Geoscience Reference
In-Depth Information
Fig. 15.7
Sulfoxidation of carboxine by the fungus Ustilage maydis (Bollag and Liu
1990
)
appropriate position before oxygenase enzymes can cause ring cleavage. Dehy-
droxylation usually is essential for enzymatic cleavage of the benzene ring under
aerobic conditions. The hydroxyl groups must be placed either ortho or para to
each other, probably to facilitate the shifts of electrons involved in the ring fission.
Dioxygenases are the enzymes responsible for ring cleavage, and they can cause
ortho-(intradiol) or meta-(extradiol) fission of a catechol, forming cis, cis-muconic
acid or 2-hydroxymuconic, and semialdehyde, respectively.
Heterocyclic ring cleavage also occurs as a metabolic, microorganism-mediated
process. For contaminants having a heterocyclic ring, the degradation path is
complicated by the heteroatoms, usually N, O, and S, contributing to decompo-
sition reactions through their individual characteristics. These compounds may
contain one or more (mostly aromatic) rings having five or six members.
Sulfoxidation reactions are characterized by enzymatic conversion of a divalent
compound to sulfoxide (Fig.
15.7
) or, in some cases, to sulfone (S ? SO ? SO
2
).
The degradation also may be catalyzed by minerals, converting organic sulfides
(thioesters) and sulfites to the corresponding sulfoxides and sulfates. Because it is
difficult to determine if the reaction is chemically or biologically induced, mi-
crobially mediated sulfoxidation in the subsurface environment can be established
only when a biocatalyst is found.
Reduction reactions mediated by microorganisms may include the reduction of
nitro bonds, sulfoxide reduction, and reductive dehalogenation. Reduction of the
nitro group to amine involves the intermediate formation of nitrase and hydrox-
yamino groups. Selected reductive reactions may involve the saturation of double
bonds, reduction of aldehydes to alcohols or ketones to secondary alcohols, or of
certain metals. The main reductive processes in the subsurface environment have
been discussed earlier in this chapter.
Hydrolytic reactions involve organic toxic molecules that have ether, ester, or
amide linkages. In the case of hydrolytic dehalogenation, a halogen is exchanged
with an hydroxyl group. This reaction is mediated by hydrolytic enzymes, excreted
outside the cells by microorganisms. In general, enzymes involved in hydrolytic
reactions include esterase, acrylamidase, phosphatase, hydrolase, and lyase. Bollag
and Liu (
1990
) emphasized that it is often difficult to determine the original
catalyst of the reaction, because specific environmental conditions or secondary
effects of microbial metabolism create conditions conducive to hydrolysis.
Table
15.3
summarizes microbially mediated hydrolytic and reductive reactions of
synthetic pesticides that reach the subsurface via land application.
