Isolation, Cultivation, and Characterization of endophytes in function of phytoremediation of DDE-contaminated soils

Abstract

2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane (DDT) is a pesticide that was used worldwide from 1943, but has been forbidden in most western countries since the 1970s. In soil, it quickly degrades to 2,2-bis(p-chlorophenyl)-1,1-dichloroethylene (DDE). Both molecules are considered as persistent organic pollutants (POPs), which are highly recalcitrant in soil, resulting in a worldwide spread of DDE contaminated soils. Cucurbita pepo has been shown to accumulate high concentrations of DDE from surrounding soil (White et al., 2005). This research focuses on optimizing the phytoremediation process with the help of plant-associated endophytic bacteria. To enhance the extraction of DDE from the soil and improve its degradation in the plant, endophytic bacteria have to be isolated and cultivated. Previously, it has been shown that only about 1-5% of the total bacterial population can be isolated and cultivated successfully (Torsvik and Øvreås, 2002). To increase the cultivable fraction of the total bacterial population, a first part of this research explores different isolation and cultivation techniques. Results reveal that the medium composition as well as the addition of plant extract can affect the extent and the diversity of the cultivable of the bacterial population. To investigate the role of endophytes during phytoremediation of DDE, in the second part, a comparison was made between the endophytic population isolated from plants that were grown with 100 µg L-1 DDE and control plants. The isolated bacteria were identified genotypically using ARDRA and were characterized phenotypically using several tests concerning their plant growth promoting capacities such as the production of indol-3-acetic acid (IAA), siderophores, organic acids, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase. Bacteria that can be beneficial to plant growth were found in both control and exposed plants, however, a general trend was noticeable where endophytes isolated from DDE-exposed plants scored better in the tests for plant growth promoting capacities. Furthermore, a screening for possible DDE degradation by endophytic bacteria was performed. Endophytes isolated from DDE-exposed plants showed a DDE degrading capacity more often than endophytes isolated from plants grown without DDE.