Nora Badawi

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.

Fine scale spatial variability of microbial pesticide degradation in soil: scales, controlling factors, and implications

Pesticide biodegradation is a soil microbial function of critical importance for modern agriculture and its environmental impact. While it was once assumed that this activity was homogeneously distributed at the field scale, mounting evidence indicates that this is rarely the case. Here, we critically examine the literature on spatial variability of pesticide biodegradation in agricultural soil. We discuss the motivations, methods, and main findings of the primary literature. We found significant diversity in the approaches used to describe and quantify spatial heterogeneity, which complicates inter-studies comparisons. However, it is clear that the presence and activity of pesticide degraders is often highly spatially variable with coefficients of variation often exceeding 50% and frequently displays non-random spatial patterns. A few controlling factors have tentatively been identified across pesticide classes: they include some soil characteristics (pH) and some agricultural management practices (pesticide application, tillage), while other potential controlling factors have more conflicting effects depending on the site or the pesticide. Evidence demonstrating the importance of spatial heterogeneity on the fate of pesticides in soil has been difficult to obtain but modeling and experimental systems that do not include soils full complexity reveal that this heterogeneity must be considered to improve prediction of pesticide biodegradation rates or of leaching risks. Overall, studying the spatial heterogeneity of pesticide biodegradation is a relatively new field at the interface of agronomy, microbial ecology, and geosciences and a wealth of novel data is being collected from these different disciplinary perspectives. We make suggestions on possible avenues to take full advantage of these investigations for a better understanding and prediction of the fate of pesticides in soil.

Centimeter-Scale Spatial Variability in 2-Methyl-4-Chlorophenoxyacetic Acid Mineralization Increases with Depth in Agricultural Soil

Mineralization of organic chemicals in soil is typically studied using large homogenized samples, but little is known about the small-scale spatial distribution of mineralization potential. We studied centimeter-scale spatial distribution of 2-methyl-4-chlorophenoxyacetic acid (MCPA) mineralization activity at different depths (8-115 cm) in a Danish agricultural soil profile using a 96-well microplate C-radiorespirometric method for small-volume samples. The heterotrophic microbial population and specific MCPA degraders decreased 10- to 100-fold from the plow layer to a depth of 115 cm. MCPA was mineralized in all samples in the plow layer, but only about 60% in the transition zone immediately below the plow layer showed mineralization; at greater depth even fewer samples showed mineralization. A patchy spatial distribution of mineralization activity was observed from right below the plow layer and in the subsoil, with a few clearly defined active zones surrounded by areas devoid of mineralization activity. Due to the patchy distribution of mineralization activity at the centimeter scale just beneath the plow layer, MCPA and presumably other weakly sorbing pesticides might be at risk of leaching to the groundwater if transported from the plow layer into the subsoil.

The gompertz function can coherently describe microbial mineralization of growth-sustaining pesticides.

Mineralization of (14)C-labeled tracers is a common way of studying the environmental fate of xenobiotics, but it can be difficult to extract relevant kinetic parameters from such experiments since complex kinetic functions or several kinetic functions may be needed to adequately describe large data sets. In this study, we suggest using a two-parameter, sigmoid Gompertz function for parametrizing mineralization curves. The function was applied to a data set of 252 normalized mineralization curves that represented the potential for degradation of the herbicide MCPA in three horizons of an agricultural soil. The Gompertz function fitted most of the normalized curves, and trends in the data set could be visualized by a scatter plot of the two Gompertz parameters (rate constant and time delay). For agricultural topsoil, we also tested the effect of the MCPA concentration on the mineralization kinetics. Reduced initial concentrations lead to shortened lag-phases, probably due to reduced need for bacterial growth. The effect of substrate concentration could be predicted by simply changing the time delay of the Gompertz curves. This delay could to some extent also simulate concentration effects for 2,4-D mineralization in agricultural soil and aquifer sediment and 2,6-dichlorobenzamide mineralization in single-species, mineral medium.

Degradation of three benzonitrile herbicides by Aminobacter MSH1 versus soil microbial communities: pathways and kinetics

BACKGROUND The herbicide dichlobenil was banned in the European Union after its metabolite 2,6-dichlorobenzamide (BAM) was encountered in groundwater. Owing to structural similarities, bromoxynil and ioxynil might be converted to persistent metabolites in a similar manner. To examine this, we used an indigenous soil bacterium Aminobacter sp. MSH1 which is capable of mineralizing dichlobenil via BAM and 2,6-dichlorobenzoic acid (2,6-DCBA). RESULTS Strain MSH1 converted bromoxynil and ioxynil to the corresponding aromatic metabolites, 3,5-dibromo-4-hydroxybenzoic acid (BrAC) and 3,5-diiodo-4-hydroxybenzoic acid (IAC) following Michaelis-Menten kinetics (adjusted R(2) between 0.907 and 0.999). However, in contrast to 2,6-DCBA, degradation of these metabolites was not detected in the pure-culture studies, suggesting that they might pose an environmental risk if similar partial degradation occurred in soil. By contrast, experiments with natural soils indicated 20-30% mineralization of ioxynil and bromoxynil within the first week. CONCLUSION The degradation pathway of the three benzonitriles is initially driven by similar enzymes, after which more specific enzymes are responsible for further degradation. Ioxynil and bromoxynil mineralization in soil is not dependent on previous benzonitrile exposure. The accumulation of dead-end metabolites, as seen for dichlobenil, is not a major problem.

