Microaerophilic degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by three Rhodococcus strains

Microaeration promotes volatile siloxanes conversion to methane and simpler monomeric products

The ubiquitous use of volatile siloxanes in a myriad of product formulations has led to a widespread distribution of these persistent contaminants in both natural ecosystems and wastewater treatment plants. Microbial degradation under microaerobic conditions is a promising approach to mitigate D4 and D5 siloxanes while recovering energy in wastewater treatment plants. This study examined D4/D5 siloxanes biodegradation under both anaerobic and microaerobic conditions ( [Formula: see text]  = 0, 1, 3 %) using wastewater sludge. Results show that the use of microaeration in an otherwise strictly anaerobic environment significantly enhances siloxane conversion to methane. 16S rRNA gene sequencing identified potential degraders, including Clostridium lituseburense, Clostridium bifermentans and Synergistales species. Furthermore, chemical analysis suggested a stepwise siloxane conversion preceding methanogenesis under microaerobic conditions. This study demonstrates the feasibility of microaerobic siloxane biodegradation, laying groundwork for scalable removal technologies in wastewater treatment plants, ultimately highlighting the importance of using bio-based approaches in tackling persistent pollutants.

Mn oxides enhanced pyrene removal with both rhizosphere and non-rhizosphere microorganisms in subsurface flow constructed wetlands

The polycyclic aromatic hydrocarbons (PAHs) are substantial wastewater pollutants emitted mostly by petroleum refineries and petrochemical industries, and their environmental fate has been of increasing concern among the public. Consequently, subsurface flow constructed wetlands (SFCWs) filled with Mn oxides (W-CW) or without Mn oxides (K-CW) were established to investigate the performance and mechanisms of pyrene (PYR) removal. The average removal rates of PYR in W-CW and K-CW were 96.00% and 92.33%, respectively. The PYR removal via other pathways (microbial degradation, photolysis, volatilisation, etc.) occupied a sizeable proportion, while the total PYR content in K-CW plant roots was significantly higher than that of W-CW. The microorganisms on the root surface and rhizosphere played an important role in PYR degradation in W-CW and K-CW and were higher in W-CW than that in K-CW in all matrix zones. The microorganisms between the 10-16 cm zone from the bottom of W-CW filled with Mn oxides (W-16) were positively correlated with PYR-degrading microorganisms, aerobic bacteria and facultative anaerobes, whereas K-16 without birnessite-coated sand was negatively correlated with these microorganisms.

Uncovering the structure and function of specialist bacterial lineages in environments routinely exposed to explosives

Environmental contamination by hexahydro-1, 3, 5-trinitro-1, 3, 5-triazine (RDX), and Octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (HMX), the two most widely used compounds for military operations, is a long-standing problem at the manufacturing and decommissioning plants. Since explosives contamination has previously been shown to favour the growth of specific bacterial communities, the present study attempts to identify the specialist bacterial communities and their potential functional and metabolic roles by using amplicon targeted and whole-metagenome sequencing approaches (WMS) in samples collected from two distinct explosives manufacturing sites. We hypothesize that the community structure and functional attributes of bacterial population are substantially altered by the concentration of explosives and physicochemical conditions. The results highlight the predominance of Planctomycetes in contrast to previous reports from similar habitats. The detailed phylogenetic analysis revealed the presence of OTU's related to bacterial members known for their explosives degradation. Further, the functional and metabolic analyses highlighted the abundance of putative genes and unidentified taxa possibly associated with xenobiotic biodegradation. Our findings suggest that microbial species capable of utilizing explosives as a carbon, energy, or electron source are favoured by certain selective pressures based on the prevailing physicochemical and geographical conditions.

