Rapid mineralization of the phenylurea herbicide diuron by Variovorax sp. strain SRS16 in pure culture and within a two-member consortium

The novel bacterial N-demethylase PdmAB is responsible for the initial step of N,N-dimethyl-substituted phenylurea herbicide degradation

The environmental fate of phenylurea herbicides has received considerable attention in recent decades. The microbial metabolism of N,N-dimethyl-substituted phenylurea herbicides can generally be initiated by mono-N-demethylation. In this study, the molecular basis for this process was revealed. The pdmAB genes in Sphingobium sp. strain YBL2 were shown to be responsible for the initial mono-N-demethylation of commonly used N,N-dimethyl-substituted phenylurea herbicides. PdmAB is the oxygenase component of a bacterial Rieske non-heme iron oxygenase (RO) system. The genes pdmAB, encoding the α subunit PdmA and the β subunit PdmB, are organized in a transposable element flanked by two direct repeats of an insertion element resembling ISRh1. Furthermore, this transposable element is highly conserved among phenylurea herbicide-degrading sphingomonads originating from different areas of the world. However, there was no evidence of a gene for an electron carrier (a ferredoxin or a reductase) located in the immediate vicinity of pdmAB. Without its cognate electron transport components, expression of PdmAB in Escherichia coli, Pseudomonas putida, and other sphingomonads resulted in a functional enzyme. Moreover, coexpression of a putative [3Fe-4S]-type ferredoxin from Sphingomonas sp. strain RW1 greatly enhanced the catalytic activity of PdmAB in E. coli. These data suggested that PdmAB has a low specificity for electron transport components and that its optimal ferredoxin may be the [3Fe-4S] type. PdmA exhibited low homology to the α subunits of previously characterized ROs (less than 37% identity) and did not cluster with the RO group involved in O- or N-demethylation reactions, indicating that PdmAB is a distinct bacterial RO N-demethylase.

Enhanced removal of a pesticides mixture by single cultures and consortia of free and immobilized Streptomyces strains

Pesticides are normally used to control specific pests and to increase the productivity in crops; as a result, soils are contaminated with mixtures of pesticides. In this work, the ability of Streptomyces strains (either as pure or mixed cultures) to remove pentachlorophenol and chlorpyrifos was studied. The antagonism among the strains and their tolerance to the toxic mixture was evaluated. Results revealed that the strains did not have any antagonistic effects and showed tolerance against the pesticides mixture. In fact, the growth of mixed cultures was significantly higher than in pure cultures. Moreover, a pure culture (Streptomyces sp. A5) and a quadruple culture had the highest pentachlorophenol removal percentages (10.6% and 10.1%, resp.), while Streptomyces sp. M7 presented the best chlorpyrifos removal (99.2%). Mixed culture of all Streptomyces spp. when assayed either as free or immobilized cells showed chlorpyrifos removal percentages of 40.17% and 71.05%, respectively, and for pentachlorophenol 5.24% and 14.72%, respectively, suggesting better removal of both pesticides by using immobilized cells. These results reveal that environments contaminated with mixtures of xenobiotics could be successfully cleaned up by using either free or immobilized cultures of Streptomyces, through in situ or ex situ remediation techniques.

Joint photomicrobial process for the degradation of the insensitive munition N-guanylurea-dinitramide (FOX-12)

N-Guanylurea-dinitramide (FOX-12) is a very insensitive energetic material intended to be used in the composition of next-generation insensitive munitions. To help predict the environmental behavior and fate of FOX-12, we conducted a study to determine its photodegradability and biodegradability. When dissolved in water, FOX-12, a guanylurea-dinitramide salt, also named GUDN, dissociated instantly to produce the dinitramide moiety and guanylurea, as demonstrated by high-performance liquid chromatography (HPLC) analysis. When an aqueous solution of FOX-12 was subjected to photolysis using a solar-simulated photoreactor, we found a rapid removal of the dinitramide with concurrent formation of N₂O, NO₂(-), and NO₃(-). The second component, guanylurea, was photostable. However, when FOX-12 was incubated aerobically with the soil isolate Variovorax strain VC1 and protected from light, the dinitramide component of FOX-12 was recalcitrant but guanylurea degraded effectively to ammonia, guanidine, and presumably CO₂. When FOX-12 was incubated with strain VC1 in the presence of light, both components of FOX-12 degraded, giving similar products to those described above. We concluded that the new insensitive explosive FOX-12 can be effectively degraded by a joint photomicrobial process and, therefore, should not cause persistent contamination of surface waters.

