Nitroaromatic compounds are released into the biosphere almost exclusi
vely from anthropogenic sources. Some compounds are produced by incomp
lete combustion of fossil fuels; others are used as synthetic intermed
iates, dyes, pesticides, and explosives. Recent research revealed a nu
mber of microbial systems capable of transforming or biodegrading nitr
oaromatic compounds. Anaerobic bacteria can reduce the nitro group via
nitroso and hydroxylamino intermediates to the corresponding amines.
Isolates of Desulfovibrio spp. can use nitroaromatic compounds as thei
r source of nitrogen. They can also reduce 2,4,6-trinitrotoluene to 2,
4,6-triaminotoluene. Several strains of Clostridium can catalyze a sim
ilar reduction and also seem to be able to degrade the molecule to sma
ll aliphatic acids. Anaerobic systems have been demonstrated to destro
y munitions and pesticides in soil. Fungi can extensively degrade or m
ineralize a variety of nitroaromatic compounds. For example, Phaneroch
aete chrysosporium mineralizes 2,4-dinitrotoluene and 2,4,6-trinitroto
luene and shows promise as the basis for bioremediation strategies. Th
e anaerobic bacteria and the fungi mentioned above mostly transform ni
troaromatic compounds via fortuitous reactions. In contrast, a number
of nitroaromatic compounds can settle as growth substrates for aerobic
bacteria. Removal or productive metabolism of nitro groups can be acc
omplished by four different strategies. (a) Some bacteria can reduce t
he aromatic ring of dinitro and trinitro compounds by the addition of
a hydride ion to form a hydride-Meisenheimer complex, which subsequent
ly rearomatizes with the elimination of nitrite. (b) Monooxygenase enz
ymes can add a single oxygen atom and eliminate the nitro group from n
itrophenols. (c) Dioxygenase enzymes can insert two hydroxyl groups in
to the aromatic ring and precipitate the spontaneous elimination of th
e nitro group from a variety of nitroaromatic compounds. (d) Reduction
of the nitro group to the corresponding hydroxylamine is the initial
reaction in the productive metabolism of nitrobenzene, 4-nitrotoluene,
and 4-nitrobenzoate. The hydroxylamines undergo enzyme-catalyzed rear
rangements to hydroxylated compounds that are substrates for ring-fiss
ion reactions. Potential applications of the above reactions include n
ot only the biodegradation of environmental contaminants, but also bio
catalysis and synthesis of valuable organic molecules.