Exploring the Distinct Distribution of Archaeal Communities in Sites Contaminated with Explosives

Most of the research on bioremediation and estimation of microbial diversity in waste contaminated sites is focused on the domain Bacteria, whereas details on the relevance of Archaea are still lacking. The present study examined the archaeal diversity and predicted metabolic pathways in two discrete sites (SITE1 and SITE2) contaminated with explosives (RDX and HMX) by amplicon-targeted sequencing of 16S rRNA genes. In total, 14 soil samples were processed, and 35,758 OTUs were observed, among which 981 OTUs were classified as Archaea, representing ~2.7% of the total microbial diversity in our samples. The majority of OTUs belonged to phyla Euryarchaeota (~49%), Crenarchaeota (~24%), and Thaumarchaeota (~23%), while the remaining (~4%) OTUs were affiliated to Candidatus Parvarchaeota, Candidatus Aenigmarchaeota, and Candidatus Diapherotrites. The comparative studies between explosives contaminated and agricultural soil samples (with no history of explosives contamination) displayed significant differences between the compositions of the archaeal communities. Further, the metabolic pathways pertaining to xenobiotic degradation were presumably more abundant in the contaminated sites. Our data provide a first comprehensive report of archaeal communities in explosives contaminated sites and their putative degradation role in such ecosystems which have been as yet unexplored.

Diversity and abundance of the functional genes and bacteria associated with RDX degradation at a contaminated site pre- and post-biostimulation

Bioremediation is becoming an increasingly popular approach for the remediation of sites contaminated with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Multiple lines of evidence are often needed to assess the success of such approaches, with molecular studies frequently providing important information on the abundance of key biodegrading species. Towards this goal, the current study utilized shotgun sequencing to determine the abundance and diversity of functional genes (xenA, xenB, xplA, diaA, pnrB, nfsI) and species previously associated with RDX biodegradation in groundwater before and after biostimulation at an RDX-contaminated Navy Site. For this, DNA was extracted from four and seven groundwater wells pre- and post-biostimulation, respectively. From a set of 65 previously identified RDX degraders, 31 were found within the groundwater samples, with the most abundant species being Variovorax sp. JS1663, Pseudomonas fluorescens, Pseudomonas putida, and Stenotrophomonas maltophilia. Further, 9 RDX-degrading species significantly (p<0.05) increased in abundance following biostimulation. Both the sequencing data and qPCR indicated that xenA and xenB exhibited the highest relative abundance among the six genes. Several genes (diaA, nsfI, xenA, and pnrB) exhibited higher relative abundance values in some wells following biostimulation. The study provides a comprehensive approach for assessing biomarkers during RDX bioremediation and provides evidence that biostimulation generated a positive impact on a set of key species and genes. KEY POINTS: • A co-occurrence network indicated diverse RDX degraders. • >30 RDX-degrading species were detected. • Nine RDX-degrading species increased following biostimulation. • Sequencing and high-throughput qPCR indicated that xenA and xenB were most abundant.

Spatially-distinct redox conditions and degradation rates following field-scale bioaugmentation for RDX-contaminated groundwater remediation

In situ bioaugmentation for cleanup of an hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)-contaminated groundwater plume was recently demonstrated. Results of a forced-gradient, field-scale cell transport test with Gordonia sp. KTR9 and Pseudomonas fluorescens strain I-C cells (henceforth "KTR9" and "Strain I-C") showed these strains were transported 13 m downgradient over 1 month. Abundances of xplA and xenB genes, respective indicators of KTR9 and Strain I-C, approached injection well cell densities at 6 m downgradient, whereas gene abundances (and conservative tracer) had begun to increase at 13 m downgradient at test conclusion. In situ push-pull tests were subsequently completed to measure RDX degradation rates in the bioaugmented wells under ambient gradient conditions. Time-series monitoring of RDX, RDX end-products, conservative tracer, xplA and xenB gene copy numbers and XplA and XenB protein abundance were used to assess the efficacy of bioaugmentation and to estimate the apparent first-order RDX degradation rates during each test. A collective evaluation of redox conditions, RDX end-products, varied RDX degradation kinetics, and biomarkers indicated that Strain I-C and KTR9 rapidly degraded RDX. Results showed bioaugmentation is a viable technology for accelerating RDX cleanup in the demonstration site aquifer and may be applicable to other sites. Full-scale implementation considerations are discussed.

