Polycyclic aromatic hydrocarbons (PAHs): improved land treatment with bioaugmentation - Bioremediation of Recalcitrant Compounds - page 230

degraders (predominantly Pseudomonas species). The cluster in the lower
right (1, 4-6, 13, 24-26, 29) consisted of fluoranthene degraders, predomi-
nantly Sphingomonas species. These results suggest that certain catabolic
characteristics are associated with specific genera. That is, Sphingomonas and
Mycobacterium could be the primary genera that are able to attack HMW
PAHs. This may be related to characteristics of their cell membranes that
allow these PAHs to diffuse inside the cells, or it may be related to the
presence of key membrane-associated dioxygenases. Interestingly, most of
the pure cultures that have been isolated for their ability to grow on pyrene
have been Mycobacterium . We are aware of only one case in which a
Gram-negative microorganism, a Pseudomonas strain, was able to grow on
pyrene (Thibault et al., 1996).
Numerous fungal species are also able to partially degrade PAHs, both
low and high molecular weight (Baldrian et al., 2000; Boonchan et al., 2000;
Mueller et al., 1997). The initial reactions of PAH degradation by fungi are
usually ascribed to their extracellular lignolytic enzymes (usually the lacca-
ses and perioxidases), and these organisms may be involved in PAH turnover
in unpolluted soils. The effectiveness of a coculture of a fungus ( Penicillium )
with the bacterium Stenotrophomonas and a mixed bacterial population in
degrading five-ring PAHs suggests that the initial oxidation products pro-
duced by the fungi are then further degraded by the bacteria. There was no
indication that this coculture affects the bioavailability of the HMW PAHs.
It may not be a particularly useful technique for bioremediation, but it is
interesting in terms of the natural way in which PAHs might be degraded
in the environment.
The most important observation is that there are a number of PAH
degraders that have relatively broad cometabolic capabilities, especially for
HMW PAHs (Dagher et al., 1996; Mahaffey et al., 1988; Mueller et al., 1997;
Schneider et al., 1996; Schocken and Gibson, 1984). The term cometabolism ,
as used in this study, is the ability to transform (oxidize) a particular PAH,
but without growth on that PAH. Presumably, partial degradation products
are generated, which cannot be further metabolized to produce carbon
and energy. There have been several reports of PAH-cometabolizing
Sphingomonas species (Dagher et al., 1996; Fredrickson et al., 1995; Kastner
et al., 1994; Mueller et al., 1990). Sphingomonas paucimobilis strain EPA 505
has been shown to have a substantial cometabolic capability for HMW PAHs
(Mueller et al., 1990; Ye et al., 1996). Sphingomonas strain B1 (formally Beijer-
inckia B1), isolated originally for its ability to grow on biphenyl, has been
shown to cometabolize benzo(a)anthracene to acid metabolites (Gibson et
al., 1973; Mahaffey et al., 1988). Whether sphingomonads are commonly
associated with PAH degradation is yet to be assessed, but a study examining
the diversity of bacteria able to degrade PAHs (Ye et al., 1996) showed that
Sphingomonas species tended to be the isolates capable of degrading fluoran-
thene, whereas the bacteria able to degrade phenanthrene were more com-
monly associated with Pseudomonas strains. Dagher et al. (1996) compared
three PAH-degrading Pseudomonas sp. with a Sphingomonas sp. and found