Identification of by-products and toxicity assessment in aqueous-phase hydrodechlorination of diuron with palladium on activated carbon catalysts

The hydrodechlorination (HDC) of diuron in aqueous phase with hydrogen using two different activated carbon-supported Pd catalysts was studied. A commercial activated carbon and one prepared by chemical activation of grape seeds with phosphoric acid (GS) were evaluated as supports, being the catalysts tested in a wide range of temperature (30-100 °C) and space-time (78-311 kgcat h mol(-1)). Diuron conversion was above 70% under all the conditions tested. The Pd catalyst supported on GS showed the highest activity in terms of diuron conversion within the temperature range studied, allowing nearly complete conversion above 50 °C. However, a gradual loss of activity with time was observed for this catalyst. A complete route of hydrogenation of diuron was elucidated. Two reaction routes one leading to fenuron and another to aniline were identified. As the temperature and space-time were increased, the formation of fenuron (via monuron) was found to be favored. The toxicity of the reaction products was evaluated, being the route to fenuron and monuron, the one giving rise to a significant decrease of ecotoxicity.

Enhanced mineralization of diuron using a cyclodextrin-based bioremediation technology

The phenylurea herbicide diuron [N-(3,4-dichlorophenyl)-N,N-dimethylurea] is widely used in a broad range of herbicide formulations and, consequently, it is frequently detected as a major soil and water contaminant in areas where there is extensive use. Diuron has the unfortunate combination of being strongly adsorbed by soil organic matter particles and, hence, slowly degraded in the environment due to its reduced bioavailability. N-Phenylurea herbicides seem to be biodegraded in soil, but it must be kept in mind that this biotic or abiotic degradation could lead to accumulation of very toxic derived compounds, such as 3,4-dichloroaniline. Research was conducted to find procedures that might result in an increase in the bioavailability of diuron in contaminated soils, through solubility enhancement. For this purpose a double system composed of hydroxypropyl-β-cyclodextrin (HPBCD), which is capable of forming inclusion complexes in solution, and a two-member bacterial consortium formed by the diuron-degrading Arthrobacter sulfonivorans (Arthrobacter sp. N2) and the linuron-degrading Variovorax soli (Variovorax sp. SRS16) was used. This consortium can achieve a complete biodegradation of diuron to CO2 with regard to that observed in the absence of the CD solution, where only a 45% biodegradation was observed. The cyclodextrin-based bioremediation technology here described shows for the first time an almost complete mineralization of diuron in a soil system, in contrast to previous incomplete mineralization based on single or consortium bacterial degradation.

Purification and characterization of a novel chlorpyrifos hydrolase from Cladosporium cladosporioides Hu-01

Chlorpyrifos is of great environmental concern due to its widespread use in the past several decades and its potential toxic effects on human health. Thus, the degradation study of chlorpyrifos has become increasing important in recent years. A fungus capable of using chlorpyrifos as the sole carbon source was isolated from organophosphate-contaminated soil and characterized as Cladosporium cladosporioides Hu-01 (collection number: CCTCC M 20711). A novel chlorpyrifos hydrolase from cell extract was purified 35.6-fold to apparent homogeneity with 38.5% overall recovery by ammoniumsulfate precipitation, gel filtration chromatography and anion-exchange chromatography. It is a monomeric structure with a molecular mass of 38.3 kDa. The pI value was estimated to be 5.2. The optimal pH and temperature of the purified enzyme were 6.5 and 40°C, respectively. No cofactors were required for the chlorpyrifos-hydrolysis activity. The enzyme was strongly inhibited by Hg²⁺, Fe³⁺, DTT, β-mercaptoethanol and SDS, whereas slight inhibitory effects (5–10% inhibition) were observed in the presence of Mn²⁺, Zn²⁺, Cu²⁺, Mg²⁺, and EDTA. The purified enzyme hydrolyzed various organophosphorus insecticides with P-O and P-S bond. Chlorpyrifos was the preferred substrate. The Km and Vmax values of the enzyme for chlorpyrifos were 6.7974 μM and 2.6473 μmol·min⁻¹, respectively. Both NH2-terminal sequencing and matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometer (MALDI-TOF-MS) identified an amino acid sequence MEPDGELSALTQGANS, which shared no similarity with any reported organophosphate-hydrolyzing enzymes. These results suggested that the purified enzyme was a novel hydrolase and might conceivably be developed to fulfill the practical requirements to enable its use in situ for detoxification of chlorpyrifos. Finally, this is the first described chlorpyrifos hydrolase from fungus.

