Degradation of simazine by microorganisms isolated from soils of Spanish olive fields

Isolation of 2 simazine-degrading bacteria and development of a microbial agent for bioremediation of simazine pollution

Simazine was one of the most commonly used herbicides and was widely used to control broadleaf weeds in agriculture and forestry. Its widespread use had caused wide public concern for its high ecological toxicity. In order to remove simazine residues, 2 strains capable of effectively degrading simazine were isolated from the soil and named SIMA-N5 and SIMA-N9. SIMA-N5 was identified as Bacillus licheniformis by 16SrRNA sequence analysis, and SIMA-N9 was Bacillus altitudinis. According to the degradation ratio of simazine in a certain period of time, the degradation ability of different strains was evaluated. The degradation efficiency of simazine (5 mg/L) by SIMA-N9 could reach about 98% in 5d, and the strain SIMA-N5 could reach 94% under the same conditions. In addition, the addition of Pennisetum rhizosphere soil during the process of degrading simazine by strain SIMA-N9 could effectively improve the degradation efficiency. The strain SIMA-N9 has been developed as a microbial agent for the bioremediation of simazine contamination in soil. The new microbial agent developed by using SIMA-N9 has achieved satisfactory application effects. Based on the research results already obtained in this study, it was considered that strain SIMA-N9 and its live bacterial agent could play an important role in bioremediation of simazine pollution. This study could not only provide a set of solutions to the simazine pollution, but also provide a reference for the treatment of other pesticide pollution.

Influence of microorganisms and leaching on simazine attenuation in an agricultural soil

Simazine is an s-triazine herbicide world widely used for the control of broadleaf weeds. The influence of leaching and microorganisms on simazine attenuation in an agricultural soil long-term treated with this herbicide was studied. To elucidate the leaching potential of simazine in this soil, undisturbed soil columns amended with simazine were placed in a specially designed system and an artificial precipitation was simulated. To evaluate the simazine removal by soil microorganisms, three soil microcosm sets were established: i) control soil; ii) soil subjected to gamma irradiation (γ-soil) and iii) γ-soil inoculated with the simazine-degrading bacterium Pseudomonas sp. strain MHP41. The simazine-degrading microorganisms in soil were estimated using an indicator for respiration combined with MPN enumeration. The simazine removal in soil was monitored by GC-ECD and HPLC. In this agricultural soil the leaching of the applied simazine was negligible. The gamma irradiation decreased in more than one order of magnitude the cultivable heterotrophic bacteria and reduced the simazine-degrading microorganisms. Simazine was almost completely depleted (97%) in control soil by natural attenuation after 23 d, whereas in γ-soil only 70% of simazine was removed. The addition of the simazine-degrading strain MHP41 to γ-soil restored and upheld high stable simazine catabolic microorganisms as well as increased the simazine removal (87%). The results indicated that simazine is subjected to microbial degradation with negligible leaching in this agricultural soil and pointed out the crucial role of native microbiota in the herbicide removal.

Complete genome sequence of the metabolically versatile plant growth-promoting endophyte Variovorax paradoxus S110

Variovorax paradoxus is a microorganism of special interest due to its diverse metabolic capabilities, including the biodegradation of both biogenic compounds and anthropogenic contaminants. V. paradoxus also engages in mutually beneficial interactions with both bacteria and plants. The complete genome sequence of V. paradoxus S110 is composed of 6,754,997 bp with 6,279 predicted protein-coding sequences within two circular chromosomes. Genomic analysis has revealed multiple metabolic features for autotrophic and heterotrophic lifestyles. These metabolic diversities enable independent survival, as well as a symbiotic lifestyle. Consequently, S110 appears to have evolved into a superbly adaptable microorganism that is able to survive in ever-changing environmental conditions. Based on our findings, we suggest V. paradoxus S110 as a potential candidate for agrobiotechnological applications, such as biofertilizer and biopesticide. Because it has many associations with other biota, it is also suited to serve as an additional model system for studies of microbe-plant and microbe-microbe interactions.

