Potential of preventive bioremediation to reduce environmental contamination by pesticides in an agricultural context: A case study with the herbicide 2,4-D

One of the major problems with pesticides is linked to the non-negligible proportion of the sprayed active ingredient that does not reach its intended target and contaminates environmental compartments. Here, we have implemented and provided new insights to the preventive bioremediation process based on the simultaneous application of the pesticide with pesticide-degrading microorganisms to reduce the risk of leaching into the environment. This study pioneers such a practice, in an actual farming context. The 2,4-dichlorophenoxyacetic acid herbicide (2,4-D) and one of its bacterial mineralizing-strains (Cupriavidus necator JMP134) were used as models. The 2,4-D biodegradation was studied in soil microcosms planted with sensitive (mustard) and insensitive (wheat) plants. Simultaneous application of a 2,4-D commercial formulation (DAM®) at agricultural recommended doses with 10⁵ cells.g^-1 dw of soil of the JMP134 strain considerably accelerated mineralization of the herbicide since its persistence was reduced threefold for soil supplemented with the mineralizing bacterium without reducing the herbicide efficiency. Furthermore, the inoculation of the Cupriavidus necator strain did not significantly affect the α- and β-diversity of the bacterial community. By tackling the contamination immediately at source, the preventive bioremediation process proves to be an effective and promising way to reduce environmental contamination by agricultural pesticides.

Environmental Concentrations of Sulfonamides Can Alter Bacterial Structure and Induce Diatom Deformities in Freshwater Biofilm Communities

Since the early 1920s, the intensive use of antibiotics has led to the contamination of the aquatic environment through diffuse sources and wastewater effluents. The antibiotics commonly found in surface waters include sulfamethoxazole (SMX) and sulfamethazine (SMZ), which belong to the class of sulfonamides, the oldest antibiotic class still in use. These antibiotics have been detected in all European surface waters with median concentrations of around 50 ng L^–1 and peak concentrations of up to 4–6 μg L^–1. Sulfonamides are known to inhibit bacterial growth by altering microbial production of folic acid, but sub-lethal doses may trigger antimicrobial resistance, with unknown consequences for exposed microbial communities. We investigated the effects of two environmentally relevant concentrations (500 and 5,000 ng L^–1) of SMZ and SMX on microbial activity and structure of periphytic biofilms in stream mesocosms for 28 days. Measurement of sulfonamides in the mesocosms revealed contamination levels of about half the nominal concentrations. Exposure to sulfonamides led to slight, transitory effects on heterotrophic functions, but persistent effects were observed on the bacterial structure. After 4 weeks of exposure, sulfonamides also altered the autotrophs in periphyton and particularly the diversity, viability and cell integrity of the diatom community. The higher concentration of SMX tested decreased both diversity (Shannon index) and evenness of the diatom community. Exposure to SMZ reduced diatom species richness and diversity. The mortality of diatoms in biofilms exposed to sulfonamides was twice that in non-exposed biofilms. SMZ also induced an increase in diatom teratologies from 1.1% in non-exposed biofilms up to 3% in biofilms exposed to SMZ. To our knowledge, this is the first report on the teratological effects of sulfonamides on diatoms within periphyton. The increase of both diatom growth rate and mortality suggests a high renewal of diatoms under sulfonamide exposure. In conclusion, our study shows that sulfonamides can alter microbial community structures and diversity at concentrations currently present in the environment, with unknown consequences for the ecosystem. The experimental set-up presented here emphasizes the interest of using natural communities to increase the ecological realism of ecotoxicological studies and to detect potential toxic effects on non-target species.

Design of a degenerate primer pair to target a bacterial functional community: The hppd bacterial gene coding for the enzyme targeted by herbicides, a study case

The present work aimed to design a degenerate primer pair to target a large part of the hppd soil bacterial community, possibly affected by herbicides. We validated these primers by qPCR and high-throughput sequencing analysis of soil samples.