Aerobic mineralization of nitroguanidine by Variovorax strain VC1 isolated from soil

Nitroguanidine (NQ) is an energetic material that is used as a key ingredient of triple-base propellants and is currently being considered as a TNT replacement in explosive formulations. NQ was efficiently degraded in aerobic microcosms when a carbon source was added. NQ persisted in unamended microcosms or under anaerobic conditions. An aerobic NQ-degrading bacterium, Variovorax strain VC1, was isolated from soil microcosms containing NQ as the sole nitrogen source. NQ degradation was inhibited in the presence of a more favorable source of nitrogen. Resting cells of VC1 degraded NQ effectively (54 μmol h(-1) g(-1) protein) giving NH(3) (50.0%), nitrous oxide (N(2)O) (48.5%) and CO(2) (100%). Disappearance of NQ was accompanied by the formation of a key intermediate product that we identified as nitrourea by comparison with a reference material. Nitrourea is unstable in water and suffered both biotic and abiotic decomposition to eventually give NH(3), N(2)O, and CO(2). However, we were unable to detect urea. Based on products distribution and reaction stoichiometry, we suggested that degradation of NQ, O(2)NN═C(NH(2))(2), might involve initial enzymatic hydroxylation of the imine, -C═N- bond, leading first to the formation of the unstable α-hydroxynitroamine intermediate, O(2)NNHC(OH)(NH(2))(2), whose decomposition in water should lead to the formation of NH(3), N(2)O, and CO(2). NQ biodegradation was induced by nitroguanidine itself, L-arginine, and creatinine, all being iminic compounds containing a guanidine group. This first description of NQ mineralization by a bacterial isolate demonstrates the potential for efficient microbial remediation of NQ in soil.

Enhancement of cypermethrin degradation by a coculture of Bacillus cereus ZH-3 and Streptomyces aureus HP-S-01

Degradation of cypermethrin was significantly enhanced in a coculture of Bacillus cereus ZH-3 and Streptomyces aureus HP-S-01. In the pure culture, longer half-lives (t(1/2)=32.6-43.0h) of cypermethrin were observed, as compared to the mixed cocultures (t(1/2)=13.0h). The optimal degradation conditions were determined to be 28.2°C and pH 7.5 based on response surface methodology (RSM). Under these conditions, the mixed cultures completely metabolized cypermethrin (50mgL(-1)) within 72h. Analysis of degradation products of cypermethrin indicated that the microbial consortium converted cypermethrin to α-hydroxy-3-phenoxy-benzeneacetonitrile, 3-phenoxybenzaldehyde and 4-phenoxyphenyl-2,2-dimethyl-propiophenone, and subsequently transformed these compounds with a maximum specific degradation rate (q(max)), half-saturation constant (K(s)) and inhibition constant (K(i)) of 0.1051h(-1), 31.2289mgL(-1) and 220.5752mgL(-1), respectively. This is the first report of a proposed pathway of degradation of cypermethrin by hydrolysis of ester linkage and oxidization of 3-phenoxybenzyl in a coculture. Finally, this coculture is the first described mixed microbial consortium capable of metabolizing cypermethrin.