Microbial bioremediation of oil contaminated seawater: A survey of patent deposits and the characterization of the top genera applied

Petroleum has been the world's major source of energy since the middle of the twentieth century, leading to positive changes but also social, political, and environmental problems worldwide. Oil contamination affects all ecosystems, and the remediation of polluted sites using environmentally friendly strategies is crucial. Here, we report an analysis of the patent documents of potential petroleum bioremediation techniques that use microbes to clean seawater. The patent search was performed using Orbit Intelligence®, SciFinder® and the Derwent World Patents Index®. A group of 500 patent documents were validated according to the search objective and carefully studied. Increases in patent deposits coincide with periods following widely reported oil spills, suggesting a relationship between media disclosure and stimulation of innovation activities. China leads the list of countries with patent applications in bioremediation with 152 deposits, followed by Russia with 133 and the US with 48. These three countries have completely different temporal deposit profiles, influenced by their historical, political, and economic scenarios. A total of 368 patents described degradation of the oil compounds exclusively by bacteria, 24 by fungi and yeasts, 1 by Archaea, 1 using a microalgal strain, and 32 by mixed consortia. The leading microbial genera found in the patents are Pseudomonas (114 patents), Bacillus (75), and Rhodococcus (60). In the top-10 list of microbial strains mostly cited/claimed, no genera are obligate hydrocarbonoclastic bacteria. This fact, together with the broad pattern found in the main International Patent Classification (IPC) codes, suggest that most of the documents are general bioremediation approaches and not focused on oil-polluted seawater. This work highlights the importance of stimulating the development of innovative environmentally friendly strategies focused on the degradation of oil hydrocarbons in marine ecosystems.

Biodegradation of decabromodiphenyl ether (BDE 209) by a newly isolated bacterium from an e-waste recycling area

Polybrominated diphenyl ethers (PBDEs) have become widespread environmental pollutants all over the world. A newly isolated bacterium from an e-waste recycling area, Stenotrophomonas sp. strain WZN-1, can degrade decabromodiphenyl ether (BDE 209) effectively under aerobic conditions. Orthogonal test results showed that the optimum conditions for BDE 209 biodegradation were pH 5, 25 °C, 0.5% salinity, 150 mL minimal salt medium volume. Under the optimized condition, strain WZN-1 could degrade 55.15% of 65 μg/L BDE 209 under aerobic condition within 30 day incubation. Moreover, BDE 209 degradation kinetics was fitted to a first-order kinetics model. The biodegradation mechanism of BDE 209 by strain WZN-1 were supposed to be three possible metabolic pathways: debromination, hydroxylation, and ring opening processes. Four BDE 209 degradation genes, including one hydrolase, one dioxygenase and two dehalogenases, were identified based on the complete genome sequencing of strain WZN-1. The real-time qPCR demonstrated that the expression level of four identified genes were significantly induced by BDE 209, and they played an important role in the degradation process. This study is the first to demonstrate that the newly isolated Stenotrophomonas strain has an efficient BDE 209 degradation ability and would provide new insights for the microbial degradation of PBDEs.

Complete Genome Sequence of Stenotrophomonas sp. Strain WZN-1, Which Is Capable of Degrading Polybrominated Diphenyl Ethers

Stenotrophomonas sp. strain WZN-1, isolated from an e-waste recycling area in Tianjin, China, is capable of degrading polybrominated diphenyl ethers (PBDEs). The complete genome of strain WZN-1 consists of 4,512,703 bp. This genome information will provide important information about the biodegradation pathways and mechanisms of PBDEs.