Environmental Fate of the Herbicide Fluazifop-P-butyl and Its Degradation Products in Two Loamy Agricultural Soils: A Combined Laboratory and Field Study

The herbicide fluazifop-P-butyl (FPB) is used against grasses in agricultural crops such as potato, oilseed rape, and sugar beet. Limited information is available in scientific literature on its environmental fate, therefore extensive monitoring at two agricultural test fields was combined with laboratory studies to determine leaching and the underlying degradation and sorption processes. Water samples from drains, suction cups, and groundwater wells showed leaching of the degradation products fluazifop-P (FP) and 2-hydroxy-5-trifluoromethyl-pyridin (TFMP) following FPB treatment. Laboratory experiments with soil from each field revealed a rapid degradation of FPB to FP. The degradation was almost exclusively microbial, and further biodegradation to TFMP occurred at a slower rate. Both degradation products were sorbed to the two soils to a small extent and were fairly persistent to degradation during the two-month incubation period. Together, the field and laboratory results from this study showed that the biodegradation of FPB in loamy soils gave rise to the production of two major degradation products that sorbed to a small extent. In this study, both degradation products leached to drainage and groundwater during precipitation. It is therefore recommended that these degradation products be included in programs monitoring water quality in areas with FPB use.

Degradation and sorption of the fungicide tebuconazole in soils from golf greens

The fungicide tebuconazole (TBZ) is used to repress fungal growth in golf greens and ensure their playability. This study determined the degradation and sorption of TBZ applied as an analytical grade compound, a commercial fungicide formulation or in combination with a surfactant product in thatch and soils below two types of greens (USGA and push-up greens) in 12-cm vertical profiles covered by three different types of turf grass. Only minor TBZ degradation was observed and it was most pronounced in treatments with the commercial fungicide product or in combination with the surfactant compared to the analytical grade compound alone. A tendency for higher TBZ sorption when applied as the formulated product and lowest sorption when applied as a formulated product in combination with the surfactant was observed, with this effect being most distinct on USGA greens. No correlation between occurrence of degradation and soil depth, green type or grass type was observed. Sorption seemed to be the main process governing the leaching risk of TBZ from the greens and a positive correlation to the organic matter content was shown. In light of these findings, organic matter content should be taken into consideration during the construction of golf courses, especially when following USGA guidelines.

Novel Method Reveals a Narrow Phylogenetic Distribution of Bacterial Dispersers in Environmental Communities Exposed to Low-Hydration Conditions

ABSTRACT In this study, we developed a method that provides profiles of community-level surface dispersal from environmental samples under controlled hydration conditions and enables us to isolate and uncover the diversity of the fastest bacterial dispersers. The method expands on the porous surface model (PSM), previously used to monitor the dispersal of individual bacterial strains in liquid films at the surface of a porous ceramic disc. The novel procedure targets complex communities and captures the dispersed bacteria on a solid medium for growth and detection. The method was first validated by distinguishing motile Pseudomonas putida and Flavobacterium johnsoniae strains from their nonmotile mutants. Applying the method to soil and lake water bacterial communities showed that community-scale dispersal declined as conditions became drier. However, for both communities, dispersal was detected even under low-hydration conditions (matric potential, −3.1 kPa) previously proven too dry for P. putida strain KT2440 motility. We were then able to specifically recover and characterize the fastest dispersers from the inoculated communities. For both soil and lake samples, 16S rRNA gene amplicon sequencing revealed that the fastest dispersers were substantially less diverse than the total communities. The dispersing fraction of the soil microbial community was dominated by Pseudomonas species cells, which increased in abundance under low-hydration conditions, while the dispersing fraction of the lake community was dominated by Aeromonas species cells and, under wet conditions (−0.5 kPa), also by Exiguobacterium species cells. The results gained in this study bring us a step closer to assessing the dispersal ability within complex communities under environmentally relevant conditions. IMPORTANCE Dispersal is a key process of bacterial community assembly, and yet, very few attempts have been made to assess bacterial dispersal at the community level, as the focus has previously been on pure-culture studies. A crucial factor for dispersal in habitats where hydration conditions vary, such as soils, is the thickness of the liquid films surrounding solid surfaces, but little is known about how the ability to disperse in such films varies within bacterial communities. Therefore, we developed a method to profile community dispersal and identify fast dispersers on a rough surface resembling soil surfaces. Our results suggest that within the motile fraction of a bacterial community, only a minority of the bacterial types are able to disperse in the thinnest liquid films. During dry periods, these efficient dispersers can gain a significant fitness advantage through their ability to colonize new habitats ahead of the rest of the community.