Characterization of the propanil biodegradation pathway in Sphingomonas sp. Y57 and cloning of the propanil hydrolase gene prpH

In our previous study, the isoproturon-degrading strain Sphingomonas sp. Y57 was isolated from the wastewater treatment system of an herbicide factory. Interestingly, this strain also showed the ability to degrade propanil (3,4-dichloropropionamilide). The present work reveals that Y57 degrades propanil via the following pathway: propanil was initially hydrolyzed to 3,4-dichloroaniline (3,4-DCA) and then converted to 4,5-dichlorocatechol, which was then subjected to aromatic ring cleavage and further processing. N-acylation and N-deacylation of 3,4-DCA also occurred, and among N-acylation products, 3,4-dichloropropionanilide was found for the first time. The gene encoding the propanil hydrolase responsible for transforming propanil into 3,4-DCA was cloned from Y57 and was designated as prpH. PrpH was expressed in Escherichia coli BL21 and purified using Ni-nitrilotriacetic acid affinity chromatography. PrpH displayed the highest activity against propanil at 40°C and at pH 7.0. The effect of metal ions on the propanil-degrading activity of PrpH was also determined. To our knowledge, this is the first report of a strain that can degrade both propanil and 3,4-DCA and the first identification of a gene encoding a propanil hydrolase in bacteria.

Detoxification of a carcinogenic paint preservative by Blumea malcolmii Hook cell cultures

Phytoremediation is considered as an effective viable alternative to remediate the contaminated sites, industrially hazardous chemicals and other toxic pollutants. This bioremediation option offers a safe, cheap and eco friendly alternative to existing physical and chemical remediation technologies as well as other biological sources. The wall paint preservatives consist of several harmful and carcinogenic compounds causing serious environmental concerns. In the present study, an actively growing Blumea malcolmii Hook cell suspensions were established successfully on MS+CM (20%) +2,4-D (5 mg l(-1))+Gln (100 mg l(-1))+sucrose (3%) and were used to detoxify a paint preservative Troysan S 89 (a mixture of carbendazim, diuron and ochthilinone). FTIR and UV spectral analytical studies revealed the phytotransformation of Troysan S 89 by Blumea cell suspension cultures. The non-toxic nature of the products formed after phytotransformation was confirmed by phytotoxicity, cytogenotoxicity while non-carcinogenic nature by Ames tests. The novelty of the present study is effective communal degradation of a mixture of three toxicants in Troysan S 89 by cell suspension cultures of Blumea. This work suggested that Blumea cell suspensions might be able to contribute to the wider and safer application of phytoremediation.

Evaluation of carcinogenic potential of diuron in a rat mammary two-stage carcinogenesis model

This study aimed to evaluate the carcinogenic potential of the herbicide Diuron in a two-stage rat medium-term mammary carcinogenesis model initiated by 7,12-dimethylbenz(a)anthracene (DMBA). Female seven-week-old Sprague-Dawley (SD) rats were allocated to six groups: groups G1 to G4 received intragastrically (i.g.) a single 50 mg/kg dose of DMBA; groups G5 and G6 received single administration of canola oil (vehicle of DMBA). Groups G1 and G5 received a basal diet, and groups G2, G3, G4, and G6 were fed the basal diet with the addition of Diuron at 250, 1250, 2500, and 2500 ppm, respectively. After twenty-five weeks, the animals were euthanized and mammary tumors were histologically confirmed and quantified. Tumor samples were also processed for immunohistochemical evaluation of the expressions of proliferating cell nuclear antigen (PCNA), cleaved caspase-3, estrogen receptor-α (ER-α), p63, bcl-2, and bak. Diuron treatment did not increase the incidence or multiplicity of mammary tumors (groups G2 to G4 versus Group G1). Also, exposure to Diuron did not alter tumor growth (cell proliferation and apoptosis indexes) or immunoreactivity to ER-α, p63 (myoephitelial marker), or bcl-2 and bak (apoptosis regulatory proteins). These findings indicate that Diuron does not have a promoting potential on mammary carcinogenesis in female SD rats initiated with DMBA.

Degradation of dichloroaniline isomers by a newly isolated strain, Bacillus megaterium IMT21

An efficient 3,4-dichloroaniline (3,4-DCA)-mineralizing bacterium has been isolated from enrichment cultures originating from a soil sample with a history of repeated exposure to diuron, a major metabolite of which is 3,4-DCA. This bacterium, Bacillus megaterium IMT21, also mineralized 2,3-, 2,4-, 2,5- and 3,5-DCA as sole sources of carbon and energy. These five DCA isomers were degraded via two different routes. 2,3-, 2,4- and 2,5-DCA were degraded via previously unknown dichloroaminophenol metabolites, whereas 3,4- and 3,5-DCA were degraded via dichloroacetanilide.