Prevalence of the gene trzN and biogeographic patterns among atrazine-degrading bacteria isolated from 13 Colombian agricultural soils

The following study evaluated the diversity and biogeography of 83 new atrazine-degrading bacteria and the composition of their atrazine degradation genes. These strains were isolated from 13 agricultural soils and grouped according to rep-PCR genomic fingerprinting into 11 major clusters, which showed biogeographic patterns. Three clusters (54 strains) belonged to the genus Arthrobacter, seven clusters (28 strains) were similar to the genus Nocardioides and only one strain was a gram-negative from the genus Ancylobacter. PCR assays for the detection of the genes atzA, B, C, D, E, F and trzN conducted with each of the 83 strains revealed that 82 strains (all gram positive) possessed trzN, 74 of them possessed the combination of trzN, atzB and atzC, while only the gram-negative strain had atzA. A similar PCR assay for the two analogous genes, atzA and trzN, responsible for the first step of atrazine degradation, was performed with DNA extracted directly from the enrichment cultures and microcosms spiked with atrazine. In these assays, the gene trzN was detected in each culture, while atzA was detected in only six out of 13 soils. These results raise an interesting hypothesis on the evolutionary ecology of the two atrazine chlorohydrolase genes (i.e. atzA and trzN) and about the biogeography of atrazine-degrading bacteria.

Agronomic and environmental implications of enhanced s-triazine degradation

Novel catabolic pathways enabling rapid detoxification of s-triazine herbicides have been elucidated and detected at a growing number of locations. The genes responsible for s-triazine mineralization, i.e. atzABCDEF and trzNDF, occur in at least four bacterial phyla and are implicated in the development of enhanced degradation in agricultural soils from all continents except Antarctica. Enhanced degradation occurs in at least nine crops and six crop rotation systems that rely on s-triazine herbicides for weed control, and, with the exception of acidic soil conditions and s-triazine application frequency, adaptation of the microbial population is independent of soil physiochemical properties and cultural management practices. From an agronomic perspective, residual weed control could be reduced tenfold in s-triazine-adapted relative to non-adapted soils. From an environmental standpoint, the off-site loss of total s-triazine residues could be overestimated 13-fold in adapted soils if altered persistence estimates and metabolic pathways are not reflected in fate and transport models. Empirical models requiring soil pH and s-triazine use history as input parameters predict atrazine persistence more accurately than historical estimates, thereby allowing practitioners to adjust weed control strategies and model input values when warranted.

Removal of simazine in a UV/TiO2 heterogeneous system

The degradation of simazine by photocatalytic oxidation in a TiO2 suspension was studied. The influence of various parameters such as wavelength sources, light intensity, TiO2 dosage, and initial pH has been investigated, and the optimum conditions for the degradation of simazine have been identified. The photocatalytic degradation of simazine was observed to follow a pseudo-first-order reaction. The overdose of light intensity and photocatalyst does not always guarantee a beneficial effect on the photocatalytic reaction, and the optimum TiO2 dosage was found to be 0.1 g/L in this study. The optimum pH value is 9.0 for the photocatalytic degradation of simazine, whereas extremely acidic and alkaline conditions inhibit photocatalytic efficiency. Simazine can be fully destroyed, but ring-opening and mineralization are not observed in this system. In addition, seven simazine derivatives (CEAT, OEET, CAAT, ODET, OEAT, OAAT, OOOT) were detected by LC-ESI/MS. It is suggested that dealkylation is the major pathway of simazine photodecay in UV/TiO2 systems. The final product was found to be cyanuric acid.

Treatment of bromoamine acid wastewater using combined process of micro-electrolysis and biological aerobic filter

The wastewater originated from the production of bromoamine acid was treated in a sequential system of micro-electrolysis (ME) and biological aerobic filter (BAF). Decolorization and COD(Cr) removal rate of the proposed system was investigated with full consideration of the influence of two major controlling factors such as organic loading rate (OLR) and hydraulic retention time (HRT). The removal rate of COD(Cr) was 81.2% and that of chrominance could be up to 96.6% at an OLR of 0.56 kg m(-3)d(-1) when the total HRT was 43.4h. Most of the chrominance was removed by the ME treatment, however, the BAF process was more effective for COD(Cr) removal. The GC-MS and HPLC-MS analysis of the contaminants revealed that 1-aminoanthraquinone, bromoamine acid and mono-sulfonated 1,2-dichlorobenzene were the main organic components in the wastewater. The reductive transformation of the anthraquinone derivatives in the ME reactor improved the biodegradability of the wastewater, and rendered the decolorization. After long-term of operation, it was observed that the predominant microorganisms immobilized on the BAF carriers were rod-shaped and globular. Four bacterial strains with apparent 16S rDNA fragments in the Denaturing Gradient Gel Electrophoresis (DGGE) profiles of BAF samples were identified as Variovorax sp., Sphingomonas sp., Mycobacterium sp., and Microbacterium sp.

