Microbiomes of a specialist caterpillar are consistent across different habitats but also resemble the local soil microbial communities

BACKGROUND

Insect-associated microorganisms can provide a wide range of benefits to their host, but insect dependency on these microbes varies greatly. The origin and functionality of insect microbiomes is not well understood. Many caterpillars can harbor symbionts in their gut that impact host metabolism, nutrient uptake and pathogen protection. Despite our lack of knowledge on the ecological factors driving microbiome assemblages of wild caterpillars, they seem to be highly variable and influenced by diet and environment. Several recent studies have shown that shoot-feeding caterpillars acquire part of their microbiome from the soil. Here, we examine microbiomes of a monophagous caterpillar (Tyria jacobaeae) collected from their natural host plant (Jacobaea vulgaris) growing in three different environments: coastal dunes, natural inland grasslands and riverine grasslands, and compare the bacterial communities of the wild caterpillars to those of soil samples collected from underneath each of the host plants from which the caterpillars were collected.

RESULTS

The microbiomes of the caterpillars were dominated by Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes. Only 5% of the total bacterial diversity represented 86.2% of the total caterpillar's microbiome. Interestingly, we found a high consistency of dominant bacteria within the family Burkholderiaceae in all caterpillar samples across the three habitats. There was one amplicon sequence variant belonging to the genus Ralstonia that represented on average 53% of total community composition across all caterpillars. On average, one quarter of the caterpillar microbiome was shared with the soil.

CONCLUSIONS

We found that the monophagous caterpillars collected from fields located more than 100 km apart were all dominated by a single Ralstonia. The remainder of the bacterial communities that were present resembled the local microbial communities in the soil in which the host plant was growing. Our findings provide an example of a caterpillar that has just a few key associated bacteria, but that also contains a community of low abundant bacteria characteristic of soil communities.

Responses of Acidobacteria Granulicella sp. WH15 to High Carbon Revealed by Integrated Omics Analyses

The phylum Acidobacteria is widely distributed in soils, but few representatives have been cultured. In general, Acidobacteria are oligotrophs and exhibit slow growth under laboratory conditions. We sequenced the genome of Granulicella sp. WH15, a strain obtained from decaying wood, and determined the bacterial transcriptome and proteome under growth in poor medium with a low or high concentration of sugar. We detected the presence of 217 carbohydrate-associated enzymes in the genome of strain WH15. Integrated analysis of the transcriptomic and proteomic profiles showed that high sugar triggered a stress response. As part of this response, transcripts related to cell wall stress, such as sigma factor σW and toxin-antitoxin (TA) systems, were upregulated, as were several proteins involved in detoxification and repair, including MdtA and OprM. KEGG metabolic pathway analysis indicated the repression of carbon metabolism (especially the pentose phosphate pathway) and the reduction of protein synthesis, carbohydrate metabolism, and cell division, suggesting the arrest of cell activity and growth. In summary, the stress response of Granulicella sp. WH15 induced by the presence of a high sugar concentration in the medium resulted in the intensification of secretion functions to eliminate toxic compounds and the reallocation of resources to cell maintenance instead of growth.

Nitrous oxide emission related to ammonia-oxidizing bacteria and mitigation options from N fertilization in a tropical soil

Nitrous oxide (N2O) from nitrogen fertilizers applied to sugarcane has high environmental impact on ethanol production. This study aimed to determine the main microbial processes responsible for the N2O emissions from soil fertilized with different N sources, to identify options to mitigate N2O emissions, and to determine the impacts of the N sources on the soil microbiome. In a field experiment, nitrogen was applied as calcium nitrate, urea, urea with dicyandiamide or 3,4 dimethylpyrazone phosphate nitrification inhibitors (NIs), and urea coated with polymer and sulfur (PSCU). Urea caused the highest N2O emissions (1.7% of N applied) and PSCU did not reduce cumulative N2O emissions compared to urea. NIs reduced N2O emissions (95%) compared to urea and had emissions comparable to those of the control (no N). Similarly, calcium nitrate resulted in very low N2O emissions. Interestingly, N2O emissions were significantly correlated only with bacterial amoA, but not with denitrification gene (nirK, nirS, nosZ) abundances, suggesting that ammonia-oxidizing bacteria, via the nitrification pathway, were the main contributors to N2O emissions. Moreover, the treatments had little effect on microbial composition or diversity. We suggest nitrate-based fertilizers or the addition of NIs in NH4(+)-N based fertilizers as viable options for reducing N2O emissions in tropical soils and lessening the environmental impact of biofuel produced from sugarcane.