Biotransformation and biomonitoring of phenylurea herbicide diuron

A Gram-positive, Micrococcus sp. strain PS-1 isolated from diuron storage site was studied for its capability of biotransformation of phenylurea herbicide diuron to a secondary metabolite, 1-(3,4-dichlorophenyl)urea (DCPU) for bioconjugation and antibody development applications. The metabolite formed associated with profound changes in bacterial cell morphology demonstrated increase in the degradation kinetics of diuron in presence of small quantity of a surfactant. The synthesized metabolite identified by chromatographic and mass spectrometry techniques was conjugated with carrier protein, and used as an immunogen for antibodies production. The generated antibody was highly specific, demonstrating excellent sensitivity against diuron. The antibody was used as receptor molecules in standard fluorescence immunoassay (FIA) format showing detection limit of 0.01 ng/mL in the optimum working concentration range of diuron with good signal precision (∼2%). The study presented first time the degradation pathway of herbicide by specific microorganism to synthesize hapten for bioconjugation and immunoassay development.

Degradation mechanisms of phoxim in river water

Degradation of phoxim in river water was fully explored in this paper. Effects of pH, temperature, and photoirradiation on the degradation were investigated in detail. The results indicated that the degradation was characterized by a first-order process; UV irradiation and the increase of pH and temperature substantially accelerated the degradation. To fully characterize the degradation mechanism, HPLC-MS/MS was utilized to identify the degradation intermediates. Five intermediates were identified as phoxom, phoxom dimer, O,O,O',O'-tetraethyldithiopyrophosphate, O,O,O'-triethyl-O'-2-hydroxyethyldisulfinylpyrophosphate, and O,O,O'-triethyl-O'-2-hydroxyethyldithiopyrophosphate. On the basis of the results of the intermediate analysis, the degradation pathways of phoxim under the present experimental conditions were proposed. Through conversion of a thiophosphoryl into a phosphoryl group, some phoxim was converted to phoxom, most of which further formed dimer. Another portion of phoxim transformed to O,O,O',O'-tetraethyldithiopyrophosphate via nucleophilic substitution and photolysis. Thereafter, O,O,O',O'-tetraethyldithiopyrophosphate underwent hydroxylation to form O,O,O'-triethyl-O'-2-hydroxyethyldithiopyrophosphate or sulfur oxidation first and then hydroxylation to produce O,O,O'-triethyl-O'-2-hydroxyethyldisulfinylpyrophosphate. The understanding of phoxim's degradation mechanism in this study will be critical to its safety assessment and increase the understanding of the fate of phoxim in environment water.

Diuron biotransformation and its effects on biofilm bacterial community structure

The effect of realistic environmental contamination of diuron on natural epilithic biofilms dwelling bacterial communities and their transformation capacities were investigated by using microcosm experiments. Cobbles carrying biofilms from two sites ("Pau" and "Lacq") located in areas of contrasting pesticide use (urban and agricultural) on the Gave de Pau river (South-West France) were analysed. The water of the upstream site, Pau, was characterised by fewer pesticides than the water of Lacq, whereas concentrations were higher at Pau. The sampled cobbles were exposed to diuron (10 μg L(-1)) in microcosms. After 3 weeks of exposure, pesticides were analysed and bacterial community structures were assessed with terminal-restriction fragment length polymorphism (T-RFLP). Diuron was biotransformed during contact with biofilms, revealing that these communities contribute to the production of DCPMU (1-(3,4-dichlorophenyl)-3-methylurea) and DCPU metabolites (1-(3,4-dichlorophenyl) urea) in the river ecosystems. Bacterial communities from the most contaminated site appeared to be more resistant to diuron exposure. Correlation analyses combining chemical data with molecular fingerprinting showed that past in situ exposure drove the response of the bacterial communities.