Different substrate specificities of two triazine hydrolases (TrzNs) from Nocardioides species

Nocardioides sp. strain MTD22 degraded atrazine, ametryn and atraton, as did Arthrobacter aurescens strain TC1 and Nocardioides sp. strain C190. These strains contain trzN, a gene coding for TrzN, triazine hydrolase showing a broad substrate range. However, Nocardioides sp. strain AN3 degraded only atrazine despite containing trzN. These differences in s-triazine degradation are presumed to be due to differences in the amino acid sequences of TrzNs. Consequently, 1371 nucleotides of the trzN coding sequences of strains AN3 and MTD22 were determined. Comparisons of the amino acid sequences of TrzNs indicated that three residues of strain AN3 (Thr(214), His(215) and Gln(241)) were distinct from those of the other three strains (Pro(214), Tyr(215) and Glu(241)). To confirm the relationships between these amino acid sequences and the substrate specificities of TrzNs, wild and chimera trzN genes were constructed and expressed in Escherichia coli cells. Cells expressing wild MTD22 trzN (Pro(214)Tyr(215)Glu(241)) and chimera AN3-MTD22 trzN (Thr(214)His(215)Glu(241)) degraded all s-triazines, but the degradation rate was markedly decreased in AN3-MTD22 trzN. Wild AN3 trzN (Thr(214)His(215)Gln(241)) and chimera MTD22-AN3 trzN (Pro(214)Tyr(215)Gln(241)) degraded only atrazine. These results suggest that the substitution of Glu(241) for Gln(241) significantly decreases enzyme affinity for ametryn and atraton.

Isolation and characterization of a novel simazine-degrading bacterium from agricultural soil of central Chile, Pseudomonas sp. MHP41

s-Triazine herbicides are used extensively in South America in agriculture and forestry. In this study, a bacterium designated as strain MHP41, capable of degrading simazine and atrazine, was isolated from agricultural soil in the Quillota valley, central Chile. Strain MHP41 is able to grow in minimal medium, using simazine as the sole nitrogen source. In this medium, the bacterium exhibited a growth rate of mu=0.10 h(-1), yielding a high biomass of 4.2 x 10(8) CFU mL(-1). Resting cells of strain MHP41 degrade more than 80% of simazine within 60 min. The atzA, atzB, atzC, atzD, atzE and atzF genes encoding the enzymes of the simazine upper and lower pathways were detected in strain MHP41. The motile Gram-negative bacterium was identified as a Pseudomonas sp., based on the Biolog microplate system and comparative sequence analyses of the 16S rRNA gene. Amplified ribosomal DNA restriction analysis allowed the differentiation of strain MHP41 from Pseudomonas sp. ADP. The comparative 16S rRNA gene sequence analyses suggested that strain MHP41 is closely related to Pseudomonas nitroreducens and Pseudomonas multiresinovorans. This is the first s-triazine-degrading bacterium isolated in South America. Strain MHP41 is a potential biocatalyst for the remediation of s-triazine-contaminated environments.

Application of fluorescence in situ hybridization technique to detect simazine-degrading bacteria in soil samples

We propose a new approach to evaluate the natural attenuation capacity of soil by using fluorescence in situ hybridization (FISH). A specific oligonucleotide probe AtzB1 was designed based on the sequence data of the atzB gene involved in the hydrolytic deamination of s-triazines; this gene, located in a multiple copy plasmid was detected by the optimized FISH protocol. Two agricultural soils (Lodi and Henares) with a history of simazine treatments, and two natural soils (Soto and Monza), without previous exposure to simazine, were studied. AtzB1 probe-target cells were found only in the agricultural soils and, in a greater percentage, in the Lodi soil, compared to the Henares one. Moreover, the greatest percentage of AtzB1 probe-target cells in Lodi was accompanied by a greater mineralization rate, compared to the Henares soil. The FISH method used in this study was suitable for the detection of simazine-degrading bacteria and could be a useful indicator of the potential of soil bioremediation.

