Remediation of PAH-Contaminated Soils and Sediments: A ...

Cometabolism is explained by three basic reasons (Alexander, 1999):
� Initial microbial enzymes are used to convert a substrate into an organic
product which is not further transformed by other enzymes specific to that
microorganism. No metabolic intermediates used for biosynthesis or
energy production are produced in this scenario and accumulation occurs.
� Initial substrate is transformed into enzyme inhibiting or growth
suppressing organic products.
� An additional substrate that is not available at the time of the reaction is
needed by the microorganism to complete the degradation of both
substrates.
In the first part of the cometabolism degradation reaction, the following equation
can be used to represent substrates (A, B, C, D) and enzymes (a, b, c, d):
a b c
A B C D →→→→ CO2 + energy + cell-carbon
Enzyme “a” might have low substrate specificity and can act on A or on the
product of A (or A’). The product of B (or B’) might differ in the same way that A
and A’ differ, but enzyme “b” has higher substrate specificity and cannot degrade
B’. So, B’ would accumulate as represented by the following equation:
a
A’ B’
The substrate of these cometabolic reactions persists in the soil and is not
mineralized into CO2, microbial energy and cell-carbon. In the case of PAHs, the
product of naphthalene (A’) may continue to be degraded, but the product of
Benzo[a]pyrene (B’), or metabolite, may not be further degraded and accumulate
in the soils. Though Benzo[a]pyrene may be degraded as mentioned earlier, the
problem with this situation is that metabolites can be more carcinogenic than the
initial substrate (Alexander, 1999).
Scientifically supported degradation of HMW PAHs in the presence of LMW
PAHs is discussed or reviewed in the following literature:
� Juhasz and Naidu (2000) reviewed several studies where the presence of
LMW PAHs enhanced the degradation of benzo[a]pyrene through cometabolism.
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