Rhizosphere ecology of PCB-degrading bacteria at a contaminated site and implications for bioremediation.

Abstract

Rhizosphere bioremediation is an emerging technology that relies on plant roots to enhance bacterial biodegradation of soil contaminants like polychlorinated biphenyls (PCBs), providing an inexpensive alternative to conventional cleanup methods. Ecological investigations were conducted at two naturally-vegetated, PCB-contaminated field sites in the Czech Republic to identify PCB-tolerant plant species with the ability to enrich indigenous PCB-degrading bacteria in their root zones and to identify taxa of bacteria significant to rhizoremediation. Vegetation analyses at the contaminated sites revealed that 51 different plant species, including trees, grasses and forbs, and 6 different mushroom species were growing in soil contaminated with PCBs in concentrations from 1--500 mg/kg DW soil, indicating that PCBs are not toxic to a broad spectrum of plant and fungal species at these concentrations. Two out of five trees investigated, Austrian pine (Pinus nigra) and goat willow (Salix caprea), fostered significantly higher numbers of PCB-degrading bacteria in their root zones compared to other plant species or nonrooted soil due, due to an apparent enrichment effect. The abundant aromatic compounds produced by pine and willow may be one of the driving forces impacting populations of bacteria that degrade both plant aromatic compounds and aromatic contaminants in soil. The impacts of abiotic factors on populations of PCB-degraders was also investigated, and there was no significant influence detected by soil moisture (ranging from 1.8--18.5%) or soil PCB concentrations (0--500 mg/kg). Of the 163 PCB-metabolizing bacteria isolated from root-zone soils and 80 isolates from the rhizoplane, 93% and 100% of them were Gram-positive, respectively. Taxonomic identification of PCB-degrading bacterial isolates was performed using 16S rDNA sequence analysis. Rhodococcus spp. were the most prevalent PCB-degraders at the site, and were isolated from the roots of every plant species investigated. Several isolates exhibited high PCB degradation rates and broad congener specificities when subjected to a PCB degradation assay. The findings that some plant species enrich populations of indigenous bacteria with outstanding PCB-degradation abilities in the root-zone under field conditions support the rhizoremediation concept that growing certain plants in contaminated sites can provide a sustained, ecologically-stable strategy for the bioremediation of PCBs.