Selective modification of clay minerals for the adsorption of herbicides widely used in olive groves

Ground and surface water contamination by herbicides applied to olive groves in Spain and other Mediterranean countries is demanding strategies to prevent and remediate the environmental problems repeatedly caused by such herbicides. In this study, six different organic cations (L-carnitine, spermine, hexadimethrine, tyramine, phenyltrimethylammonium, and hexadecyltrimethylammonium) were incorporated into Na-rich Wyoming montmorillonite (SWy-2) and Ca-rich Arizona montmorillonite (SAz-1) at two different loadings (50% and 100% of the cation exchange capacity of the clays) as a strategy to enhance the affinity of the clay minerals for three herbicides widely used in olive groves: terbuthylazine, diuron, and MCPA. The modified montmorillonites were characterized and tested as adsorbents of the herbicides through batch adsorption tests. At the experimental conditions used, some of the modified montmorillonites removed more than 95% of the herbicide initially present in aqueous solution, whereas the unmodified clays removed less than 15%. All three herbicides displayed very strong affinities for SAz-1 exchanged with hexadecyltrimethylammonium cations, particularly when these were incorporated at 100% of the cation exchange capacity of the clay mineral. Terbuthylazine and diuron also displayed very strong affinities for SWy-2 exchanged with L-carnitine and spermine, respectively. The chemical characteristics of the organic cation greatly influenced the adsorptive properties of the resultant organoclay. The herbicides were in general reversibly adsorbed by the modified clays. The results indicate that some of the tested modified clays could be suitable for the removal of the assayed herbicides from contaminated water and also as possible supports for the design of slow release formulations of such herbicides to attenuate their environmental impact when used in high-risk scenarios such as olive groves.

Isolation and identification of atrazine-degrading bacteria from corn field soil in Fars province of Iran

In this study several agricultural fields with a long history of atrazine application in Fars province of Iran have been explored for their potential of atrazine biodegradation. After several subculturing for a period of 300 days acclimation, leads to an enhancement of atrazine biodegradation rate. A successful enrichment culture with a high capability for atrazine degradation was obtained (88%). A combination of enrichment culture technique, in a basal salt medium containing atrazine and carbon sources under nitrogen limitation and plating on indicator atrazine agar, have permitted the isolation of bacterial consortium with high capability of using atrazine as a nitrogen source. Seven gram-negative and one gram-positive bacterial strain, which were able to use this herbicide as a sole source of nitrogen, were isolated from Darehasalouie Kavar corn field soil. Based on physiological, biochemical and nutritional characteristics, the isolated bacteria were identified as Pseudomonas alcaligenes, Acidovorax sp., Pseudomonas putida, Ralstonia eutrophus, Pseudomonas syiringe, Erwinia tracheiphila, Entrobacter agglomerans and Micrococcus varians. Therefore, the bacterial consortium in liquid culture containing carbon sources and atrazine as a sole source of nitrogen, degrade added atrazine more than 80%.

Simazine treatment history determines a significant herbicide degradation potential in soils that is not improved by bioaugmentation with Pseudomonas sp. ADP

AIMS

To study biological removal of the herbicide simazine in soils with different history of herbicide treatment and to test bioaugmentation with a simazine-degrading bacterial strain.

METHODS AND RESULTS

Simazine removal was studied in microcosms prepared with soils that had been differentially exposed to this herbicide. Simazine removal was much higher in previously exposed soils than in unexposed ones. Terminal restriction fragment length polymorphism analysis and multivariate analysis showed that soils previously exposed to simazine contained bacterial communities that were significantly impacted by simazine but also had an increased resilience. The biodegradation potential was also related to the presence of high levels of the atz-like gene sequences involved in simazine degradation. Bioaugmentation with Pseudomonas sp. ADP resulted in an increased initial rate of simazine removal, but this strain scarcely survived. After 28 days, residual simazine removals were the same in bioaugmented and not bioaugmented microcosms.

CONCLUSIONS

In soils with a history of simazine treatment bacterial communities were able to overcome subsequent impacts with the herbicide. The success of bioaugmentation was limited by the low survival of the introduced strain.

SIGNIFICANCE AND IMPACT OF THE STUDY

Conclusions from this work provided insights on simazine biodegradation potential of soils and the convenience of bioaugmentation.