Sulfate-Dependent Anaerobic Degradation of Herbicide Acetochlor by a Sulfate-Reducing Bacterium Cupidesulfovibrio sp. SRB-5

Enhanced Dechlorination of the Herbicide Acetochlor by an Anaerobic Consortium via Sulfate Acclimation

Acetochlor residues can contaminate anoxic habitats where anaerobic microbial transformation dominates. Herein, a highly efficient anaerobic acetochlor-degrading consortium ACT6 was enriched using sulfate and acetochlor as selection pressures. The acclimated consortium ACT6 showed an 8.7-fold increase in its ability to degrade acetochlor compared with the initial consortium ACT1. Two degradation pathways of acetochlor were found: reductive dechlorination and thiol-substitution dechlorination in the chloroacetyl group, in which the latter dominated. Acclimation enhanced the abundances of Desulfovibrio, Proteiniclasticum, and Lacrimispora from 0.7 to 28.0% (40-fold), 4.7 to 18.1% (4-fold), and 2.3 to 12.3% (5-fold), respectively, which were positively correlated with sulfate concentrations and acetochlor degradation ability. Three acetochlor-degrading anaerobes were isolated from the acclimated consortium ACT6, namely Cupidesulfovibrio sp. SRB-5, Proteiniclasticum sp. BAD-10, and Lacrimispora sp. BAD-7. This study provides new insights into the anaerobic catabolism of acetochlor and the anaerobic treatment of acetochlor in wastewater.

Regulatory roles of APS reductase in Citrobacter sp. XT1-2-2 as a response mechanism to cadmium immobilization in rice

Citrobacter sp. XT1-2-2, a functional microorganism with potential utilization, has the ability to immobilize soil cadmium. In this study, the regulatory gene cysH, as a rate-limiting enzyme in the sulfur metabolic pathway, was selected for functional analysis affecting cadmium immobilization in soil. To verify the effect of APS reductase on CdS formation, the ΔAPS and ΔAPS-com strains were constructed by conjugation transfer. Through TEM analysis, it was found that the adsorption of Cd2+ was affected by the absence of APS reductase in XT1-2-2 strain. The difference analysis of biofilm formation indicated that APS reductase was necessary for cell aggregation and biofilm formation. The p-XRD, XPS and FT-IR analysis revealed that APS reductase played an important role in the cadmium immobilization process of XT1-2-2 strain and promoting the formation of CdS. According to the pot experiments, the cadmium concentration of roots, culms, leaves and grains inoculated with ΔAPS strain was significantly higher than that of wild-type and ΔAPS-com strains, and the cadmium removal ability of ΔAPS strain was significantly lower than that of wild-type strain. The study provided insights into the exploration of new bacterial assisted technique for the remediation and safe production of rice in cadmium-contaminated paddy soils.

Novel Bifunctional Amidase Catalyzing the Degradation of Propanil and Aryloxyphenoxypropionate Herbicides in Rhodococcus sp. C-1

Propanil residues can contaminate habitats where microbial degradation is predominant. In this study, an efficient propanil-degrading strain C-1 was isolated from paddy and identified as Rhodococcus sp. It can completely degrade 10 μg/L-150 mg/L propanil within 0.33-10 h via the hydrolysis of the amide bond, forming 3,4-dichloroaniline. A novel bifunctional amidase, PamC, was identified in strain C-1. PamC can catalyze the hydrolysis of the amide bond of propanil to produce 3,4-dichloroaniline as well as the hydrolysis of the ester bonds of aryloxyphenoxypropionate herbicides (APPHs, clodinafop-propargyl, cyhalofop-butyl, fenoxaprop-p-ethyl, fluazifop-p-butyl, haloxyfop-p-methyl, and quizalofop-p-ethyl) to form aryloxyphenoxypropionic acids. Molecular docking and site-directed mutagenesis confirmed that the catalytic triad Lys82-Ser157-Ser181 was the active center for PamC to hydrolyze propanil and cyhalofop-butyl. This study presents a novel bifunctional amidase with capabilities for both amide and ester bond hydrolysis and enhances our understanding of the molecular mechanisms underlying the degradation of propanil and APPHs.

Current insights into environmental acetochlor toxicity and remediation strategies

Acetochlor is a selective pre-emergent herbicide that is widely used to control annual grass and broadleaf weeds. However, due to its stable chemical structure, only a small portion of acetochlor exerts herbicidal activity in agricultural applications, while most of the excess remains on the surfaces of plants or enters ecosystems, such as soil and water bodies, causing harm to the environment and human health. In recent years, researchers have become increasingly focused on the repair of acetochlor residues. Compared with traditional physical and chemical remediation methods, microorganisms are the most effective way to remediate chemical pesticide pollution, such as acetochlor, because of their rich species, wide distribution, and diverse metabolic pathways. To date, researchers have isolated and identified many high-efficiency acetochlor-degrading strains, such as Pseudomonas oleovorans, Klebsiella variicola, Bacillus subtilus, Rhodococcus, and Methylobacillus, among others. The microbial degradation pathways of acetochlor include dechlorination, hydroxylation, N-dealkylation, C-dealkylation, and dehydrogenation. In addition, the microbial enzymes, including hydrolase (ChlH), debutoxylase (Dbo), and monooxygenase (MeaXY), responsible for acetochlor biodegradation are also being investigated. In this paper, we review the migration law of acetochlor in the environment, its toxicity to nontarget organisms, and the main metabolic methods. Moreover, we summarize the latest progress in the research on the microbial catabolism of acetochlor, including the efficient degradation of microbial resources, biodegradation metabolic pathways, and key enzymes for acetochlor degradation. At the end of the article, we highlight the existing problems in the current research on acetochlor biodegradation, provide new ideas for the remediation of acetochlor pollution in the environment, and propose future research directions.