Biostimulation and microbial community profiling reveal insights on RDX transformation in groundwater

Hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) is a high explosive released to the environment as a result of weapons manufacturing and testing worldwide. At Los Alamos National Laboratory, the Technical Area (TA) 16 260 Outfall discharged high‐explosives‐bearing water from a high‐explosives‐machining facility to Cañon de Valle during 1951 through 1996. These discharges served as a primary source of high‐explosives and inorganic‐element contamination in the area. Data indicate that springs, surface water, alluvial groundwater, and perched‐intermediate groundwater contain explosive compounds, including RDX (hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine); HMX (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine); and TNT (2,4,6‐trinitrotoluene). RDX has been detected in the regional aquifer in several wells, and a corrective measures evaluation is planned to identify remedial alternatives to protect the regional aquifer. Perched‐intermediate groundwater at Technical Area 16 is present at depths from 650 ft to 1200 ft bgs. In this study, we examined the microbial diversity in a monitoring well completed in perched‐intermediate groundwater contaminated by RDX, and examined the response of the microbial population to biostimulation under varying geochemical conditions. Results show that the groundwater microbiome was dominated by Actinobacteria and Proteobacteria. A total of 1,605 operational taxonomic units (OTUs) in 96 bacterial genera were identified. Rhodococcus was the most abundant genus (30.6%) and a total of 46 OTUs were annotated as Rhodococcus. One OTU comprising 25.2% of total sequences was closely related to a RDX ‐degrading strain R. erythropolis HS4. A less abundant OTU from the Pseudomonas family closely related to RDX‐degrading strain P. putida II‐B was also present. Biostimulation significantly enriched Proteobacteria but decreased/eliminated the population of Actinobacteria. Consistent with RDX degradation, the OTU closely related to the RDX‐degrading P. putida strain II‐B was specifically enriched in the RDX‐degrading samples. Analysis of the accumulation of RDX‐degradation products reveals that during active RDX degradation, there is a transient increase in the concentration of the degradation products MNX, DNX, TNX, and NDAB. The accumulation of these degradation products suggests that RDX is degraded via sequential reduction of the nitro functional groups followed by abiotic ring‐cleavage. The results suggest that strict anaerobic conditions are needed to stimulate RDX degradation under the TA‐16 site‐specific conditions.

Impact of glycerin and lignosulfonate on biodegradation of high explosives in soil

Soil microcosms were constructed and monitored to evaluate the impact of substrate addition and transient aerobic and anaerobic conditions on TNT, RDX and HMX biodegradation in grenade range soils. While TNT was rapidly biodegraded under both aerobic and anaerobic conditions with and without organic substrate, substantial biodegradation of RDX, HMX, and RDX daughter products was not observed under aerobic conditions. However, RDX and HMX were significantly biodegraded under anaerobic conditions, without accumulation of TNT or RDX daughter products (2-ADNT, 4-ADNT, MNX, DNX, and TNX). In separate microcosms containing grenade range soil, glycerin and lignosulfonate addition enhanced oxygen consumption, increasing the consumption rate >200% compared to untreated soils. Mathematical model simulations indicate that oxygen consumption rates of 5 to 20g/m³/d can be achieved with reasonable amendment loading rates. These results indicate that glycerin and lignosulfonate can be potentially used to stimulate RDX and HMX biodegradation by increasing oxygen consumption rates in soil.

Evaluation of Biostimulation and Bioaugmentation To Stimulate Hexahydro-1,3,5-trinitro-1,3,5,-triazine Degradation in an Aerobic Groundwater Aquifer

Hexahydro-1,3,5-trinitro-1,3,5,-triazine (RDX) is a toxic and mobile groundwater contaminant common to military sites. This study compared in situ RDX degradation rates following bioaugmentation with Gordonia sp. strain KTR9 (henceforth KTR9) to rates under biostimulation conditions in an RDX-contaminated aquifer in Umatilla, OR. Bioaugmentation was achieved by injecting site groundwater (6000 L) amended with KTR9 cells (10(8) cells mL(-1)) and low carbon substrate concentrations (<1 mM fructose) into site wells. Biostimulation (no added cells) was performed by injecting groundwater amended with low (<1 mM fructose) or high (>15 mM fructose) carbon substrate concentrations in an effort to stimulate aerobic or anaerobic microbial activity, respectively. Single-well push-pull tests were conducted to measure RDX degradation rates for each treatment. Average rate coefficients were 1.2 day(-1) for bioaugmentation and 0.7 day(-1) for high carbon biostimulation; rate coefficients for low carbon biostimulation were not significantly different from zero (p values ≥0.060). Our results suggest that bioaugmentation with KTR9 is a feasible strategy for in situ biodegradation of RDX and, at this site, is capable of achieving RDX concentration reductions comparable to those obtained by high carbon biostimulation while requiring ~97% less fructose. Bioaugmentation has potential to minimize substrate quantities and associated costs, as well as secondary groundwater quality impacts associated with anaerobic biostimulation processes (e.g., hydrogen sulfide, methane production) during full-scale RDX remediation.