Assessment of the ecotoxicological impact of natural and synthetic β-triketone herbicides on the diversity and activity of the soil bacterial community using omic approaches

The emergence of pesticides of natural origin appears as an environmental-friendly alternative to synthetic pesticides for managing weeds. To verify this assumption, leptospermone, a natural β-triketone herbicide, and sulcotrione, a synthetic one, were applied to soil microcosms at 0× (control), 1× or 10× recommended field dose. The fate of these two herbicides (i.e. dissipation and formation of transformation products) was monitored to assess the scenario of exposure of soil microorganisms to natural and synthetic herbicides. Ecotoxicological impact of both herbicides was explored by monitoring soil bacterial diversity and activity using next-generation sequencing of 16S rRNA gene amplicons and soil metabolomics. Both leptospermone and sulcotrione fully dissipated over the incubation period. During their dissipation, transformation products of natural and synthetic β-triketone were detected. Hydroxy-leptospermone was almost completely dissipated by the end of the experiment, while CMBA, the major metabolite of sulcotrione, remained in soil microcosms. After 8 days of exposure, the diversity and structure of the soil bacterial community treated with leptospermone was significantly modified, while less significant changes were observed for sulcotrione. For both herbicides, the diversity of the soil bacterial community was still not completely recovered by the end of the experiment (45 days). The combined use of next-generation sequencing and metabolomic approaches allowed us to assess the ecotoxicological impact of natural and synthetic pesticides on non-target soil microorganisms and to detect potential biomarkers of soil exposure to β-triketones.

The dissipation and microbial ecotoxicity of tebuconazole and its transformation products in soil under standard laboratory and simulated winter conditions

Tebuconazole (TBZ) is a widely used triazole fungicide at EU level on cereals and vines. It is relatively persistent in soil where it is transformed to various transformation products (TPs) which might be environmentally relevant. We assessed the dissipation of TBZ in soil under contrasting incubation conditions (standard vs winter simulated) that are relevant to its application scheme, determined its transformation pathway using advanced analytical tools and ¹⁴C-labeled TBZ and assessed its soil microbial toxicity. Mineralization of ¹⁴C-triazole-ring-labeled TBZ was negligible but up to 11% of ¹⁴C-penyl-ring-labeled TBZ evolved as ¹⁴CO₂ within 150 days of incubation. TBZ persistence increased at higher dose rates (×10 compared to the recommended agronomical dose ×1) and under winter simulated conditions compared to standard incubation conditions (at ×1 dose rate DT₅₀ of 202 and 88 days, respectively). Non-target suspect screening enabled the detection of 22 TPs of TBZ, among which 17 were unknown. Mass spectrometry analysis led to the identification of 1-(4-chlorophenyl) ethanone, a novel TP of TBZ, the formation of which and decay in soil was determined by gas chromatography mass spectrometry. Three hypothetical transformation pathways of TBZ, all converging to 1H-1,2,4-triazole are proposed based on suspect screening. The ecotoxicological effect of TBZ and of its TPs was assessed by measuring by qPCR the abundance of the total bacteria and the relative abundance of 11 prokaryotic taxa and 4 functional groups. A transient impact of TBZ on the relative abundance of all prokaryotic taxa (except α-proteobacteria and Bacteroidetes) and one functional microbial group (pcaH-carrying microorganisms) was observed. However the direction of the effect (positive or negative) varied, and in certain cases, depended on the incubation conditions. Proteobacteria was the most responsive phylum to TBZ with recovery observed 20 days after treatment. The ecotoxicological effects on the soil microorganisms were not correlated with 1-(4-chlorophenyl) ethanone.

