Bacteria associated with Oak and Ash on a TCE-contaminated site: characterization of isolates with potential to avoid evapotranspiration of TCE

Environmental Biology, Hasselt University, Agoralaan Building D, 3590, Diepenbeek, Belgium.
Environmental Science and Pollution Research (Impact Factor: 2.83). 05/2009; 16(7):830-43. DOI: 10.1007/s11356-009-0154-0
Source: PubMed


Along transects under a mixed woodland of English Oak (Quercus robur) and Common Ash (Fraxinus excelsior) growing on a trichloroethylene (TCE)-contaminated groundwater plume, sharp decreases in TCE concentrations were observed, while transects outside the planted area did not show this remarkable decrease. This suggested a possibly active role of the trees and their associated bacteria in the remediation process. Therefore, the cultivable bacterial communities associated with both tree species growing on this TCE-contaminated groundwater plume were investigated in order to assess the possibilities and practical aspects of using these common native tree species and their associated bacteria for phytoremediation. In this study, only the cultivable bacteria were characterized because the final aim was to isolate TCE-degrading, heavy metal resistant bacteria that might be used as traceable inocula to enhance bioremediation.
Cultivable bacteria isolated from bulk soil, rhizosphere, root, stem, and leaf were genotypically characterized by amplified rDNA restriction analysis (ARDRA) of their 16S rRNA gene and identified by 16S rRNA gene sequencing. Bacteria that displayed distinct ARDRA patterns were screened for heavy metal resistance, as well as TCE tolerance and degradation, as preparation for possible future in situ inoculation experiments. Furthermore, in situ evapotranspiration measurements were performed to investigate if the degradation capacity of the associated bacteria is enough to prevent TCE evapotranspiration to the air.
Between both tree species, the associated populations of cultivable bacteria clearly differed in composition. In English Oak, more species-specific, most likely obligate endophytes were found. The majority of the isolated bacteria showed increased tolerance to TCE, and TCE degradation capacity was observed in some of the strains. However, in situ evapotranspiration measurements revealed that a significant amount of TCE and its metabolites was evaporating through the leaves to the atmosphere.
The characterization of the isolates obtained in this study shows that the bacterial community associated with Oak and Ash on a TCE-contaminated site, was strongly enriched with TCE-tolerant strains. However, this was not sufficient to degrade all TCE before it reaches the leaves. A possible strategy to overcome this evapotranspiration to the atmosphere is to enrich the plant-associated TCE-degrading bacteria by in situ inoculation with endophytic strains capable of degrading TCE.

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    • "From all purified bacterial strains total genomic DNA was extracted using the DNeasy R Blood and Tissue Kit (Qiagen , Valencia, CA, USA). Polymerase chain reaction (PCR) amplification of 16S rRNA, amplified 16S rRNA restriction analysis (ARDRA) and gel electrophoresis were performed according to Weyens et al. (2009b). Bacterial strains with the same ARDRA patterns were grouped and the purified PCR product (QIAquick 96 PCR Purification Kit (Qiagen, Valencia, CA, USA)) of 1 representative isolate of each group was sequenced by Macrogen (Seoul, Korea) using an Automatic Sequencer 3730XL. "
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    • "While it has already been shown that endophytes from eudicots can have strong beneficial impacts on monocots, and endophytes from monocots can benefit dicots (Khan et al. 2012; Knoth et al. 2012; Rodriguez et al. 2008), it is still encouraging that the CAP9 consortium confers resistance to TNT in common bent grass, a monocot species that is taxonomically distant from A. pseudoplatanus, a dicotyledonous tree species. Given that there are clearly different populations of associated, cultivable bacteria even between tree species (Weyens et al. 2009a), it would be interesting to test the ability of acer trees inoculated with the CAP9 consortium to remediate TNT. Levels of endophytes with xenobiotic-detoxifying abilities appear to correlate with the concentration of the xenobiotic in the soil in which the host plants are growing (Siciliano et al. 2001) and, as shown in this study, TNT-detoxifying endophytes are present in TNTcontaminated soil (Thijs et al. 2014a). "

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