Environmental Engineering Reference
In-Depth Information
is unclear. Perhaps it is related to the organic matter that each species nor-
mally utilizes in nature. It is assumed that EPA 505 will hydroxylate other
HMW PAHs in the same manner and produce ring-opened intermediates
that cannot be further metabolized. Thus, this strain appears to have a
remarkable breadth of cometabolic capability due to very loose specificity
of the dioxygenase enzymes that are responsible for metabolizing phenan-
threne and fluoranthene.
7.1.3.5.4 Biodegradation pathways.
As microorganisms in the envi-
ronment are confronted with PAHs of increasing numbers of aromatic rings,
the biochemical strategy for removing carbon fragments that can be oxidized
through the intermediary metabolism of the organisms becomes more com-
plex. In many cases, the activity of PAH-degrading enzymes is constitutive,
yet the presence of certain growth PAHs seems to increase enzyme activity
(Aitken et al., 1998). Stringfellow and Aitken (1995) and Stringfellow et al.
(1995) have shown that the growth substrate phenanthrene (and salicylate)
was able to induce the cometabolism of fluoranthene and pyrene, both of
which were not growth substrates in
Pseudomonas saccharophilia
strain P-15,
an isolate from PAH-contaminated soil. Studies with the same organism also
revealed that chrysene and benzo(a)anthracene could be mineralized and
the mineralization was stimulated by pregrowth on phenanthrene (Chen and
Aitken, 1999). Chrysene was not a metabolic inducer. Benzo(a)pyrene was
not mineralized significantly regardless of whether cells were pregrown on
phenanthrene or not. In soils, it is possible that an inducer such as salicylate
could be added directly to the soil to induce greater PAH degradation activ-
ity, but it is probably impractical in the final analysis (Chen and Aitken,
1999). The more likely scenario is to make the HMW PAHs as available as
possible during the time when the inducing PAHs, most likely phenanthrene,
are actively being degraded.
The potential of sequential degradation of the PAHs complicates issues
of cometabolic induction. Phenanthrene is usually in high concentration in
PAH-contaminated soil. If it is competing with the cometabolized PAH for
the same active site of an enzyme, cometabolism is likely to be slowed by
phenanthrene, assuming it is the preferred substrate for the enzymes. In
studies examining the effect of PAH mixtures on cometabolism, Luning Prak
and Pritchard (2002a) have shown that phenanthrene does inhibit the come-
tabolism of pyrene in strain EPA 505. These experiments were carried out
in the presence of the chemical surfactant Tween 80 in order to produce
concentrations of PAHs that could be readily followed by HPLC analysis.
In these studies, the rate of phenanthrene degradation is the same in the
presence and absence of pyrene, suggesting that if only one enzyme system
is responsible for the initial PAH metabolism, then phenanthrene is clearly
the preferred PAH substrate. The presence of phenanthrene does not totally
preclude pyrene cometabolism (i.e., there is some decay over time), but
pyrene metabolism is much slower in the absence of phenanthrene.
