Polychlorinated biphenyl (PCB)-degrading bacteria associated with trees in a PCB-contaminated site

Abstract

The abundance, identities, and degradation abilities of indigenous polychlorinated biphenyl (PCB)-degrading bacteria associated with five species of mature trees growing naturally in a contaminated site were investigated to identify plants that enhance the microbial PCB degradation potential in soil. Culturable PCB degraders were associated with every plant species examined in both the rhizosphere and root zone, which was defined as the bulk soil in which the plant was rooted. Significantly higher numbers of PCB degraders (2.7- to 56.7-fold-higher means) were detected in the root zones of Austrian pine (Pinus nigra) and goat willow (Salix caprea) than in the root zones of other plants or non-root-containing soil in certain seasons and at certain soil depths. The majority of culturable PCB degraders throughout the site and the majority of culturable PCB degraders associated with plants were identified as members of the genus Rhodococcus by 16S rRNA gene sequence analysis. Other taxa of PCB-degrading bacteria included members of the genera Luteibacter and Williamsia, which have not previously been shown to include PCB degraders. PCB degradation assays revealed that some isolates from the site have broad congener specificities; these isolates included one Rhodococcus strain that exhibited degradation abilities similar to those of Burkholderia xenovorans LB400. Isolates with broad congener specificity were widespread at the site, including in the biostimulated root zone of willow. The apparent association of certain plant species with increased abundance of indigenous PCB degraders, including organisms with outstanding degradation abilities, throughout the root zone supports the notion that biostimulation through rhizoremediation is a promising strategy for enhancing PCB degradation in situ.

FIG. 1.

Seasonal trends in the mean numbers of PCB degraders (a) and total heterotrophs (b) at a depth of 20 to 40 cm. The mean values are based on four to six replicate plate counts for each plant species on each date. gDW, gram (dry weight).

FIG. 2.

Relationship between soil moisture and numbers of PCB degraders (a) and total heterotrophs (b) (n = 88). gDW, gram (dry weight).

FIG. 3.

PCB degradation abilities of bacteria isolated from the root zones or rhizospheres of plants growing in PCB-contaminated soil. Species names in parentheses indicate the nearest matches in the RDP II database. The previously characterized strains B. xenovorans LB400 and Rhodococcus sp. strain RHA1 are included for reference.