Environmental Engineering Reference
In-Depth Information
corresponding increase of the chlorobenzoic acids, was detected during
another 72 h of incubation, suggesting that meta-cleavage products formed
by NY05 and VP44 were stable. Recovery of about 5% of 2,4′-CB as 4-CBA
by both strains showed that they could also oxidize the ortho-chlorinated
ring.
Advantageously to NY05 and VP44, strains LB400 and RHA1 efficiently
degraded 80 to 100% of 2,2′- and 2,4,2′-CB (chlorine in ortho-position on
each ring of the biphenyl moiety). Both strains transformed 2,4,2′-CB into
equimolar amounts of 2,4-CBA, and there was a 60 to 80% transformation
rate of 2,2′-CB into 2-CBA. In the latter case, RHA1 accumulated HOPDA
(maximum of 394 nm), accounting for the difference in the depleted CB and
accumulated 2-CBA, whereas LB400 partially depleted 2-CBA, similar to
2-CB oxidation. NY05 and VP44, again, were much less active, depleting
only 5 to 30% of 2,2′- and 2,4,2′-CB, with equimolar yields of 2-CBA and
2,4-CBA. It should be noted that LB400 advantageously exhibited only tran-
sient appearance of (Cl)HOPDAs, whereas strain RHA1 accumulated
HOPDA from 2-CB and 2,2′-CB. This difference between two strains with
similar PCB degradative ability might be due to the different substrate spec-
ificity of their respective HOPDA hydrolases, which have only about 30%
sequence identity (Hofer et al., 1993; Masai et al., 1997). Poor turnover of
HOPDAs can exclude some CBs from further productive metabolism.
Two principal and complementary modes of PCB metabolism (prefer-
ential oxygenation of ortho- or para-chlorinated ring of biphenyl moiety)
were demonstrated by
Rhodococcus
strains NY05 and RHA1 and have been
found most attractive for use in bioremediation. Consumption rates and
products of metabolism of the eight PCB congeners by strains NY05, VP44,
and LB400 were determined. These findings were summarized in Pellizari
et al. (1996), Hrywna et al. (1999), and Maltseva et al. (1999). We have studied
metabolism of defined PCB mixtures simulating most extensive (pattern C)
and average (pattern M) anaerobic dechlorination products accumulating in
PCB-contaminated soils and sediments. Strains NY05, RHA1, and LB400
were used in resting cell assays and showed from 40 to 80% depletion of
mix M after 24 h of incubation, with 65 to 95% of the expected recovery as
chlorobenzoates, mostly 2-CBA and 4-CBA (Figure 6.7). Similarly, the bac-
teria depleted 60 to 75% of mix C after 24 h of incubation, with 70 to 90%
recovery as chlorobenzoates, predominantly 2-CBA (Figure 6.8). 2,2′-CB,
2,6-CB, 2,4,2′-CB, and 2,4,4′-CB were the most resistant to microbial attack
when supplied in mixtures M and C. Accumulation of yellow color was
detected during degradation of mixtures M and C by
Rhodococcus
strains
NY05 and RHA1, in accordance with HOPDA production found in trans-
formation of individual congeners.
Not surprisingly, ortho-directed RHA1 and LB400 exhibited higher activ-
ity toward mixtures M (65 and 80%, respectively) and C (70 and 75%, respec-
tively). Degradation rates of PCB congeners containing chlorine on both
biphenyl rings decreased when they were supplied in mixtures, possibly due
to competition from easily degradable congeners with one nonchlorinated
