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. 2005 May;71(5):2484-92.
doi: 10.1128/AEM.71.5.2484-2492.2005.

Shifts in Rhizoplane Communities of Aquatic Plants After Cadmium Exposure

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Shifts in Rhizoplane Communities of Aquatic Plants After Cadmium Exposure

Lisa M Stout et al. Appl Environ Microbiol. .
Free PMC article

Abstract

In this study we present the comparative molecular analysis of bacterial communities of the aquatic plant Lemna minor from a contaminated site (RCP) and from a laboratory culture (EPA), as well as each of these with the addition of cadmium. Plants were identified as L. minor by analysis of the rpl16 chloroplast region. Comparative bacterial community studies were based on the analyses of 16S rRNA clone libraries, each containing about 100 clones from the root surfaces of plants. Bacterial communities were compared at three phylogenetic levels of resolution. At the level of bacterial divisions, differences in diversity index scores between treatments, with and without cadmium within the same plant type (EPA or RCP), were small, indicating that cadmium had little effect. When we compared genera within the most dominant group, the beta-proteobacteria, differences between unamended and cadmium-amended libraries were much larger. Bacterial diversity increased upon cadmium addition for both EPA and RCP libraries. Analyses of diversity at the phylotype level showed parallel shifts to more even communities upon cadmium addition; that is, percentage changes in diversity indices due to cadmium addition were the same for either plant type, indicating that contamination history might be independent of disturbance-induced diversity shifts. At finer phylogenetic levels of resolution, the effects of cadmium addition on bacterial communities were very noticeable. This study is a first step in understanding the role of aquatic plant-associated microbial communities in phytoremediation of heavy metals.

Figures

FIG. 1.
FIG. 1.
Phylogenetic identification of the plants used in this study. Phylogenetic analysis of the rpl16 chloroplast intron region was used for identification of members of the family Lemnaceae. EPA plants and RCP plants group with the Lemna minor rpl16 gene, while other species of Lemna form separate clusters. The rpl16 region from Pistia stratiotes was used as an outgroup. The phylogenetic tree was constructed using neighbor-joining analysis with 1,000 bootstrap replicates. Bootstrap values are indicated at nodes. The scale bar indicates 0.02 nucleotide substitution.
FIG. 2.
FIG. 2.
Abundance of clones from bacterial divisions present in clone libraries of (A) EPA plants (black bars) or EPA plants plus Cd (gray bars). RCP clone libraries are shown in panel B for RCP plants (black bars) or RCP plants plus Cd (gray bars). The number of clones used for each clone library analysis is indicated.
FIG. 3.
FIG. 3.
Relative abundance of genera within the dominant group, β-proteobacteria, in plant-associated communities. The influence of cadmium addition is shown for (A) EPA plants in EPA water, (B) EPA plants in EPA water amended with Cd, (C) RCP plants in RCP water, and (D) RCP plants in RCP water amended with Cd. Genera are assigned based on closest RDP II matches. Genera listed in bold are dominant groups within each library. Percentages are based on the number of clones (n) of β-proteobacteria in each library.
FIG. 4.
FIG. 4.
Phylogenetic relationships of bacteria detected on EPA plants (EPA libraries A and B, left) and on RCP plants (RCP libraries C and D, right) inferred from 16S rRNA sequence analysis. Trees were constructed using the neighbor-joining algorithm with 1,000 bootstrap replicates. For each tree, ▪ represents no Cd added and ▴ represents the addition of Cd. Numbers in parentheses represent the number of clones represented by each phylotype (≥97% similarity). Staphylococcus aureus is included as an outgroup for each tree. The scale bar represents nucleotide substitutions.
FIG. 5.
FIG. 5.
Rarefaction curves for clone libraries from EPA (♦), EPA plus Cd (▪), RCP (▴), and RCP plus Cd (•). Phylotypes were determined by ≥97% similarity.

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