Geoscience Reference
In-Depth Information
proliferation of active microbial populations and consequently increase the rate of
decomposition of additional contaminants that enter the system.
Cometabolic transformations include the degradation of toxic organic mole-
cules by microorganisms that grow at the expense of a substrate other than the
toxic organic one. This process, in which enzymes involved in catalyzing the
initial reaction are lacking in substrate specificity, may lead to the formation of
intermediate products that, in some cases, are more toxic than the parent material.
These products cause an adverse environmental impact and may inhibit microbial
growth and metabolism. Environmental factors and the contaminant concentration
may affect the nature of the metabolism; different microorganisms can metabolize
different toxic organic molecules. Wang et al. (
1984
) found that isopropyl N-
phenylcarbamate (IPC) is mineralized at low concentrations, while at higher
concentrations, it is converted to organic products by cometabolism. In general,
cometabolism does not result in extensive degradation of a specific organic con-
taminant. However, different microorganisms can transform a molecule by
sequential cometabolic attacks, and cometabolic products of one organism can be
used as a growth substrate for another organism.
Polymerization, or conjugation, is the process in which toxic organic molecules
undergo microbially mediated transformation by oxidative coupling reactions. In
this case, a contaminant or its intermediate product(s) combines with itself or other
organic molecules (e.g., xenobiotic residues, naturally occurring compounds) to
form larger molecular polymers that can be incorporated in subsurface humic
substances.
Cellular accumulation is an additional microorganism-mediated transformation
pathway of organic contaminants in the subsurface environment. The rate of
accumulation differs for various organisms and depends on the type and concen-
tration of toxic organic chemical in the surrounding medium. Microbial uptake is a
passive absorption process and not active metabolism. Studies by, for example,
Johnson and Kennedy (
1973
) and Paris and Lewis (
1976
) proved that dead,
autoclaved cells accumulate similar amounts of toxic organic molecules to living
organisms. These results suggest that cell accumulation is not an induced meta-
bolic process but an absorptive one. For some specific cases, such as the pesticides
fenitrothion and DDT, accumulation is greater in dead cells than living organisms
(Kikuchi et al.
1984
). Once accumulated in microbial cells, toxic organic chem-
icals may be degraded. The organophosphate pesticide fensulfothion, for example,
is metabolized by the bacterium Klebsiella pneumoniae, the transformation
product fensulfothion sulfide being also bound by both living and dead cells
(Timms and MacRae
1982
).
Nonenzymatic transformation of toxic organics is an indirect process that
occurs in the subsurface as a result of microbially induced changes in environ-
mental parameters such as pH and redox potential. The activity of microorganisms
leads to changes in pH, for example, due to biochemical processes such as deg-
radation of proteins or oxidation of organic N to nitrite and nitrate, sulfide to
elemental S, or ferrous sulfate to ferric ion. Changes in pH may induce the
transformation of exogenous organic contaminants in the aqueous and solid
