Co-occurrence of a native and invasive wetland plant species increases rhizosphere bacterial diversity in both species

Background/Question/Methods  

Assemblages of bacterial species that coexist in close association with plant roots (the rhizosphere) are distinct from the surrounding environment and are strongly influenced by plant species identity. A prominent invasive plant in many human-disturbed systems across North America is purple loosestrife (Lythrum salicaria) which can exclude native wetland species in comparison to the native broadleaf cattail (Typha latifolia). The objective of this study was to compare the structural differences of rhizosphere bacterial communities associated with these two ecologically significant wetland plants. Root tissue and associated sediment material from 48 plants (24 of each species) was collected from three sites in Northwestern PA. Of the 16 samples at each site, eight samples were taken from plants that co-occurred directly with the other species (i.e., their root systems occupied the same volume of soil).Microbial DNA was isolated from root-associated sediments and the 16S rRNA taxonomic marker gene was amplified using barcoded primers. Bacterial communities were characterized through targeted metagenomic sequencing of the 16S amplicons using the Illumina MiSeq platform. Bacterial alpha and beta diversity patterns were compared against three factors: site, plant species, and co-occurrence using univariate ANOVAs and permutational MANOVAs, respectively.

Results/Conclusions

At a broad taxonomic level, both plant species had rhizospheres colonized primarily by the Proteobacteria, Planctomycetes, Chloroflexi, Bacteroidetes, Verrucomicrobia, Acidobacteria, Firmicutes, and Actinobacteria. Diversity of bacterial operational taxonomic units (OTUs) in rhizosphere communities was higher in plants that co-occurred with the other species (Simpson evenness: F_1,38=4.43, p=0.04; Chao richness: F_1,38=12.1, p=0.001). Site and plant species were not significant predictors of bacterial alpha diversity but were the strongest factors along which communities differentiated. This pattern was consistent by both abundance-based measures (Bray-Curtis: F_Site=2.16, p_Site=0.005 and F_Sp=1.91, p_Sp=0.029) and presence-absence measures (Jaccard: F_Site=1.65, p_Site=0.005 and F_Sp=1.43, p_Sp=0.029). Co-occurring plants shared the same proportion of bacterial OTUs as plants occurring separately (51.6% against 49.3%) but 20% more OTUs were attributed to co-occurring plant rhizospheres than the rhizospheres of separate plants. These results suggest that sediments with multi-species root systems favor more diverse communities and highlight the need for further studies of the wetland rhizosphere habitat.