Acidobacteria strains from subdivision 1 act as plant growth-promoting bacteria

Acidobacteria is one of the most abundant phyla in soils and has been detected in rhizosphere mainly based on cultivation-independent approaches such as 16S rRNA gene survey. Although putative interaction of Acidobacteria with plants was suggested, so far no plant–bacterial interactions were shown. Therefore, we performed several in vitro tests to evaluate Acidobacteria–plant interactions and the possible mechanisms involved in such interaction. We observed that Arabidopsis thaliana inoculated with three strains belonging to Acidobacteria subdivision 1 showed increase in biomass of roots and shoots as well as morphological changes in root system. Our results indicate that the plant hormone indole-3-acetic acid production and iron acquisition are plausibly involved in the plant and Acidobacteria interactions. Here, we confirm for the first time that Acidobacteria can actively interact with plants and act as plant growth-promoting bacteria. In addition, we show that Acidobacteria strains produce exopolysaccharide which supports the adhesion of bacteria to the root surfaces.

The Ecology of Acidobacteria: Moving beyond Genes and Genomes

The phylum Acidobacteria is one of the most widespread and abundant on the planet, yet remarkably our knowledge of the role of these diverse organisms in the functioning of terrestrial ecosystems remains surprisingly rudimentary. This blatant knowledge gap stems to a large degree from the difficulties associated with the cultivation of these bacteria by classical means. Given the phylogenetic breadth of the Acidobacteria, which is similar to the metabolically diverse Proteobacteria, it is clear that detailed and functional descriptions of acidobacterial assemblages are necessary. Fortunately, recent advances are providing a glimpse into the ecology of members of the phylum Acidobacteria. These include novel cultivation and enrichment strategies, genomic characterization and analyses of metagenomic DNA from environmental samples. Here, we couple the data from these complementary approaches for a better understanding of their role in the environment, thereby providing some initial insights into the ecology of this important phylum. All cultured acidobacterial type species are heterotrophic, and members of subdivisions 1, 3, and 4 appear to be more versatile in carbohydrate utilization. Genomic and metagenomic data predict a number of ecologically relevant capabilities for some acidobacteria, including the ability to: use of nitrite as N source, respond to soil macro-, micro nutrients and soil acidity, express multiple active transporters, degrade gellan gum and produce exopolysaccharide (EPS). Although these predicted properties allude to a competitive life style in soil, only very few of these prediction shave been confirmed via physiological studies. The increased availability of genomic and physiological information, coupled to distribution data in field surveys and experiments, should direct future progress in unraveling the ecology of this important but still enigmatic phylum.

Bacterial Community Succession in Pine-Wood Decomposition

Though bacteria and fungi are common inhabitants of decaying wood, little is known about the relationship between bacterial and fungal community dynamics during natural wood decay. Based on previous studies involving inoculated wood blocks, strong fungal selection on bacteria abundance and community composition was expected to occur during natural wood decay. Here, we focused on bacterial and fungal community compositions in pine wood samples collected from dead trees in different stages of decomposition. We showed that bacterial communities undergo less drastic changes than fungal communities during wood decay. Furthermore, we found that bacterial community assembly was a stochastic process at initial stage of wood decay and became more deterministic in later stages, likely due to environmental factors. Moreover, composition of bacterial communities did not respond to the changes in the major fungal species present in the wood but rather to the stage of decay reflected by the wood density. We concluded that the shifts in the bacterial communities were a result of the changes in wood properties during decomposition and largely independent of the composition of the wood-decaying fungal communities.

Bacterial chitinolytic communities respond to chitin and pH alteration in soil

Chitin amendment is a promising soil management strategy that may enhance the suppressiveness of soil toward plant pathogens. However, we understand very little of the effects of added chitin, including the putative successions that take place in the degradative process. We performed an experiment in moderately acid soil in which the level of chitin, next to the pH, was altered. Examination of chitinase activities revealed fast responses to the added crude chitin, with peaks of enzymatic activity occurring on day 7. PCR-denaturing gradient gel electrophoresis (DGGE)-based analyses of 16S rRNA and chiA genes showed structural changes of the phylogenetically and functionally based bacterial communities following chitin addition and pH alteration. Pyrosequencing analysis indicated (i) that the diversity of chiA gene types in soil is enormous and (i) that different chiA gene types are selected by the addition of chitin at different prevailing soil pH values. Interestingly, a major role of Gram-negative bacteria versus a minor one of Actinobacteria in the immediate response to the added chitin (based on 16S rRNA gene abundance and chiA gene types) was indicated. The results of this study enhance our understanding of the response of the soil bacterial communities to chitin and are of use for both the understanding of soil suppressiveness and the possible mining of soil for novel enzymes.