Application of (13)C and (15)N stable isotope probing to characterize RDX degrading microbial communities under different electron-accepting conditions

This study identified microorganisms capable of using the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) or its metabolites as carbon and/or nitrogen sources under different electron-accepting conditions using (13)C and (15)N stable isotope probing (SIP). Mesocosms were constructed using groundwater and aquifer solids from an RDX-contaminated aquifer. The mesocosms received succinate as a carbon source and one of four electron acceptors (nitrate, manganese(IV), iron(III), or sulfate) or no additional electron acceptor (to stimulate methanogenesis). When RDX degradation was observed, subsamples from each mesocosm were removed and amended with (13)C3- or ring-(15)N3-, nitro-(15)N3-, or fully-labeled (15)N6-RDX, followed by additional incubation and isolation of labeled nucleic acids. A total of fifteen 16S rRNA sequences, clustering in α- and γ-Proteobacteria, Clostridia, and Actinobacteria, were detected in the (13)C-DNA fractions. A total of twenty seven sequences were derived from different (15)N-DNA fractions, with the sequences clustered in α- and γ-Proteobacteria, and Clostridia. Interestingly, sequences identified as Desulfosporosinus sp. (in the Clostridia) were not only observed to incorporate the labeled (13)C or (15)N from labeled RDX, but also were detected under each of the different electron-accepting conditions. The data suggest that (13)C- and (15)N-SIP can be used to characterize microbial communities involved in RDX biodegradation, and that the dominant pathway of RDX biodegradation may differ under different electron-accepting conditions.

Stable isotope probing reveals the importance of Comamonas and Pseudomonadaceae in RDX degradation in samples from a Navy detonation site

This study investigated the microorganisms involved in hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) degradation from a detonation area at a Navy base. Using Illumina sequencing, microbial communities were compared between the initial sample, samples following RDX degradation, and controls not amended with RDX to determine which phylotypes increased in abundance following RDX degradation. The effect of glucose on these communities was also examined. In addition, stable isotope probing (SIP) using labeled ((13)C3, (15)N3-ring) RDX was performed. Illumina sequencing revealed that several phylotypes were more abundant following RDX degradation compared to the initial soil and the no-RDX controls. For the glucose-amended samples, this trend was strong for an unclassified Pseudomonadaceae phylotype and for Comamonas. Without glucose, Acinetobacter exhibited the greatest increase following RDX degradation compared to the initial soil and no-RDX controls. Rhodococcus, a known RDX degrader, also increased in abundance following RDX degradation. For the SIP study, unclassified Pseudomonadaceae was the most abundant phylotype in the heavy fractions in both the presence and absence of glucose. In the glucose-amended heavy fractions, the 16S ribosomal RNA (rRNA) genes of Comamonas and Anaeromxyobacter were also present. Without glucose, the heavy fractions also contained the 16S rRNA genes of Azohydromonas and Rhodococcus. However, all four phylotypes were present at a much lower level compared to unclassified Pseudomonadaceae. Overall, these data indicate that unclassified Pseudomonadaceae was primarily responsible for label uptake in both treatments. This study indicates, for the first time, the importance of Comamonas for RDX removal.

