Behavior of the Chiral Herbicide Imazamox in Soils: Enantiomer Composition Differentiates between Biodegradation and Photodegradation

Synthesizing Laboratory and Field Experiments to Quantify Dominant Transformation Mechanisms of 2,4-Dichlorophenoxyacetic Acid (2,4-D) in Aquatic Environments

Laboratory studies used to assess the environmental fate of organic chemicals such as pesticides fail to replicate environmental conditions, resulting in large errors in predicted transformation rates. We combine laboratory and field data to identify the dominant loss processes of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in lakes for the first time. Microbial and photochemical degradation are individually assessed using laboratory-based microcosms and irradiation studies, respectively. Field campaigns are conducted in six lakes to quantify 2,4-D loss following large-scale herbicide treatments. Irradiation studies show that 2,4-D undergoes direct photodegradation, but modeling efforts demonstrated that this process is negligible under environmental conditions. Microcosms constructed using field inocula show that sediment microbial communities are responsible for degradation of 2,4-D in lakes. Attempts to quantify transformation products are unsuccessful in both laboratory and field studies, suggesting that their persistence is not a major concern. The synthesis of laboratory and field experiments is used to demonstrate best practices in designing laboratory persistence studies and in using those results to mechanistically predict contaminant fate in complex aquatic environments.

Indigenous functional microbial communities for the preferential degradation of chloroacetamide herbicide S-enantiomers in soil

This study investigated indigenous functional microbial communities associated with the degradation of chloroacetamide herbicides acetochlor (ACE), S-metolachlor (S-MET) and their enantiomers in repeatedly treated soils. The results showed that biodegradation was the main process for the degradation of ACE, S-MET and their enantiomers. Eight dominant bacterial genera associated with the degradation were found: Amycolatopsis, Saccharomonospora, Mycoplasma, Myroides, Mycobacterium, Burkholderia, Afipia, and Kribbella. The S-enantiomers of ACE and S-MET were preferentially degraded, which mainly relied on Amycolatopsis, Saccharomonospora and Kribbella for the ACE S-enantiomer and Amycolatopsis and Saccharomonospora for the S-MET S-enantiomer. Importantly, the relative abundances of Amycolatopsis and Saccharomonospora increased by 146.3%-4467.2% in the S-enantiomer treatments of ACE and S-MET compared with the control, which were significantly higher than that in the corresponding R-enantiomer treatments (25.3%-4168.2%). Both metagenomic and qPCR analyses demonstrated that four genes, ppah, alkb, benA, and p450, were the dominant biodegradation genes (BDGs) potentially involved in the preferential degradation of the S-enantiomers of ACE and S-MET. Furthermore, network analysis suggested that Amycolatopsis, Saccharomonospora, Mycoplasma, Myroides, and Mycobacterium were the potential hosts of these four BDGs. Our findings indicated that Amycolatopsis and Saccharomonospora might play pivotal roles in the preferential degradation of the S-enantiomers of ACE and S-MET.

Evaluation of exploitive potential for higher bioactivity and lower residue risk enantiomer of chiral fungicide pydiflumetofen

BACKGROUND

Pydiflumetofen, as a new succinate dehydrogenase inhibitor (SDHI) chiral fungicide, has been used in crop production because of its broad-spectrum and high-efficiency antifungal activity. However, little is known about pydiflumetofen at the chiral level. The stereoselective bioactivity and degradation of pydiflumetofen enantiomers were therefore investigated.

RESULTS

Pydiflumetofen presented effective bioactivity against the eight tested phytopathogens, and its enantiomers showed significant differences in activity. The bioactivity of R-pydiflumetofen was 9.0-958.8 times higher than that of the S enantiomer. Treatment with R-pydiflumetofen increased the cell membrane permeability of Sclerotinia sclerotiorum and decreased exopolysaccharide and oxalic acid production more than treatment with S-pydiflumetofen. Furthermore, R-pydflumetofen exhibited better inhibitory activity against the succinate dehydrogenase enzyme of S. sclerotiorum than S-pydiflumetofen by 584-fold. According to homology modeling and molecular docking studies, the binding affinities of the R and S enantiomers were -7.0 and -5.3 kcal mol^-1 , respectively. Additionally, the degradation half-lives of S- and R-pydiflumetofen in three vegetables (cucumber, eggplant, and cowpea) under field conditions were 2.56-3.12 days and 2.48-2.76 days, respectively, which reveals that R-pydiflumetofen degrades faster than S-pydiflumetofen.

CONCLUSION

Based on the results obtained, R-pydiflumetofen not only exhibited a higher bactericidal activity, but also posed fewer residual risks in the environment. The mechanism of the stereoselective bioactivity was correlated with the stereoselective inhibition activity of the target enzyme and affected the cell membrane permeability and the production of exopolysaccharide and oxalic acid. This research could provide a foundation for the systematic evaluation of pydiflumetofen from an enantiomeric view. © 2021 Society of Chemical Industry.

