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Understanding how species interaction and assembly processes structure the abundant and rare bacterial biosphere in soils is crucial for predicting how biodiversity influences ecosystem functioning. Here, we profiled the bacterial communities across a salt marsh ecosystem gradient to investigate the co-occurrence patterns across taxa and the relative influence of ecological processes mediating the assembly of the abundant and rare biospheres in soil. Our results revealed abundant taxa to be ubiquitous across all sample sites, whereas the distributions of the rare taxa were relatively more site-specific. The alpha-diversity indices and beta-diversity of rare sub-communities were significantly higher than those of the abundant sub-communities. Besides, both the taxonomic and functional composition of soil bacterial communities significantly differed between the two biospheres. Furthermore, the influence of stochasticity differed in each sub-community. In particular, stochastic processes were relatively more important in constraining the rare taxa assembly. Co-occurrence network analysis revealed that a few abundant taxa occupy central nodes within the networks, possibly indicating crucial roles as keystone taxa. Collectively, these findings suggest that abundant and rare bacterial biospheres have distinct distributions underpinned by a dynamic interplay of ecological processes and taxa co-occurrence patterns. [ABSTRACT FROM AUTHOR]

Bacterial communities play essential roles in estuarine marsh ecosystems, but the interplay of ecological processes underlying their community assembly are poorly understood. Here, we studied the sediment bacterial communities along a linear gradient extending from the water-land junction toward a high-marsh, using the 16S rRNA gene amplicon sequencing. Bacterial community composition differed significantly between sediment transects. Physico-chemical properties, particularly sediment nutrient levels [i.e., total nitrogen (TN) and available phosphorus (AP)], as well as sediment physical structure and pH (P < 0.05) were strongly associated with the overall community variations. In addition, the topological properties of bacterial co-occurrence networks varied with the distance to the water-land junction. Both node- and network-level topological features revealed the bacterial network of sediments farthest from the junction to be less intense in complexity and interactions compared with other sediments. Phylogenetic null modelling analysis showed a progressive transition from stochastic to deterministic community assembly for the water-land junction sites toward the emerging terrestrial system. Taken together, this study provides a detailed outline of the distribution pattern of sediment bacterial community across the estuarine marsh and inform on the mechanisms and processes mediating bacterial community assembly in marsh soils. [ABSTRACT FROM AUTHOR]

Although mangroves represent ecosystems of global importance, the genetic diversity and abundance of functional genes that are key to their functioning scarcely have been explored. Here, we present a survey based on the nifli gene across transects of sediments of two mangrove systems located along the coast line of Säo Paulo state (Brazil) which differed by degree of disturbance, i.e., an oil-spill-affected and an unaffected mangrove. The diazotrophic communities were assessed by denaturing gradient gel electrophoresis (DGGE), quantitative PCR (qPCR), and clone libraries. The nifli gene abundance was similar across the two mangrove sediment systems, as evidenced by qPCR. However, the nifli-based PCR-DGGE profiles revealed clear differences between the mangroves. Moreover, shifts in the nifli gene diversities were noted along the land-sea transect within the previously oiled mangrove. The nifli gene diversity depicted the presence of nitrogen-fixing bacteria affiliated with a wide range of taxa, encompassing members oi the Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Firmicutes, and also a group of anaerobic sulfate-reducing bacteria. We also detected a unique mangrove-specific cluster of sequences denoted Mgv-nifli. Our results indicate that nitrogen-fixing bacterial guilds can be partially endemic to mangroves, and these communities are modulated by oil contamination, which has important implications for conservation strategies. [ABSTRACT FROM AUTHOR]