Effects of Perchlorate and Other Groundwater Inorganic Co-Contaminants on Aerobic RDX Degradation

Transformation of 2, 4, 6-trinitrotoluene by Stenotrophomonas strain SG1 under aerobic and anaerobic conditions

TNT, or 2,4,6-trinitrotoluene, is a common explosive that can contaminate soil and groundwater in production sites, military training areas, and disposal locations. The compound is highly toxic; therefore, there is an urgent need to rehabilitate the impacted environments. Harnessing the microbial ability to biodegrade TNT into environmentally harmless compound(s) is one approach to remediating contaminated sites. In our study, we report on the genomic and metabolic ability of Stenotrophomonas strain SG1 to degrade TNT under aerobic and anaerobic conditions. The bacterial strain SG1 was first isolated as a contaminant from a culture of Diaphorobacter sp. strain DS2 over minimal media supplemented with TNT. The draft genome assembly of strain SG1 is ∼4.7 Mb and is distributed among 358 contigs. The homology search against the custom database of enzymes responsible for TNT biodegradation revealed the presence of three N-ethylmaleimide reductases (NemA) with a defined KEGG ortholog and KEGG pathway of TNT degradation. The presence of respiratory nitrate reductases has also been mapped, which supports denitrification under anaerobic conditions. Experimentally, the TNT transformation rate accelerated when carbon sources, such as sodium acetate, sodium citrate, sodium succinate, sucrose, and glucose (final concentration of 5 mM), were added. Citrate promoted the highest growth and TNT transformation ratio (88.35%) in 120 h. With the addition of 5 mM ammonium chloride, TNT completely disappeared in the citrate and sucrose-containing treatments in 120 h. However, higher biomass was obtained in the sucrose and glucose-containing treatments in 120 h. During incubation, the formation of amino dinitrotoluene isomers, dinitrotoluene isomers, trinitrobenzene, azoxy isomers, diaryl hydroxylamines, and corresponding secondary amines was confirmed by GC/MS and UPLC/MS. 2-Amino-4-nitrotoluene, 4-amino-2-nitrotoluene, and 2-amino-6-nitrotoluene were also identified in the culture supernatant by GC/MS. Under anaerobic conditions, TNT completely disappeared in the citrate and citrate plus nitrate treatments. Since the strain shows the ability to remove TNT, this research should be useful in basic research and practical applications for removing TNT from wastewater.

Inhibition effect of 2,4,6-trinitrotoluene (TNT) on RDX degradation by rhodococcus strains isolated from contaminated soil and water

2,4,6-trinitrotoluene (TNT) is a highly toxic explosive that contaminates soil and water and may interfere with the degradation of co-occurring compounds, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). We proposed that TNT may influence RDX-degrading bacteria via either general toxicity or a specific effect on the |RDX degradation mechanisms. Thus, we examined the impact of TNT on RDX degradation by Rhodococcus strains YH1, T7, and YY1, which were isolated from an explosives-polluted environment. Although partly degraded, TNT did not support the growth of any of the strains when used as either sole carbon or sole nitrogen sources, or as carbon and nitrogen sources. The incubation of a mixture of TNT (25 mg/l) and RDX (20 mg/l) completely inhibited RDX degradation. The effect of TNT on the cytochrome P450, catalyzing RDX degradation, was tested in a resting cell experiment, proving that TNT inhibits XplA protein activity. A dose-response experiment showed that the IC50/trans values for YH1, T7, and YY1 were 7.272, 5.098, and 9.140 (mg/l of TNT), respectively, illustrating variable sensitivity to TNT among the strains. The expression of xplA was also strongly suppressed by TNT. Cells that were pre-grown with RDX (allowing xplA expression) and incubated with ammonium chloride, glucose, and TNT, completely transformed into their amino dinitrotoluene isomers and formed azoxy toluene isomers. The presence of oxygen-insensitive nitroreductase that enable reduction of the nitro group in the presence of O2 in the genomes of these strains suggests that they are responsible for TNT transformation in the cultures. The experimental results concluded that TNT has an adverse effect on RDX degradation by the examined strains. It inhibits RDX degradation due to the direct impact on cytochrome P450, xplA, or its expression. The tested strains can transform TNT independently of RDX. Thus, degradation of both compounds is possible if TNT concentrations are below their IC50 values.