Improved RDX detoxification with starch addition using a novel nitrogen-fixing aerobic microbial consortium from soil contaminated with explosives

In this work, we developed and characterized a novel nitrogen-fixing aerobic microbial consortium for the complete detoxification of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Aerobic RDX biodegradation coupled with microbial growth and nitrogen fixation activity were effectively stimulated by the co-addition of starch and RDX under nitrogen limiting conditions. In the starch-stimulated nitrogen-fixing RDX degradative consortium, the RDX degradation activity was correlated with the xplA and nifH gene copy numbers, suggesting the involvement of nitrogen fixing populations in RDX biodegradation. Formate, nitrite, nitrate, and ammonia were detected as aerobic RDX degradation intermediates without the accumulation of any nitroso-derivatives or NDAB (4-nitro-2,4-diazabutanal), indicating nearly complete mineralization. Pyrosequencing targeting the bacterial 16S rRNA genes revealed that the Rhizobium, Rhizobacter and Terrimonas population increased as the RDX degradation activity increased, suggesting their involvement in the degradation process. These findings imply that the nitrogen-fixing aerobic RDX degrading consortium is a valuable microbial resource for improving the detoxification of RDX-contaminated soil or groundwater, especially when combined with rhizoremediation.

Application of (13)C-stable isotope probing to identify RDX-degrading microorganisms in groundwater

We employed stable isotope probing (SIP) with (13)C-labeled hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) to identify active microorganisms responsible for RDX biodegradation in groundwater microcosms. Sixteen different 16S rRNA gene sequences were derived from microcosms receiving (13)C-labeled RDX, suggesting the presence of microorganisms able to incorporate carbon from RDX or its breakdown products. The clones, residing in Bacteroidia, Clostridia, α-, β- and δ-Proteobacteria, and Spirochaetes, were different from previously described RDX degraders. A parallel set of microcosms was amended with cheese whey and RDX to evaluate the influence of this co-substrate on the RDX-degrading microbial community. Cheese whey stimulated RDX biotransformation, altered the types of RDX-degrading bacteria, and decreased microbial community diversity. Results of this study suggest that RDX-degrading microorganisms in groundwater are more phylogenetically diverse than what has been inferred from studies with RDX-degrading isolates.

Insight on RDX degradation mechanism by Rhodococcus strains using 13C and 15N kinetic isotope effects

The explosive Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is known to be degraded aerobically by various isolates of the Rhodococcus species, with denitration being the key step, mediated by Cytochrome P450. Our study aimed at gaining insight into the RDX degradation mechanism by Rhodococcus species and comparing isotope effects associated with RDX degradation by distinct Rhodococcus strains. For these purposes, enrichment in (13)C and (15)N isotopes throughout RDX denitration was studied for three distinct Rhodococcus strains, isolated from soil and groundwater in an RDX-contaminated site. The observable (15)N enrichment throughout the reaction, together with minor (13)C enrichment, suggests that N-N bond cleavage is likely to be the key rate-limiting step in the reaction. The similarity in the kinetic (15)N isotope effect between the three tested strains suggests that either isotope-masking effects are negligible, or are of a similar extent for all tested strains. The lack of variability in the kinetic (15)N isotope effect allows the interpretation of environmental studies with greater confidence.

Microbial remediation of explosive waste

Explosives are synthesized globally mainly for military munitions. Nitrate esters, such as GTN and PETN, nitroaromatics like TNP and TNT and nitramines with RDX, HMX and CL20, are the main class of explosives used. Their use has resulted in severe contamination of environment and strategies are now being developed to clean these substances in an economical and eco-friendly manner. The incredible versatility inherited in microbes has rendered these explosives as a part of the biogeochemical cycle. Several microbes catalyze mineralization and/or nonspecific transformation of explosive waste either by aerobic or anaerobic processes. It is likely that ongoing genetic adaptation, with the recruitment of silent sequences into functional catabolic routes and evolution of substrate range by mutations in structural genes, will further enhance the catabolic potential of bacteria toward explosives and ultimately contribute to cleansing the environment of these toxic and recalcitrant chemicals. This review summarizes information on the biodegradation and biotransformation pathways of several important explosives. Isolation, characterization, utilization and manipulation of the major detoxifying enzymes and the molecular basis of degradation are also discussed. This may be useful in developing safer and economic microbiological methods for clean up of soil and water contaminated with such compounds. The necessity of further investigations concerning the microbial metabolism of these substances is also discussed.

