Biological degradation of 2,4,6-trinitrotoluene

Abstract

Nitroaromatic compounds are xenobiotics that have found multiple applications in the synthesis of foams, pharmaceuticals, pesticides, and explosives. These compounds are toxic and recalcitrant and are degraded relatively slowly in the environment by microorganisms. 2,4,6-Trinitrotoluene (TNT) is the most widely used nitroaromatic compound. Certain strains of Pseudomonas and fungi can use TNT as a nitrogen source through the removal of nitrogen as nitrite from TNT under aerobic conditions and the further reduction of the released nitrite to ammonium, which is incorporated into carbon skeletons. Phanerochaete chrysosporium and other fungi mineralize TNT under ligninolytic conditions by converting it into reduced TNT intermediates, which are excreted to the external milieu, where they are substrates for ligninolytic enzymes. Most if not all aerobic microorganisms reduce TNT to the corresponding amino derivatives via the formation of nitroso and hydroxylamine intermediates. Condensation of the latter compounds yields highly recalcitrant azoxytetranitrotoluenes. Anaerobic microorganisms can also degrade TNT through different pathways. One pathway, found in Desulfovibrio and Clostridium, involves reduction of TNT to triaminotoluene; subsequent steps are still not known. Some Clostridium species may reduce TNT to hydroxylaminodinitrotoluenes, which are then further metabolized. Another pathway has been described in Pseudomonas sp. strain JLR11 and involves nitrite release and further reduction to ammonium, with almost 85% of the N-TNT incorporated as organic N in the cells. It was recently reported that in this strain TNT can serve as a final electron acceptor in respiratory chains and that the reduction of TNT is coupled to ATP synthesis. In this review we also discuss a number of biotechnological applications of bacteria and fungi, including slurry reactors, composting, and land farming, to remove TNT from polluted soils. These treatments have been designed to achieve mineralization or reduction of TNT and immobilization of its amino derivatives on humic material. These approaches are highly efficient in removing TNT, and increasing amounts of research into the potential usefulness of phytoremediation, rhizophytoremediation, and transgenic plants with bacterial genes for TNT removal are being done.

FIG. 1

Mechanisms for the reduction of nitro groups in nitroaromatic compounds. The first step in nitro group reduction can be achieved through one-electron transfer (solid line) or two-electron transfer (dashed line). The first mechanism produces a nitroanion radical that could react with oxygen to form a superoxide radical and the original nitroaromatic compound through a futile cycle (dotted line). If the mechanism occurs via the transfer of two electrons, the nitroso derivative formed is the first putative intermediate; following two consecutive electron transfers, a hydroxylamine and an aromatic amine are produced. The scheme has been adapted from reference .

FIG. 2

TNT-Meisenheimer complex formation. The hydride ion can be donated by NAD(P)H, giving rise to a Meisenheimer complex. Aromaticity is restored on the release of a nitro group.

FIG. 3

Pathways for the aerobic metabolism of TNT in aerobic microorganisms. The scheme is based on articles cited in the text, which are indicated in the figure in square brackets. When the conversion of a compound to another is believed to occur through a series of intermediates, the steps are indicated by two arrows. TCA, trichloroacetic acid.

FIG. 4

Proposed mechanisms for anaerobic TNT metabolism in bacteria. The scheme is based on articles cited in the text, which are indicated in the figure in square brackets. When the conversion of a compound to another is believed to occur through a series of intermediates, the steps are indicated by two arrows.

FIG. 5

Scheme showing the coupling of electron donor compounds, TNT oxidoreduction, and ATP synthesis. Pi, inorganic phosphate. The scheme has been adapted from reference (69).

FIG. 6

Mechanisms for fungal degradation of TNT. The scheme is based on articles cited in the text, which are indicated in the figure in s brackets.