Biodegradation kinetics of the benzimidazole fungicide thiophanate-methyl by bacteria isolated from loamy sand soil

Degradation of the fungicide thiophanate-methyl (TM) by Enterobacter sp. TDS-1 and Bacillus sp. TDS-2 isolated from sandy soil previously treated with TM was studied in mineral salt medium (MSM) and soil. Both strains were able to grow in MSM supplemented with TM (50 mg l(-1)) as the sole carbon source. Over a 16 days incubation period, 60 and 77% of the initial dose of TM were degraded by strains TDS-1 and TDS-2, respectively, and disappearance of TM was described by first-order kinetics. Medium supplementation with glucose markedly stimulated bacterial growth; while the final rate of TM degradation was reduced by 21 and 27% for strains TDS-1 and TDS-2, respectively as compared to medium with TM only. Moreover, this additional carbon source changed the TM degradation kinetics, which proceeded according to a zero-order model. This effect was linked to substrate competition and/or a strong decrease of medium pH. Isolates degraded TM (100 mg kg(-1)) in soil with rate constants of 0.186 and 0.210 day(-1), following first-order rate kinetics, and the time in which the initial TM concentration was reduced by 50% (DT50) in soils inoculated with strains TDS-1 and TDS-2 were 6.3 and 5.1 days, respectively. Analysis of TM degradation products in soil showed that the tested strains may have the potential to transform carbendazim (MBC) to 2-aminobenzimidazole (2-AB), and may be useful for a bioremediation of MBC-polluted soils.

Biological treatment of propanil and 3,4-dichloroaniline: kinetic and microbiological characterisation

Propanil (3,4-dichloropropionanilide) is a widely used herbicide, applied worldwide in rice paddies. Propanil is primarily transformed in nature to 3,4-dichloroaniline (DCA), which is more slowly biodegradable. Both compounds have adverse health and ecotoxicity effects. This work investigated the microbial ecology and kinetics of propanil-degrading enrichments obtained from soil in a sequencing batch reactor (SBR) operated with different feeding strategies, aiming at the enhanced biological removal of propanil and DCA from contaminated waters. During SBR operation with a dump feeding strategy, a high propanil concentration led to DCA accumulation, which was only fully degraded after 5 days, likely due to DCA inhibition. For this reason, the operational mode was changed to fed-batch operation with lower initial propanil concentrations, which resulted in faster propanil and DCA biodegradation. Thus a fed-batch operation seems more appropriate for the acclimatisation of an effective propanil- and DCA-degrading population. The changes in performance were accompanied by a shift in the microbial population structure, as determined by DGGE of the 16S rRNA gene, particularly after a feed of DCA as the sole carbon source. Isolates obtained from the acclimatised population included members of the genera Enterococcus and Rhodococcus, as well as Brevundimonas, which displayed >90% propanil biodegradation efficiency.

Diuron mineralisation in a Mediterranean vineyard soil: impact of moisture content and temperature

BACKGROUND

The diuron-mineralising ability of the microbiota of a Mediterranean vineyard soil exposed each year to this herbicide was measured. The impact of soil moisture and temperature on this microbial activity was assessed.

RESULTS

The soil microbiota was shown to mineralise diuron. This mineralising activity was positively correlated with soil moisture content, being negligible at 5% and more than 30% at 20% soil moisture content. According to a double Gaussian model applied to fit the dataset, the optimum temperature/soil moisture conditions were 27.9 degrees C/19.3% for maximum mineralisation rate and 21.9 degrees C/18.3% for maximum percentage mineralisation. The impact of temperature and soil moisture content variations on diuron mineralisation was estimated. A simulated drought period had a suppressive effect on subsequent diuron mineralisation. This drought effect was more marked when higher temperatures were used to dry (40 degrees C versus 28 degrees C) or incubate (28 degrees C versus 20 degrees C) the soil. The diuron kinetic parameters measured after drought conditions were no longer in accordance with those estimated by the Gaussian model.

CONCLUSION

Although soil microbiota can adapt to diuron mineralisation, its activity is strongly dependent on climatic conditions. It suggests that diuron is not rapidly degraded under Mediterranean climate, and that arable Mediterranean soils are likely to accumulate diuron residues.

Mineralisation of the herbicide linuron by Variovorax sp. strain RA8 isolated from Japanese river sediment using an ecosystem model (microcosm)

BACKGROUND

Linuron is a globally used phenylurea herbicide, and a large number of studies have been made on the microbial degradation of the herbicide. However, to date, the few bacteria able individually to mineralise linuron have been isolated only from European agricultural soils. An attempt was made to isolate linuron-mineralising bacteria from Japanese river sediment using a uniquely designed river ecosystem model (microcosm) treated with (14)C-ring-labelled linuron (approximately 1 mg L(-1)).