Low impact of phenanthrene dissipation on the bacterial community in grassland soil

The effect of phenanthrene on the bacterial community was studied on permanent grassland soil historically presenting low contamination (i.e. less than 1 mg kg(-1)) by polycyclic aromatic hydrocarbons (PAHs). Microcosms of soil were spiked with phenanthrene at 300 mg kg(-1). After 30 days of incubation, the phenanthrene concentration decreased rapidly until its total dissipation within 90 days. During this incubation period, significant changes of the total bacterial community diversity were observed, as assessed by automated-ribosomal intergenic spacer analysis fingerprinting. In order to get a deeper view of the effect of phenanthrene on the bacterial community, the abundances of ten phyla and classes (Actinobacteria, Acidobacteria, Bacteroidetes, Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Firmicutes, Verrucomicrobiales, Gemmatimonadetes, and Planctomycetes) were monitored by quantitative polymerase chain reaction performed on soil DNA extracts. Interestingly, abundances of some bacterial taxa significantly changed as compared with controls. Moreover, among these bacterial groups impacted by phenanthrene spiking, some of them presented the potential of phenanthrene degradation, as assessed by PAH-ring hydroxylating dioxygenase (PAH-RHDα) gene detection. However, neither the abundance nor the diversity of the PAH-RHDα genes was significantly impacted by phenanthrene spiking, highlighting the low impact of this organic contaminant on the functional bacterial diversities in grassland soil.

Mapping field spatial distribution patterns of isoproturon-mineralizing activity over a three-year winter wheat/rape seed/barley rotation

The temporal and spatial variability of the activity of soil microorganisms able to mineralize the herbicide isoproturon (IPU) pesticide was investigated over a three-year long crop rotation between 2008 and 2010. Isoproturon mineralization was higher in 2008, when winter wheat was treated with this herbicide, than in 2009 and 2010, when rape seed and barley were treated with different herbicides. Under laboratory conditions, we showed that isoproturon mineralization was not promoted by sulfonylurea herbicide applied on barley crop in 2010. IPU mineralization was shown to be highly variable at the field scale in years 2009 and 2010. Principal component analyses and analyses of similarities revealed that soil pH and equivalent humidity, and to a lesser extent soil organic matter content and cation exchange capacity (CEC) were the main drivers of isoproturon-mineralizing activity variance. Using a rather simple model that yields the rate of isoproturon mineralization as a function of soil pH and equivalent humidity, we explained up to 85% of the variance observed. Mapping field-scale distribution of isoproturon mineralization over the three-year survey indicated higher variability in 2009 and in 2010 as compared to 2008, suggesting that isoproturon treatment applied to winter wheat promoted isoproturon mineralization activity and reduced its spatial variability. Field-scale distribution of isoproturon mineralization showed important similarity to the distribution of soil pH, equivalent humidity and to a lesser extent to soil organic matter and cation exchange capacity (CEC) thereby confirming our model.

Evidence for 2,4-D mineralisation in Mediterranean soils: impact of moisture content and temperature

BACKGROUND

The 2,4-D degradation ability of the microbiota of three arable Mediterranean soils was estimated. The impact of soil moisture and temperature on 2,4-D degradation was investigated.

RESULTS

The microbiota of the three soils regularly exposed to 2,4-D were able rapidly to mineralise this herbicide. The half-life of 2,4-D ranged from 8 to 30 days, and maximum mineralisation of (14)C-2,4-D ranged from 57 to 71%. Extractable (14)C-2,4-D and (14)C-bound residues accounted for less than 1 and 15% respectively of the (14)C-2,4-D initially added. The highest amounts of (14)C-2,4-D bound residues were recorded in the soil with the lowest 2,4-D-mineralising ability. Although all three soils were able to mineralise 2,4-D, multivariate analysis revealed that performance of this degrading microbial activity was dependent on clay content and magnesium oxide. Soil temperature affected the global structure of soil microbial community, but it had only a moderate effect on 2,4-D-mineralising ability. 2,4-D-mineralising ability was positively correlated with soil moisture content. Negligible 2,4-D mineralisation occurred in all three soils when incubated at 10 or 15% soil moisture content, i.e. within the range naturally occurring under the Mediterranean climate of Algeria.

CONCLUSION

This study shows that, although soil microbiota can adapt to rapid mineralisation of 2,4-D, this microbial activity is strongly dependent on climatic parameters. It suggests that only limited pesticide biodegradation occurs under Mediterranean climate, and that arable Mediterranean soils are therefore fragile and likely to accumulate pesticide residues.