Characterizing Bacterial Community Structure And Diversity In Rhizospheric Soils Of Dominant Plants In Jiuduansha Wetlands, The Yangtze River Estuary

Soil microbial communities often reflect the biotic and abiotic properties of ecosystems such as plant community composition and plant traits, and thus any shifts in plant composition may lead to the changes in soil microbial communities. The objective of this study was to investigate the effects of marsh succession and plant invasion on the composition of bacterial communities. A combined use of DGGE (denaturing gradient gel electrophoresis) and cloning / sequencing was applied in this study to characterize bacterial community structure and dynamic changes in the rhizospheric soils of three marsh plants in Jiuduansha wetlands located in the Yangtze River estuary. The main results are summarized as follows:1. The 16S rDNA fragments were amplified by PCR from total community DNA extracted from the rhizospheric soils of Phragmites australis, Scirpus mariqueter and Spartina alterniflora with primers 8f (GC-clamp) and 534r. We analyzed the rhizosphere DGGE patterns for all sampling times corresponding to major plant phonological stages (dormancy, vegetative growth, reproduction, senescence). Although some sequences, such as those similar to Enterobacter, Serratia and Rhodobacter, were detected at all sampling times, considerable shifts in the bacterial communities were found, which might have resulted from differences in the microenvironments that were specific to the rhizospheric soils of three plants2. In this study, the composition of bacterial communities of rhizospheric soils of Phragmites australis, Scirpus mariqueter and Spartina alterniflora in Jiuduansha wetlands was investigated by constructing 16S ribosomal DNA clone libraries. A variety of bacterial taxa were identified, including Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Acidobacteria, Epsilonproteobacteria, Bacteroidetes, Chloroflexi, Nitrospira, Planctomycetes, Spirochaetes, Verrucomicrobia, Actinobateria, Firmicutes and some sequences ascribed to unclassified bacteria. The members of Proteobacteria were the most abundant in all rhizospheric soils. The rhizobacterial communities of three marsh plants contained similar major bacterial taxa, but phylogenetic analysis showed that the composition of these taxa was different among the plant species. This suggests that different bacteria might have selected the specific rhizospheric environments thatdifferent plants have created.3. Clones were placed into operational taxonomic unit (OTU) groups at the level of sequence similarity of >97% in order to quantify bacterial diversity. Chao 1 non-parametric diversity estimator coupled with the reciprocal of Simpson's index (1/D) was applied to sequence data obtained from each library to evaluate total sequence diversity and to quantitatively compare the level of dominance. Nonparametric estimations of bacterial richness showed that the rhizospheric soils of Phragmites australis, Scirpus mariqueter and Spartina alterniflora contained 200, 668 and 382 OTUs, respectively. The reciprocal of Simpson's index showed that bacterial communities in the Spartina alterniflora and Phragmites australis rhizospheric soils displayed species dominance, while those in Scirpus mariqueter rhizospheric soil had uniform distributions of species abundance. The differences in the observed OTU richness and evenness index, to some extent, are expected to reflect the combined effects of host plant and edaphic environmental selection pressures on bacteria in root microenvironments.4. The 16S rDNA fragments were amplified by PCR with group-specific primers. Sulfate-reducing bacterial communities and ammonia-oxidizing bacterial communities of rhizospheric soils were investigated via PCR-DGGE approach. The results showed that the composition of ammonia-oxidizing bacterial communities in rhizosphere of three plants was relatively simple. According to the data obtained in this study, ammonia-oxidizing bacterial community structure showed little change at different stages of plant growth and development. Some sequences ascribed to Desulfovibrio and Desulfuromonas were detected at all sampling times, but sequences ascribed to Desulfobulbus, Desulfosarcina, Desulfocapsa were present or absent in rhizosphere soils of different plants and at different stages of plant growth. Temporal and spatial shifts in the sulfate-reducing bacterial communities are also expected to reflect the effects of plants rhizospheric bacteria, and might indicate an association with SO₄^2- reduction rates.In conclusion, the results obtained in this study suggest that plants are capable of altering the structure and diversity of rhizobacterial communities. Phylogenetic analysis of the resultant sequences indicates that the salt marshes have a great diversity of bacteria, and that the bacterial diversity changes as the plant community experiences a rapid succession in wetlands. In addition, Spartina invasions might haveaffected the composition of native bacterial communities. Our results provide valuable information for better understanding the important roles of bacteria in wetland ecosystem functioning and evaluating the ecological impacts of plant invasions on native ecosystems.