Environmental Engineering Reference
In-Depth Information
any event, the result was that ortho-only-substituted congeners increased
from less than 1% in Aroclor 1260 to 39%, with sequential inoculations at
the end of the experiment.
Another biological barrier to PCB bioremediation is that there are no
bacteria that have been found in nature that can grow on important PCB
congeners as a growth substrate. Anaerobic cometabolic reductive dechlori-
nation of highly chlorinated PCBs produces less chlorinated congeners
(mono-, di-, and trichlorinated biphenyls), preferentially ortho-chlorobiphe-
nyls and ortho- + para-derivatives. Aerobic biphenyl-degrading bacteria can
nonspecifically co-oxidize less chlorinated PCBs, but this process requires
biphenyl or related other sources as a growth substrate. The major product
of this dead-end cometabolism is the respective chlorobenzoate. If the barrier
of growth on PCBs can be overcome, then a natural enrichment by growth
on PCBs should occur, increasing the rate of PCB removal. This approach
should be a more desired solution for PCB remediation, because it would
avoid the need to manage cometabolism, which can be difficult and costly.
Previously, we obtained a collection of bacterial isolates that aerobically
degrade biphenyl and cometabolize certain PCB congeners. We have cloned
and characterized the genes specifying anoxic hydrolytic para-dechlorina-
tion (
fcb
operon from
Arthrobacter globiformis
KZT1) and oxygenolytic
ortho-dechlorination (
ohb
operon from
Pseudomonas aeruginosa
142). These
genes encode enzymes that remove chlorine from chlorobenzoates, funneling
nonchlorinated products into common degradative pathways for nonchlo-
rinated aromatics. Combining these dehalogenase genes and biphenyl oxi-
dation pathways should result in engineered pathways for ortho-, para-, and
ortho- + para-chlorinated PCB congeners that are of a major concern in the
proposed anaerobic-aerobic bioremediation scheme (Figure 6.5). In contrast
to approaches employed by other groups, using the specific chlorobenzoate
dehalogenases not only allows the choice of a desirable host (for example,
PCB-tolerant bacteria, gram positive or gram negative) but also prevents
accumulation of toxic-aromatic-ring meta-cleavage products that would be
produced from using broad specificity benzoate oxygenases that produce
chlorocatechol from chlorobenzoate.
Another potential problem we envision for
in situ
bioremediation is that
combining the anaerobic phase for reductive dechlorination of highly chlo-
rinated PCBs and the aerobic phase for oxidation of less chlorinated PCB
congeners in the same remediation scheme might be too complicated to
manage if the common aerobe, such as gram-negative bacteria, were used
for the aerobic phase. To overcome this barrier, we propose to use not only
gram-negative but also gram-positive bacteria for construction of PCB deg-
radation pathways. The well-known ability of gram-positive microaerophilic
bacteria, such as bacilli, corynebacteria, and rhodococcaceae, to persist in
harsh environments and survive anaerobic conditions should reduce reme-
diation costs and increase chances of success.
