Biodegradation of glyceryl trinitrate by Penicillium corylophilum Dierckx

Oxic and Anoxic Organic Polymer Degradation Potential of Endophytic Fungi From the Marine Macroalga, Ecklonia radiata

Cellulose and chitin are the most abundant polymeric, organic carbon source globally. Thus, microbes degrading these polymers significantly influence global carbon cycling and greenhouse gas production. Fungi are recognized as important for cellulose decomposition in terrestrial environments, but are far less studied in marine environments, where bacterial organic matter degradation pathways tend to receive more attention. In this study, we investigated the potential of fungi to degrade kelp detritus, which is a major source of cellulose in marine systems. Given that kelp detritus can be transported considerable distances in the marine environment, we were specifically interested in the capability of endophytic fungi, which are transported with detritus, to ultimately contribute to kelp detritus degradation. We isolated 10 species and two strains of endophytic fungi from the kelp Ecklonia radiata. We then used a dye decolorization assay to assess their ability to degrade organic polymers (lignin, cellulose, and hemicellulose) under both oxic and anoxic conditions and compared their degradation ability with common terrestrial fungi. Under oxic conditions, there was evidence that Ascomycota isolates produced cellulose-degrading extracellular enzymes (associated with manganese peroxidase and sulfur-containing lignin peroxidase), while Mucoromycota isolates appeared to produce both lignin and cellulose-degrading extracellular enzymes, and all Basidiomycota isolates produced lignin-degrading enzymes (associated with laccase and lignin peroxidase). Under anoxic conditions, only three kelp endophytes degraded cellulose. We concluded that kelp fungal endophytes can contribute to cellulose degradation in both oxic and anoxic environments. Thus, endophytic kelp fungi may play a significant role in marine carbon cycling via polymeric organic matter degradation.

Biodegradation of nitroglycerin from propellant residues on military training ranges

Nitroglycerin (NG) is often present in soils and sometimes in pore water at antitank firing positions due to incomplete combustion of propellants. Various degradation processes can contribute to the natural attenuation of NG in soils and pore water, thus reducing the risks of groundwater contamination. However, until now these processes have been sparsely documented. This study aimed at evaluating the ability of microorganisms from a legacy firing position to degrade dissolved NG, as well as NG trapped within propellant particles. Results from the shake-flask experiments showed that the isolated culture is capable of degrading dissolved NG but not the nitrocellulose matrix of propellant particles, so that the deeply embedded NG molecules cannot be degraded. Furthermore, the results from column experiments showed that in a nutrient-poor sand, degradation of dissolved NG may not be sufficiently rapid to prevent groundwater contamination. Therefore, the results from this study indicate that, under favorable soil conditions, biodegradation can be an important natural attenuation process for NG dissolving out of fresh propellant residues. In contrast, biodegradation does not contribute to the long-term attenuation of NG within old, weathered propellant residues. Although NG in these old residues no longer poses a threat to groundwater quality, if soil clean-up of a legacy site is required, active remediation approaches should be sought.

Biodegradation of nitroglycerin and ethylene glycol dinitrate by free and immobilized mixed cultures

Aerobic biodegradation of nitroglycerin (NG) and ethylene glycol dinitrate (EGDN), both as individual substrates and in their mixture, was tested using batch or fed-batch cultivation with free suspended cells enriched from a soil sample subjected to a long-term contamination with explosives. EGDN was degraded only in the presence of glycerol as a co-substrate whereas NG could serve as a sole carbon, energy and nitrogen source for growth, its degradation being only slightly boosted by either glycerol or pyruvate. NG was not sufficient as a co-substrate for microbial growth on EGDN; furthermore, the presence of EGDN inhibited the NG degradation. The growth inhibition by both NG and EGDN was alleviated by the addition of glycerol. At an optimum nitroglycerin concentration of 30 mg/L, a maximum specific degradation rate of 60.9 ± 1.8 mg/gdw/h was observed. The biodegradation of both pollutants occurred with a release of nitrite. A method was developed for growing substantial amounts of NG-degrading biomass in the presence of glycerol for its immobilization on expanded slate in a pot-scale packed-bed reactor. Preliminary reactor tests were conducted in a continuous operation mode yielding a 70-90% NG biodegradation up to a load of 20 mg/L/h, with a removal rate up to 16 mg/L/h.

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.

