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.Polychlorinated biphenyls (PCBs) are toxic, persistent pollutants of worldwide concern whose cleanup using conventional methods like incineration or relocation to specialized landfills is often prohibitively expensive. An alternative strategy for in situ PCB removal is biodegradation by microorganisms capable of metabolizing PCBs. Although bioaugmentation of soil with degradative bacteria has been largely unsuccessful in achieving significant aerobic PCB degradation in the field (29), efforts to biostimulate indigenous PCB-degrading bacteria have been promising. Analogue enrichment with biphenyl has been shown to increase the numbers of aerobic PCB-degrading bacteria in soil microcosms (12, 51) and to enhance PCB degradation rates in soils (6, 12) and in situ sediments (18). Unfortunately, as a field remedial strategy, addition of biphenyl is problematic due to the low water solubility of biphenyl, the necessity of repeated application, and concerns about biphenyl toxicity. By capitalizing on the innate ability of plants to alter soil microbial community structure, rhizoremediation offers an attractive and affordable alternative means for long-term biostimulation of aerobic PCB degradation in situ.Rhizostimulation of aromatic poll...

Degradation of PCB congeners by bacterial strains

“…However, similarly to our study, Leigh et al (2006) found naturally occurring PCB degraders in significantly higher numbers in the rhizosphere of several plant species compared to soil (PCB contaminated site).…”

“…Microbial degradation of PCBs in contaminated soil has been the subject of extensive research (Furukawa and Matsumura 1976;Unterman 1988;Leigh et al 2006). In the environment, PCBs may be degraded by aerobic bacteria through the biphenyl catabolic pathway, following a cometabolic process (Seeger et al 1995;Abramowicz 1990).…”

“…Rhodococci, such as Rhodococcus sp. strain RHA1, are known to be effective PCB degraders surviving well in soil (Masai et al 1995, Seto et al 1995, Leigh et al 2006. Furthermore, many studies centred on Burkholderia sp.…”

“…However, the practical importance of such procedures is limited due to technical difficulties (repeated application) or the potential toxicity and low water solubility of biostimulating substances such as biphenyl (e.g. Leigh et al 2006). An alternative solution might be to insert the bph genes encoding the biphenyl pathway into a bacterial host with high survival capability in specific soil compartments.…”

“…Thus, rhizoremediation, i.e. the use of microbes in conjunction with plants, is a promising bioremediation strategy (Trapp and Karlson 2001;Leigh et al 2006;Yee et al 1998). Consequently, efforts were undertaken to expand the degradation capacities of rhizospherecompetent bacteria (Villacieros et al 2005;Brazil et al 1995;Yee et al 1998).…”