RESULTS

A linuron-mineralising bacterium that inhabits river sediment was successfully isolated. The isolate belongs to the genera Variovorax and was designated as strain RA8. Strain RA8 gradually used linuron in basal salt medium (5.2 mg L(-1)) with slight growth. In 15 days, approximately 25% of (14)C-linuron was mineralised to (14)CO(2), with 3,4-dichloroaniline as an intermediate. Conversely, in 100-fold diluted R2A broth, strain RA8 rapidly mineralised (14)C-linuron (5.5 mg L(-1)) and more than 70% of the applied radioactivity was released as (14)CO(2) within 3 days, and a trace amount of 3,4-dichloroaniline was detected. Additionally, the isolate also degraded monolinuron, metobromuron and chlorobromuron, but not diuron, monuron or isoproturon.

CONCLUSION

Although strain RA8 can grow on linuron, some elements in the R2A broth seemed significantly to stimulate its growth and ability to degrade. The isolate strictly recognised the structural difference between N-methoxy-N-methyl and N,N-dimethyl substitution of various phenylurea herbicides.

Metabolites of the phenylurea herbicides chlorotoluron, diuron, isoproturon and linuron produced by the soil fungus Mortierella sp

Phenylurea herbicides are used worldwide, and often pollute surface- and groundwater in concentrations exceeding the limit value for drinking water (0.1 microg l(-1)). Bacteria degrade phenylurea herbicides by successive N-dealkylation to substituted aniline products. Little is known about the corresponding fungal pathways, however. We here report degradation of chlorotoluron, diuron, isoproturon and linuron by the soil fungus Mortierella sp. Gr4. Degradation was fastest with linuron and resulted in successively dealkylated metabolites and 3,4-dichloroaniline. A major new metabolite was detected that has not yet been fully identified. Thin layer chromatography and nuclear magnetic resonance spectroscopy indicate that it is a non-aromatic diol. Degradation of isoproturon, chlorotoluron and diuron involved successive N-demethylation and, in the case of isoproturon and chlorotoluron, additional hydroxylation. A new hydroxylated isoproturon metabolite was detected. The study thus shows that the fungal pathways differ from the bacterial pathways and yield new metabolites of possible environmental concern.

Effect of immobilization of a bacterial consortium on diuron dissipation and community dynamics

This work intended to study the relationship between diuron herbicide dissipation and the population dynamics of co-cultivated Delftia acidovorans WDL34 (WDL34) and Arthrobacter sp. N4 (N4) for different cell formulations: free cells or immobilization in Ca-alginate beads of one or both strains. GFP-tagged WDL34 and N4 Gram staining allowed analyzing the cell growth and distribution of each strain in both beads and culture medium in the course of the time. Compared to the free cell co-culture of WDL34 and N4, immobilization of WDL34 in Ca-alginate beads co-cultivated with free N4 increased the dissipation rate of diuron by 53% (0.141 mg ml(-1) h(-1)). In that case, immobilization strongly modified the final equilibrium among both strains (highest total N4 to WDL34 ratio). Our results demonstrated that the inoculant formulation played a major role in the cell growth of each cultivated strain possibly increasing diuron dissipation. This optimized cell formulation may allow improving water and soil treatment.

Biodegradation of the organophosphorus insecticide diazinon by Serratia sp. and Pseudomonas sp. and their use in bioremediation of contaminated soil

An enrichment culture technique was used for the isolation of bacteria responsible for biodegradation of diazinon in soil. Three bacterial strains were screened and identified by MIDI-FAME profiling as Serratia liquefaciens, Serratia marcescens and Pseudomonas sp. All isolates were able to grow in mineral salt medium (MSM) supplemented with diazinon (50 mgL(-1)) as a sole carbon source, and within 14d 80-92% of the initial dose of insecticide was degraded by the isolates and their consortium. Degradation of diazinon was accelerated when MSM was supplemented with glucose. However, this process was linked with the decrease of pH values, after glucose utilization. Studies on biodegradation in sterilized soil showed that isolates and their consortium exhibited efficient degradation of insecticide (100mg kg(-1) soil) with a rate constant of 0.032-0.085d(-1), and DT(50) for diazinon was ranged from 11.5d to 24.5d. In contrast, degradation of insecticide in non-sterilized soil, non-supplemented earlier with diazinon, was characterized by a rate constant of 0.014d(-1) and the 7-d lag phase, during which only 2% of applied dose was degraded. The results suggested a strong correlation between microbial activity and chemical processes during diazinon degradation. Moreover, isolated bacterial strains may have potential for use in bioremediation of diazinon-contaminated soils.

