Dry Season Constrains Bacterial Phylogenetic Diversity in a Semi-Arid Rhizosphere System.

The rhizosphere is viewed as a deterministic environment led by the interaction between plants and microorganisms. In the case of semi-arid plants, this interaction is strengthened by the harshness of the environment. We tested the hypothesis that dry season represents a constraint on the bacterial diversity of the rhizosphere from semi-arid plants. To accomplish this, we sampled two leguminous species at five locations during the dry and rainy seasons in the Caatinga biome and characterised bacterial community structures using qPCR and 16S rRNA sequencing. We found that the main differences between seasons were due to reduced phylogenetic diversity caused by dryness. Variation partitioning indicated that environmental characteristics significant impacts in ß-diversity. Additionally, distance decay relationship and taxa area relationship indicate a higher spatial turnover at the rainy season. During the dry season, decreased bacterial abundance is likely due to the selection of resistant or resilient microorganisms; with the return of the rain, the sensitive populations start to colonise the rhizosphere by a process that is strongly influenced by environmental characteristics. Thus, we propose that the reduction of PD and strong influence of environmental parameters on the assemblage of these communities make them prone to functional losses caused by climatic disturbances.

Water regime influences bulk soil and Rhizosphere of Cereus jamacaru bacterial communities in the Brazilian Caatinga biome.

We used the T-RFLP technique combined with Ion Torrent (PGM) sequencing of 16S rRNA and multivariate analysis to study the structure of bulk soil and rhizosphere bacterial communities of a cactus, Cereus jamacaru, from the Brazilian Caatinga biome, which is unique to Brazil. The availability of water shapes the rhizosphere communities, resulting in different patterns during the rainy and dry seasons. Taxonomic approaches and statistical analysis revealed that the phylum Actinobacteria strongly correlated with the dry season, while samples from the rainy season exhibited a strong correlation with the phylum Proteobacteria for rhizosphere samples and with the phyla Bacteroidetes, Firmicutes, Lentisphaerae, and Tenericutes for bulk soil samples. The STAMP software also indicated that the phylum Bacteroidetes, as well as two classes in the Proteobacteria phylum (γ and δ), were the most significant ones during the rainy season. The average abundance of the phylum Actinobacteria and the genus Bacillus was significantly greater during the dry season. Some significant genera found during the dry season might reflect their tolerance to the extreme conditions found in the Caatinga biome. They may also indicate the ecological function that microorganisms play in providing plants with some degree of tolerance to water stress or in assisting in their development through mechanisms of growth promotion. Alterations in microbial communities can be due to the different abilities of native microorganisms to resist and adapt to environmental changes.

Microbial community biogeographic patterns in the rhizosphere of two Brazilian semi-arid leguminous trees.

Arid environments are regular and well distributed over all continents and display drought characteristics whether full-time or seasonal. This study aims to characterize how the microbial communities of the rhizosphere of two leguminous trees from the Brazilian semi-arid biome the Caatinga are geographically and seasonally shaped, as well as the factors driving this variation. With that purpose, the soil rhizosphere from two leguminous trees (Mimosa tenuiflora and Piptadenia stipulacea (Benth.) Ducke) were sampled in two different seasons: rainy and drought at five different sites. Assessment of bacterial and archaeal communities occurred by T-RFLP analysis of 16S rRNA and archaeal amoA genes. By these means, it was observed that the seasons (wet and dry periods) are the factors that most influence the composition of the microbial community from both analyzed plants, except for the results obtained from the CCA applied to Archaeas. Furthermore, soil physical-chemical factors also had a significant influence on the community and indicated a geographical pattern of the bacterial community. It was not possible to observe significant modifications in the composition in relation to the plant species. We have seen that soil characteristics and rainfall were the factors that most influenced the microbial composition. Also, the bacterial community had a significant correlation with soil characteristics that indicates that these rhizosphere communities might be selected by environmental characteristics. Furthermore, the data suggest that climate plays a key role in structuring the microbial community of this biome.