Using compound-specific isotope analysis to identify the mechanism of acetochlor degradation during oxygenation of hyporheic zone sediment

Biodegradation is recognized as the main pathway for acetochlor attenuation in aquatic environments. However, the potential abiotic degradation of acetochlor by hydroxyl radicals (•OH) generated during oxygenation of hyporheic zone sediments has not been investigated. This study aims to examine the production of •OH during oxygenation of hyporheic zone sediments and its effects on acetochlor attenuation. A significant decrease of acetochlor, ranging from 77.9% to 100%, was observed in the water-sediment systems with extensive •OH production. The primary sources of •OH production were found to be the oxidation of Fe(II) and reduced humic acids. Furthermore, a •OH quenching experiment suggests that •OH driven oxidation is the dominant pathway for acetochlor attenuation. Carbon isotope fractionation of acetochlor degradation during oxygenation of sediments (εbulk,C ranged from -1.5‰ to -0.5 ± 0.3‰) was close to that during acetochlor degradation by •OH in a H2O2-Fe3O4 Fenton system (εbulk,C = -0.5 ± 0.1‰), but significantly smaller than that during acetochlor biodegradation (εbulk,C = -5.8 ± 0.9‰). Compound-specific isotope analysis (CSIA) further suggests that •OH produced by sediment oxygenation plays a critical role in acetochlor attenuation in aquatic environments. Results of calculated apparent kinetic isotope effect of carbon (AKIEC) and transformation products indicate that SN1 and SN2-type nucleophilic substitution are the first steps in acetochlor attenuation through •OH driven oxidation (AKIEC = 1.007 ± 0.001) and aerobic biodegradation (AKIEC = 1.088 ± 0.013), respectively. Our findings highlight the potential of CSIA to assess the acetochlor degradation in water-sediment system, which can help to elucidate the fate of herbicide in aquatic environments.

Effect of alkaline leaching of phosphogypsum on sulfate reduction activity and bacterial community composition using different sources of anaerobic microbial inoculum

Phosphogypsum (PG), a by-product of the phosphate industry, is high in sulfate, (SO42-), which makes it an excellent substrate for sulfate-reducing bacteria (SRB) to produce hydrogen sulfide. This work aimed to optimize SO42- leaching from PG to achieve a high biological reduction of SO42- and generate high sulfide concentrations for subsequent use in the biological recovery of elemental sulfur. Five SRB consortia were isolated and enriched from: IS (Industrial sludges), MS (Marine sediments), WC (Winogradsky column), SNV (petroleum industry sediments) and PG (stored Phosphogypsum). The five consortia showed reduction activity when using PG leachate (with water) as source of SO42- and lactate, acetate, or glucose as the electron donor. The highest reduction rate (81.5 %) was registered using lactate and the IS consortium (81.5 %) followed by MS (79 %) and PG (71 %). To enhance the concentration of leached SO42- from PG for future utilization with the isolated consortia, PG was treated with NaOH solutions (2 % and 5 %). SO42- release of 97 % was achieved with a 5 % concentration and the resulting leachate was further diluted to target a SO42- concentration of 12.4 g·L-1 for utilization with the isolated consortia. Compared to water leachate, a significantly higher reduction rate was registered (2 g·L-1 of SO42) using the IS consortium, demonstrating limited inhibition effect of sulfide- concentration on SRB functionalities. Moreover, metagenomic analysis of the consortia revealed that using PG as a source of SO42- increased the abundance of Deltaproteobacteria, including known SRB like Desulfovibrio, Desulfomicrobium, and Desulfosporosinus, as well as novel SRB genera (Cupidesulfovibrio, Desulfocurvus, Desulfococcus) that showed, for the first time, significant potential as novel sulfate-reducers using PG as a SO42- source.

Insights into the metabolic pathways and biodegradation mechanisms of chloroacetamide herbicides

Chloroacetamide herbicides are widely used around the world due to their high efficiency, resulting in increasing levels of their residues in the environment. Residual chloroacetamides and their metabolites have been frequently detected in soil, water and organisms and shown to have toxic effects on non-target organisms, posing a serious threat to the ecosystem. As such, rapid and efficient techniques that eliminate chloroacetamide residues from the ecosystem are urgently needed. Degradation of these herbicides in the environment mainly occurs through microbial metabolism. Microbial strains such as Acinetobacter baumannii DT, Bacillus altitudinis A16, Pseudomonas aeruginosa JD115, Sphingobium baderi DE-13, Catellibacterium caeni DCA-1, Stenotrophomonas acidaminiphila JS-1, Klebsiella variicola B2, and Paecilomyces marquandii can effectively degrade chloroacetamide herbicides. The degradation pathway of chloroacetamide herbicides in aerobic bacteria is mainly initiated by an N/C-dealkylation reaction, followed by aromatic ring hydroxylation and cleavage processes, whereas dechlorination is the initial reaction in anaerobic bacteria. The molecular mechanisms associated with bacterial degradation of chloroacetamide herbicides have been explored, with amidase, hydrolase, reductase, ferredoxin and cytochrome P450 oxygenase currently known to play a pivotal role in the catabolic pathways of chloroacetamides. The fungal pathway for the degradation of these herbicides is more complex with more diversified products, and the degradation enzymes and genes involved remain to be discovered. However, there are few reviews specifically summarizing the microbial degrading species and biochemical mechanisms of chloroacetamide herbicides. Here, we briefly summarize the latest progress resulting from research on microbial strain resources and enzymes involved in degradation of these herbicides and their corresponding genes. Furthermore, we explore the biochemical pathways and molecular mechanisms for biodegradation of chloroacetamide herbicides in depth, thereby providing a reference for further research on the bioremediation of such herbicides.