Diuron biodegradation in activated sludge batch reactors under aerobic and anoxic conditions

Diuron biodegradation was studied in activated sludge reactors and the impacts of aerobic and anoxic conditions, presence of supplemental substrate and biomass acclimatization on its removal were investigated. Diuron and three known metabolites, namely DCPMU (1-(3,4-dichlorophenyl)-3-methylurea), DCPU (1-3,4-dichlorophenylurea) and DCA (3,4-dichloroaniline), were extracted by solid-phase extraction (dissolved phase) or sonication (particulate phase) and determined using High Performance Liquid Chromatography-Diode Array Detector (HPLC-DAD). During the experiments only a minor part of these compounds was associated with the suspended solids. Under aerobic conditions, almost 60% of Diuron was biodegraded, while its major metabolite was DCA. The existence of anoxic conditions increased Diuron biodegradation to more than 95%, while the major metabolite was DCPU. Mass balance calculation showed that a significant fraction of Diuron is mineralized or biotransformed to other unknown metabolites. The presence of low concentrations of supplemental substrate did not affect Diuron biodegradation, whereas the acclimatization of biomass slightly accelerated its elimination under anoxic conditions. Calculation of half-lives showed that under aerobic conditions DCPMU, DCPU and DCA are biodegraded much faster than the parent compound. In the future, the sequential use of anoxic and aerobic conditions could provide sufficient removal of Diuron and its metabolites from runoff waters.

Characterization of the phenylurea hydrolases A and B: founding members of a novel amidohydrolase subgroup

Mycobacterium brisbanense strain JK1, a bacterium capable of degrading the herbicide diuron, was isolated from herbicide-exposed soil. A gene/enzyme system with diuron hydrolase activity was isolated from this strain and named PUH (phenylurea hydrolase) B (puhB/PuhB) because of its close similarity to the previously characterized PUH A (puhA/PuhA). Both PUHs were heterologously expressed, purified and characterized. The PUHs were found to oligomerize as hexamers in solution, with each monomer containing a mononuclear Zn2+ active site. Sequence analysis showed that these enzymes belong to the metal-dependent amidohydrolase superfamily, although they contain a hitherto unreported Asn-X-His metal-binding motif and appear to form a novel sub-group within this superfamily. The effects of temperature and solvent on the enzymes were characterized. Determination of the kinetic parameters of the PUHs was used alongside Brønsted plots to develop a plausible catalytic mechanism, which is similar to that used by urease. In addition to the primary PUH activity, both enzymes are catalytically promiscuous, efficiently hydrolysing esters, carbamates and phosphotriesters. In fact, an analogue of diuron, in which the C–N bond was replaced by a C–O bond, was found to be turned over as efficiently as diuron, suggesting that the substrate specificity is predominantly determined by steric factors. The discovery of PuhA and PuhB on separate continents, and the absence of any other close homologues in the available sequence databases, poses a challenging question regarding the evolutionary origins of these enzymes.

Effect of different humic substances on the fate of diuron and its main metabolite 3,4-dichloroaniline in soil

Humic substances (HS) are the dominant constituents of soil organic matter (SOM). The interactions between the phenylurea herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) and several HS fractions, purified from various soil horizons, were studied. One commercial humic acid (HA) was included for comparison. Diuron was shown to adsorb significantly, but reversibly, to purified HA while sorption to fulvic acid (FA) was less pronounced. The sorption abilities of the purified HS fractions were correlated with their total aromatic content. In natural soils, SOM was the main adsorbent of diuron, but the organic matter partition coefficient was larger in sandy compared to clayey soils. Degradation of diuron in natural soils was slow and incomplete. Inoculation of a sandy C-horizon with a diuron-degrading bacterial strain led to substantial diuron degradation, but the addition of purified FA and HA to these inoculated soils decreased this degradation. The main metabolite produced during diuron degradation, 3,4-dichloroaniline, was bound irreversibly to HS within days after formation.