic, and soil physicochemical parameters with high statistical significance. We found pH (correlated with % CaO and % mineral carbon), hydrogen index (correlated with bulk gravimetric water content), and annual average number of frost days during the growing season to be among the groups of the most important environmental drivers of bacterial community structure. In contrast, bacterial community structure was only weakly stratified as a function of elevation. Contrasting patterns were discovered for individual bacterial taxa. Acidobacteria responded both positively and negatively to pH extremes. Various families within the Bacteroidetes responded to available phosphorus levels. Different verrucomicrobial groups responded to electrical conductivity, total organic carbon, water content, and mineral carbon contents. Alpine grassland bacterial communities are thus highly diverse, which is likely due to the large variety of different environmental conditions. These results shed new light on the biodiversity of mountain ecosystems, which were already identified as potentially fragile to anthropogenic influences and climate change. IMPORTANCE: This article addresses the question of how microbial communities in alpine regions are dependent on local climatic and soil physicochemical variables. We benefit from a unique 700-km2 study region in the western Swiss Alps region, which has been exhaustively studied for macro-organismal and fungal ecology, and for topoclimatic modeling of future ecological trends, but without taking into account soil bacterial diversity. Here, we present an in-depth biogeographical characterization of the bacterial community diversity in this alpine region across 100 randomly stratified sites, using 56 environmental variables. Our exhaustive sampling ensured the detection of ecological trends with high statistical robustness. Our data both confirm previously observed general trends and show many new detailed trends for a wide range of bacterial taxonomic groups and environmental parameters.

Pepino (Solanum muricatum) planting increased diversity and abundance of bacterial communities in karst area.

Soil nutrients and microbial communities are the two key factors in revegetation of barren environments. Ecological stoichiometry plays an important role in ecosystem function and limitation, but the relationships between above- and belowground stoichiometry and the bacterial communities in a typical karst region are poorly understood. We used pepino (Solanum muricatum) to examine the stoichiometric traits between soil and foliage, and determine diversity and abundance of bacteria in the karst soil. The soil had a relatively high pH, low fertility, and coarse texture. Foliar N:P ratio and the correlations with soil nitrogen and phosphorus suggested nitrogen limitation. The planting of pepino increased soil urease activity and decreased catalase activity. Higher diversity of bacteria was determined in the pepino rhizosphere than bulk soil using a next-generation, Illumina-based sequencing approach. Proteobacteria, Acidobacteria, Actinobacteria and Bacteroidetes were the dominant phyla in all samples, accounting for more than 80% of the reads. On a genus level, all 625 detected genera were found in all rhizosphere and bulk soils, and 63 genera showed significant differences among samples. Higher Shannon and Chao 1 indices in the rhizosphere than bulk soil indicated that planting of pepino increased diversity and abundance of bacterial communities in karst area.

Phosphorus depletion in forest soils shapes bacterial communities towards phosphorus recycling systems.

Phosphorus (P) is an important macronutrient for all biota on earth but similarly a finite resource. Microorganisms play on both sides of the fence as they effectively mineralize organic and solubilize precipitated forms of soil phosphorus but conversely also take up and immobilize P. Therefore, we analysed the role of microbes in two beech forest soils with high and low P content by direct sequencing of metagenomic deoxyribonucleic acid. For inorganic P solubilization, a significantly higher microbial potential was detected in the P-rich soil. This trait especially referred to Candidatus Solibacter usiatus, likewise one of the dominating species in the data sets. A higher microbial potential for efficient phosphate uptake systems (pstSCAB) was detected in the P-depleted soil. Genes involved in P starvation response regulation (phoB, phoR) were prevalent in both soils. This underlines the importance of effective phosphate (Pho) regulon control for microorganisms to use alternative P sources during phosphate limitation. Predicted genes were primarily harboured by Rhizobiales, Actinomycetales and Acidobacteriales.

Characterization of the In Situ Ecophysiology of Novel Phylotypes in Nutrient Removal Activated Sludge Treatment Plants.

