Influence of plant-associated microbial communities on heavy metal uptake by the aquatic plant <i>Lemna minor</i>
We present research aimed at determining how bacteria associated with aquatic plants influence phytoextraction of heavy metals. We focused on the aquatic plant Lemna minor and its response to cadmium (Cd) as a model. We studied bacterial communities from plants at a contaminated site, Rice City Pond (RCP), and plants provided by the U.S. Environmental Protection Agency (EPA) that had not been exposed to Cd. Initially, we performed comparative studies of the bacterial communities from these plants, with or without Cd addition, by analysis of 16S rRNA genes. We also studied Cd tolerance in both bacteria and plants in this system. Bacterial isolates were screened for metal tolerance and for the presence of Cd resistance genes, and we compared plant resposes to Cd between surface sterilized plants and plants with associated bacterial communities. In order to further determine the effects of bacteria on plants, sterile plants inoculated with bacterial isolates were grown in medium to enrich for bacterial growth. We measured plant growth rate, root length, final medium pH, bacterial biomass, and Cd concentrations in plants.
We determined through 16S rRNA analyses that both EPA and RCP plants supported diverse bacterial communities. Upon Cd addition, EPA and RCP communities showed similar shifts in diversity, both becoming more diverse upon Cd addition. When investigating isolates from these treatments, we found that exposure to Cd increased culturability on Cd-amended medium; however, increased metal tolerance was not seen in all or even most isolates from treatments with Cd exposure, and Cd resistance systems were only found in a few isolates. We saw that surface-sterilized plants accumulated slightly higher concentrations of Cd than non-sterile plants. When enriched, bacteria slightly lowered the pH of the medium, but strong effects on root elongation and Cd phytoextraction.
Bacteria seem to influence root elongation positively and influence Cd uptake negatively, keeping toxic concentrations of the metal from entering the plant. This study will allow better understanding of the role of aquatic plant-associated microbial communities in phytoremediation of heavy metals.