Optimization of DDE-phytoremediation by exploiting plant-associated bacteria and nanoparticles

Abstract

Introduction Researchers at The Connecticut Agricultural Experiment Station observed a remarkable difference in DDE accumulation at the subspecies level of Cucurbita pepo. C. pepo ssp pepo accumulates DDE, while C. pepo ssp ovifera did not possess this capacity. In a first part, this study investigates whether the the differences in DDE accumulation can be related to differences in the plant-associated bacterial populations of the different subspecies . Bacteria were isolated from both seeds and plants of accumulating and non-accumulating Cucurbita pepo subspecies. The plants were exposed to 100 μg L-1 DDE. The second part of this projects concentrates on silver nanoparticles (AgNPs) and their capability to increase the uptake of organic contaminants by plants. Here, the possibility to optimize DDE phytoremediation by exploiting both plant-associated bacteria as well as AgNPs was investigated. Plants were exposed to 100 μg L-1 DDE, 500 mg L-1 AgNPs or both. The effects of DDE and AgNPs on plant growth and on the associated bacterial populations were investigated. Plant biomasses were determined and endophytes from both subspecies in all conditions were isolated. All isolated bacteria were subjected to (1) genotypic identification, (2) characterization of their plant growth promoting capacity, (3) screening for AgNP toxicity and (4) screening for DDE degradation capacity. Results DDE proved to have a toxic effect on plant growth, while AgNPs did not cause a decrease in biomass. The effect on the total number of plant-associated bacteria that could be isolated was the opposite. While DDE increased the number of endophytes in zucchini Raven, exposure to AgNPs caused a severe decrease. Endophytes associated with zucchini Raven demonstrated more plant growth promoting capacities than squash-associated bacteria. Bacteria that were isolated from plants that were exposed to DDE or AgNPs showed to experience less toxic effects when again exposed to these products. Conclusion The DDE accumulator Zucchini Raven contains a higher number and diversity of associated bacteria and its bacteria show more plant growth promoting capacity and potential DDE degradation capacity than the bacteria associated with the non-DDE-accumulator squash Zephyr. These findings and the higher DDE-uptake of zucchini Raven makes it a suitable organism for DDE phytoremediation. On one hand, AgNPs can be used to increase the DDE uptake of Cucurbita pepo, but on the other hand it has severe toxic effects on the plant-associated bacteria. Therefore, a good balance has to be discovered between the advantages and disadvantages of AgNPs for bacteria-enhanced phytoremediation of DDE using Cucurbita pepo ssp. pepo.