An in depth understanding of the ecology of activated sludge nutrient removal wastewater treatment systems requires detailed knowledge of the community composition and metabolic activities of individual members. Recent 16S rRNA gene amplicon surveys of activated sludge wastewater treatment plants with nutrient removal indicate the presence of a core set of bacterial genera. These organisms are likely responsible for the bulk of nutrient transformations underpinning the functions of these plants. While the basic activities of some of these genera in situ are known, there is little to no information for the majority. This study applied microautoradiography coupled with fluorescence in situ hybridization (MAR-FISH) for the in situ characterization of selected genus-level-phylotypes for which limited physiological information is available. These included Sulfuritalea and A21b, both within the class Betaproteobacteria, as well as Kaga01, within sub-group 10 of the phylum Acidobacteria. While the Sulfuritalea spp. were observed to be metabolically versatile, the A21b and Kaga01 phylotypes appeared to be highly specialized.

Bacterial diversity in native habitats of the medicinal fungus Ophiocordyceps sinensis on Tibetan Plateau as determined using Illumina sequencing data.

Ophiocordyceps sinensis is one of the most well-known traditional Chinese medicinal fungi. In this study, bacterial diversity in the soils of native habitats of O. sinensis was investigated using Illumina sequencing data. A total of 525,000 sequences of V6-16S rRNA were analyzed. The number of OTUs from each sample ranged from 13,858 to 15,978 at 97% sequence similarity cut-off. The results demonstrated that the deep sequencing approach provides improved access to rare genotypes. Richness indices and Shannon's diversity index did not differ significantly between samples collected from locations where O. sinensis was present (Os1-3) and not present (NOs1-3). Classified bacterial sequences were grouped into 23 phyla including Proteobacteria, Actinobacteria, Acidobacteria, Verrucomicrobia, etc. The Venn diagram revealed that 7183 OTUs belonging to 14 phyla were shared by Os, NOs and MP (mycelial pellicle wrapping the sclerotium of O. sinensis) samples, possibly representing a core microbiome existing in native habitats of O. sinensis, and that 863 belonging to 12 phyla were shared by Os and MP samples, possibly related to the occurrence of O. sinensis. Overall, the results revealed a high bacterial diversity in the soil samples and the relationships between the bacterial diversity and O. sinensis merit further investigation.

Diversity of bacterial communities along a petroleum contamination gradient in desert soils.

Microbial communities in oil-polluted desert soils have been rarely studied compared to their counterparts from freshwater and marine environments. We investigated bacterial diversity and changes therein in five desert soils exposed to different levels of oil pollution. Automated rRNA intergenic spacer (ARISA) analysis profiles showed that the bacterial communities of the five soils were profoundly different (analysis of similarities (ANOSIM), R = 0.45, P < 0.0001) and shared less than 20 % of their operational taxonomic units (OTUs). OTU richness was relatively higher in the soils with the higher oil pollution levels. Multivariate analyses of ARISA profiles revealed that the microbial communities in the S soil, which contains the highest level of contamination, were different from the other soils and formed a completely separate cluster. A total of 16,657 ribosomal sequences were obtained, with 42-89 % of these sequences belonging to the phylum Proteobacteria. While sequences belonging to Betaproteobacteria, Gammaproteobacteria, Bacilli, and Actinobacteria were encountered in all soils, sequences belonging to anaerobic bacteria from the classes Deltaproteobacteria, Clostridia, and Anaerolineae were only detected in the S soil. Sequences belonging to the genus Terriglobus of the class Acidobacteria were only detected in the B3 soil with the lowest level of contamination. Redundancy analysis (RDA) showed that oil contamination level was the most determinant factor that explained variations in the microbial communities. We conclude that the exposure to different levels of oil contamination exerts a strong selective pressure on bacterial communities and that desert soils are rich in aerobic and anaerobic bacteria that could potentially contribute to the degradation of hydrocarbons.