Impact of long-term irrigation with municipal reclaimed wastewater on the uptake and degradation of organic contaminants in lettuce and leek

A two years drip irrigation of lettuce and leek crops with treated municipal wastewater without and with spiking with fourteen wastewater relevant contaminants at 10 μg/L concentration level was conducted under greenhouse cultivation conditions to investigate their potential accumulation in soil and leaves and to assess human health related risks. Lettuce and leek crops were selected as a worse-case scenario since leafy green vegetable has a high potential for organic contaminants uptake. The results revealed limited accumulation of contaminants in soil and plant leaves, their concentration levels being in the range of 1-30 ng/g and 1-660 ng/g range in soil and leaves, respectively. This was likely related to abiotic and biotic transformation or simply binding processes in soil, which limited contaminants plant uptake. This assumption was underpinned by studies of the enantiomeric fractionation of chiral compounds (e.g. climbazole and metoprolol) in soil as pieces of evidence of biodegradation and by the identification of transformation products or metabolites in leaves by means of liquid chromatography - high resolution - mass spectrometry using a suspect screening workflow. The high bioconcentration factors were not limited to compounds with intermediate Dow (100 to 1000) such as carbamazepine but also observed for hydrophilic compounds such as clarithromycin, hydrochlorothiazide and the food additives acesulfame and sucralose. This result assumed that accumulation was not only driven by passive processes (e.g. lipoidal diffusion through lipid bilayer cell membranes or Casparian strip) but might be supported by carrier-mediated transporters. As a whole, this study confirmed earlier reports on the a de minimis human health risk related to the consumption of raw leafy green vegetable irrigated with domestic TWW containing organic contaminants residues.

The enantioselective effects and potential risks of paclobutrazol residue during cucumber pickling process

For decades, pesticides have been widely used for controlling pests and protecting crops around the world, and the food safety issues caused by these compounds have raised widespread concern. However, the different enantioselective behaviors and biological activities of chiral pesticide enantiomers are often ignored. In this work, a novel method was put forward to investigate the enantioselective effects and potential risks of two paclobutrazol enantiomers during cucumber pickling process. The degradation kinetics indicated that when paclobutrazol reside in cucumber and was introduced into the pickling process, the half-time of paclobutrazol isomers were significantly different (9.24 d and 16.6 d), and the conversion phenomenon between the two enantiomers could also be observed. In addition, results from 16S rRNA and ITS sequencing shown that (2R, 3R)-paclobutrazol and (2S, 3S)-paclobutrazol have an obviously enantiomeric effects on microbial community of pickling system and the degradation of paclobutrazol was probably attributed to the presence of Pseudomonas and Serratia. Finally, the microorganisms functions were found to be disrupted under the exposure of (2R, 3R)-paclobutrazol and metabolic function of microorganisms to xenobiotic was inhibited, which might cause potential risks to the quality of preserved foods. In summary, we have devised a method and provided a novel insight into the potential risks of chiral pesticide residues on food safety and human health.

Behavior of imidazolinone herbicide enantiomers in earthworm-soil microcosms: Degradation and bioaccumulation

Imidazolinone herbicides are a group of chiral herbicides that are widely used to control weeds in crops. Despite their wide use, few studies on the behavior of enantiomers in terrestrial systems have been reported. In this study, the bioaccumulation of imazamox, imazapic, and imazethapyr enantiomers in earthworm and their degradation in soils were assessed using earthworm-soil microcosms. The bioaccumulation of the three herbicides in earthworm was not significantly enantioselective. Imazamox and imazethapyr did not significant stereoselective degradation in soil (p > 0.05), while the enantioselectivity of the degradation of imazapic was significant (p < 0.05). Furthermore, biota to soil accumulation factor (BSAF) values were also calculated for three herbicides. Relationships between BSAF values and organic matter content of soil and log K_OW of herbicides were investigated. The BSAFs values were negatively correlated with the log K_OW of herbicides, and were positively correlated with organic matter content of soil in earthworm-soil microcosms. These relationships indicated that chemical hydrophobicity (K_ow) and organic matter content of soil were good predictors to estimate the bioavailability of imidazolinone herbicides to earthworm.

Development of S-Fluxametamide for Bioactivity Improvement and Risk Reduction: Systemic Evaluation of the Novel Insecticide Fluxametamide at the Enantiomeric Level

Increasing numbers of novel pesticides have been applied in agriculture. However, traditional evaluation of pesticides does not distinguish between their enantiomers, which may lead to inaccurate results. In this study, systematic research on the chiral insecticide fluxametamide was conducted at the enantiomeric level. The methods for enantioseparation and semipreparative separation of fluxametamide enantiomers were developed. The optical rotation and absolute configuration of two enantiomers were determined, and their stability was verified in solvents and soils. Enantioselective bioactivities against four target pests (Plutella xylostella, Spodoptera exigua, Aphis gossypii, and Tetranychus cinnabarinus) were tested. Acute toxicities of fluxametamide enantiomers toward honeybees were also evaluated. S-(+)-Isomer exhibited 52.1-304.4 times and 2.5-3.7 times higher bioactivity than R-(-)-isomer and rac-fluxametamide, respectively. Meanwhile, rac-fluxametamide was more toxic than S/R-isomer, and S-(+)-isomer showed >30-fold higher acute toxicity than R-(-)-isomer. Molecular docking studies were performed with γ-aminobutyric acid receptor (GABAR) to monitor the mechanism of stereoselective bioactivity. The better Grid score of S-(+)-fluxametamide (-60.12 kcal/mol) than R-(-)-enantiomer (-56.59 kcal/mol) indicated higher bioactivity of S-(+)-isomer than of R-(-)-isomer. The dissipation of fluxametamide in cabbage, Chinese cabbage, and soil was nonenantioselective under field conditions. Development of S-(+)-fluxametamide could maintain the high-efficacy and low-risk properties, which should attract attention of producers, applicators, and managers of pesticides.