To improve the knowledge about the biogeographic patterns of bacteria in soils contaminated with crude oil, we studied the effects of local geochemical properties and geographic distance on bacterial community structure in oil contaminated soil in five oil refineries (46 to 360 km apart). The microbial structure was significantly affected by soil environmental factors such as pH, total petroleum hydrocarbons (TPH), total nitrogen, and cadmium level. Microbial alpha-diversity was positively correlated with pH but negatively correlated with TPH. Among sampling sites, the community dissimilarities increased with spatial distance. Variation of bacterial community was mostly attributed to simultaneous effects of spatial distance and environmental factors, and purely spatial distance contributed more to the variation. Microbial generalist OTUs were broadly distributed and dominant in contaminated soils. Their populations were in low proportion (15.75%), but they had high relative abundance (65.05%), and some were associated with TPH-degradation. Network analysis indicated that microbial communities had non-random co-occurrence patterns. Keystone taxa were Rubrivivax, Nitrospira, Methylotenera, Methyloversatilis and Acidaminobacter. Microbial taxa from the same module had strong ecological linkages and were involved in biological electron-transfer, C and N-cycles, and organic contaminant degradation. Our results indicate that the same microbial groups with TPH-degradation ability can be assembled from indigenous microorganisms in separate regions through long term exposure to contamination.

The microbiomes of rhizocompartments (nodule endophytes, root endophytes, rhizosphere and root zone) in soya bean and alfalfa were analysed using high-throughput sequencing to investigate the interactions among legume species, microorganisms and soil types. A clear hierarchical filtration of microbiota by plants was observed in the four rhizocompartments - the nodule endosphere, root endosphere, rhizosphere and root zone - as demonstrated by significant variations in the composition of the microbial community in the different compartments. The rhizosphere and root zone microbial communities were largely influenced by soil type, and the nodule and root endophytes were primarily determined by plant species. Diverse microbes inhabited the root nodule endosphere, and the corresponding dominant symbiotic rhizobia belonged to Ensifer for alfalfa and Ensifer-Bradyrhizobium for soya bean. The nonsymbiotic nodule endophytes were mainly Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes. The variation in root microbial communities was also affected by the plant growth stage. In summary, this study demonstrated that the enrichment process of nodule endophytes follows a hierarchical filtration and that the bacterial communities in nodule endophytes vary according to the plant species.

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Elucidating the driving forces behind the temporal dynamics of abundant and rare microbes is essential for understanding the assembly and succession of microbial communities. Here, we explored the successional trajectories and mechanisms of abundant and rare bacteria via soil-enrichment subcultures in response to various pollutants (phenanthrene, n-octadecane, and CdCl) using time-series Illumina sequencing datasets. The results reveal different successional patterns of abundant and rare sub-communities in eighty pollutant-degrading consortia and two original soil samples. A temporal decrease in α-diversity and high turnover rate for β-diversity indicate that deterministic processes are the main drivers of the succession of the abundant sub-community; however, the high cumulative species richness indicates that stochastic processes drive the succession of the rare sub-community. A functional prediction showed that abundant bacteria contribute primary functions to the pollutant-degrading consortia, such as amino acid metabolism, cellular responses to stress, and hydrocarbon degradation. Meanwhile, rare bacteria contribute a substantial fraction of auxiliary functions, such as carbohydrate-active enzymes, fermentation, and homoacetogenesis, which indicates their roles as a source of functional diversity. Our study suggests that the temporal succession of microbes in polluted microcosms is mainly associated with abundant bacteria rather than the high proportion of rare taxa. The major forces (i.e., stochastic or deterministic processes) driving microbial succession could be dependent on the low- or high-abundance community members in temporal microcosms with pollutants.