Biodegradation of RDX nitroso products MNX and TNX by cytochrome P450 XplA

Anaerobic transformation of the explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) by microorganisms involves sequential reduction of N-NO(2) to the corresponding N-NO groups resulting in the initial formation of MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine). MNX is further reduced to the dinitroso (DNX) and trinitroso (TNX) derivatives. In this paper, we describe the degradation of MNX and TNX by the unusual cytochrome P450 XplA that mediates metabolism of RDX in Rhodococcus rhodochrous strain 11Y. XplA is known to degrade RDX under aerobic and anaerobic conditions, and, in the present study, was found able to degrade MNX to give similar products distribution including NO(2)(-), NO(3)(-), N(2)O, and HCHO but with varying stoichiometric ratio, that is, 2.06, 0.33, 0.33, 1.18, and 1.52, 0.15, 1.04, 2.06, respectively. In addition, the ring cleavage product 4-nitro-2,4,-diazabutanal (NDAB) and a trace amount of another intermediate with a [M-H](-) at 102 Da, identified as ONNHCH(2)NHCHO (NO-NDAB), were detected mostly under aerobic conditions. Interestingly, degradation of TNX was observed only under anaerobic conditions in the presence of RDX and/or MNX. When we incubated RDX and its nitroso derivatives with XplA, we found that successive replacement of N-NO(2) by N-NO slowed the removal rate of the chemicals with degradation rates in the order RDX > MNX > DNX, suggesting that denitration was mainly responsible for initiating cyclic nitroamines degradation by XplA. This study revealed that XplA preferentially cleaved the N-NO(2) over the N-NO linkages, but could nevertheless degrade all three nitroso derivatives, demonstrating the potential for complete RDX removal in explosives-contaminated sites.

Biodegradation of RDX and MNX with Rhodococcus sp. strain DN22: new insights into the degradation pathway

Previously we demonstrated that Rhodococcus sp. strain DN22 can degrade RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) aerobically via initial denitration. The present study describes the role of oxygen and water in the key denitration step leading to RDX decomposition using (18)O(2) and H(2)(18)O labeling experiments. We also investigated degradation of MNX (hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine) with DN22 under similar conditions. DN22 degraded RDX and MNX giving NO(2)(-), NO(3)(-), NDAB (4-nitro-diazabutanal), NH(3), N(2)O, and HCHO with NO(2)(-)/NO(3)(-) molar ratio reaching 17 and ca. 2, respectively. In the presence of (18)O(2), DN22 degraded RDX and produced NO(2)(-) with m/z at 46 Da that subsequently oxidized to NO(3)(-) containing one (18)O atom, but in the presence of H(2)(18)O we detected NO(3)(-) without (18)O. A control containing NO(2)(-), DN22, and (18)O(2) gave NO(3)(-) with one (18)O, confirming biotic oxidation of NO(2)(-) to NO(3)(-). Treatment of MNX with DN22 and (18)O(2) produced NO(3)(-) with two mass ions, one (66 Da) incorporating two (18)O atoms and another (64 Da) incorporating only one (18)O atom and we attributed their formation to bio-oxidation of the initially formed NO and NO(2)(-), respectively. In the presence of H(2)(18)O we detected NO(2)(-) with two different masses, one representing NO(2)(-) (46 Da) and another representing NO(2)(-) (48 Da) with the inclusion of one (18)O atom suggesting auto-oxidation of NO to NO(2)(-). Results indicated that denitration of either RDX or MNX and denitrosation of MNX by DN22 did not involve direct participation of either oxygen or water, but both played major roles in subsequent secondary chemical and biochemical reactions of NO and NO(2)(-).