Characterization of glycerol trinitrate reductase (NerA) and the catalytic role of active-site residues

Glycerol trinitrate reductase (NerA) from Agrobacterium radiobacter, a member of the old yellow enzyme (OYE) family of oxidoreductases, was expressed in and purified from Escherichia coli. Denaturation of pure enzyme liberated flavin mononucleotide (FMN), and spectra of NerA during reduction and reoxidation confirmed its catalytic involvement. Binding of FMN to apoenzyme to form the holoenzyme occurred with a dissociation constant of ca. 10(-7) M and with restoration of activity. The NerA-dependent reduction of glycerol trinitrate (GTN; nitroglycerin) by NADH followed ping-pong kinetics. A structural model of NerA based on the known coordinates of OYE showed that His-178, Asn-181, and Tyr-183 were close to FMN in the active site. The NerA mutation H178A produced mutant protein with bound FMN but no activity toward GTN. The N181A mutation produced protein that did not bind FMN and was isolated in partly degraded form. The mutation Y183F produced active protein with the same k(cat) as that of wild-type enzyme but with altered K(m) values for GTN and NADH, indicating a role for this residue in substrate binding. Correlation of the ratio of K(m)(GTN) to K(m)(NAD(P)H), with sequence differences for NerA and several other members of the OYE family of oxidoreductases that reduce GTN, indicated that Asn-181 and a second Asn-238 that lies close to Tyr-183 in the NerA model structure may influence substrate specificity.

Complete denitration of nitroglycerin by bacteria isolated from a washwater soakaway

Four axenic bacterial species capable of biodegrading nitroglycerin (glycerol trinitrate [GTN]) were isolated from soil samples taken from a washwater soakaway at a disused GTN manufacturing plant. The isolates were identified by 16S rRNA gene sequence homology as Pseudomonas putida, an Arthrobacter species, a Klebsiella species, and a Rhodococcus species. Each of the isolates utilized GTN as its sole nitrogen source and removed nitro groups sequentially from GTN to produce glycerol dinitrates and mononitrates (GMN), with the exception of the Arthrobacter strain, which achieved removal of only the first nitro group within the time course of the experiment. The Klebsiella strain exhibited a distinct preference for removal of the central nitro group from GTN, while the other five strains exhibited no such regioselectivity. All strains which removed a second nitro group from glycerol 1,2-dinitrate showed regiospecific removal of the end nitro group, thereby producing glycerol 2-mononitrate. Most significant was the finding that the Rhodococcus species was capable of removing the final nitro group from GMN and thus achieved complete biodegradation of GTN. Such complete denitration of GTN has previously been shown only in mixed bacterial populations and in cultures of Penicillium corylophilum Dierckx supplemented with an additional carbon and nitrogen source. Hence, to the best of our knowledge, this is the first report of a microorganism that can achieve complete denitration of GTN.

Regioselectivity of nitroglycerin denitration by flavoprotein nitroester reductases purified from two Pseudomonas species

Two species of Pseudomonas capable of utilizing nitroglycerin (NG) as a sole nitrogen source were isolated from NG-contaminated soil and identified as Pseudomonas putida II-B and P. fluorescens I-C. While 9 of 13 laboratory bacterial strains that presumably had no previous exposure to NG could degrade low concentrations of NG (0.44 mM), the natural isolates tolerated concentrations of NG that were toxic to the lab strains (1.76 mM and higher). Whole-cell studies revealed that the two natural isolates produced different mixtures of the isomers of dinitroglycerol (DNG) and mononitroglycerol (MNG). A monomeric, flavin mononucleotide-containing NG reductase was purified from each natural isolate. These enzymes catalyzed the NADPH-dependent denitration of NG, yielding nitrite. Apparent kinetic constants were determined for both reductases. The P. putida enzyme had a Km for NG of 52 +/- 4 microM, a Km for NADPH of 28 +/- 2 microM, and a Vmax of 124 +/- 6 microM x min(-1), while the P. fluorescens enzyme had a Km for NG of 110 +/- 10 microM, a Km for NADPH of 5 +/- 1 microM, and a Vmax of 110 +/- 11 microM x min(-1). Anaerobic titration experiments confirmed the stoichiometry of NADPH consumption, changes in flavin oxidation state, and multiple steps of nitrite removal from NG. The products formed during time-dependent denitration reactions were consistent with a single enzyme being responsible for the in vivo product distributions. Simulation of the product formation kinetics by numerical integration showed that the P. putida enzyme produced an approximately 2-fold molar excess of 1,2-DNG relative to 1,3-DNG. This result could be fortuitous or could possibly be consistent with a random removal of the first nitro group from either the terminal (C-1 and C-3) positions or middle (C-2) position. However, during the denitration of 1,2-DNG, a 1.3-fold selectivity for the C-1 nitro group was determined. Comparable simulations of the product distributions from the P. fluorescens enzyme showed that NG was denitrated with a 4.6-fold selectivity for the C-2 position. Furthermore, a 2.4-fold selectivity for removal of the nitro group from the C-2 position of 1,2-DNG was also determined. The MNG isomers were not effectively denitrated by either purified enzyme, which suggests a reason why NG could not be used as a sole carbon source by the isolated organisms.