Anaerobic degradation of chloroacetanilide herbicides

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

Acetochlor, an important chloroacetamide herbicide (CAAH) widely used in agriculture, has resulted in environmental contamination, especially of anoxic habitats. In this study, a sulfate-reducing bacterium, designated as SRB-5, was isolated from anaerobic activated sludge and was identified as Cupidesulfovibrio sp. This bacterium possesses a novel anaerobic pathway capable of degrading acetochlor. In this pathway, sulfate is first reduced to sulfide, which attacks the C-Cl bond of acetochlor and abiotically forms acetochlor-thioalcohol and dis-S-acetochlor. These further undergo microbial degradation, producing the intermediates acetochlor ethanesulfonic acid, 2-methyl-6-ethylaniline, and 2-ethylaniline. The degradation half-times of acetochlor (100 μM) by strain SRB-5 were 2.4 and 4.2 days in industrial wastewater and paddy sludge, respectively. Strain SRB-5 could also degrade alachlor, propisochlor, butachlor, pretilachlor, and metolachlor, and the degradation kinetics fit the pseudo-first-order kinetics equation. This work highlights the potential application of strain SRB-5 for the remediation of CAAHs-contaminated sites.

Anaerobic biodegradation and detoxification of chloroacetamide herbicides by a novel Proteiniclasticum sediminis BAD-10T

Chloroacetamide herbicides (CAAHs) are important herbicides that were widely used to control agricultural weeds. However, their mass applications have seriously contaminated environment, and they are toxic to living beings. CAAHs are easy to enter anoxic environments such as subsoil, wetland sediment, and groundwater, where CAAHs are mainly degraded by anaerobic organisms. To date, there are no research on the anaerobic degradation of CAAHs by pure isolate and toxicity of anaerobic metabolites of CAAHs. In this study, the anaerobic degradation kinetics and metabolites of CAAHs by an anaerobic isolate BAD-10^T and the toxicity of anaerobic metabolites were studied. Isolate BAD-10^T could degrade alachlor, acetochlor, propisochlor, butachlor, pretilachlor and metolachlor with the degradation kinetics fitting the pseudo-first-order kinetics equation. The degradation rates of CAAHs were significantly affected by the length of N-alkoxyalkyl groups, the shorter the N-alkoxyalkyl groups, the higher the degradation rates. Four metabolites 2-ethyl-6-methyl-N-(ethoxymethyl)-acetanilide (EMEMA), N-(2-methyl-6-ethylphenyl)-acetamide (MEPA), N-2-ethylphenyl acetamide and 2-ethyl-N-carboxyl aniline were identified during acetochlor degradation, and an anaerobic catabolic pathway of acetochlor was proposed. The toxicity of EMEMA and EMPA for zebrafish, Arabidopsis and Chlorella ellipsoidea were obviously lower than that of acetochlor, indicating that the anaerobic degradation of acetochlor by isolate BAD-10^T is a detoxification process. The work reveals the anaerobic degradation kinetics and catabolic pathway of CAAHs and highlights a potential application of Proteiniclasticum sediminis BAD-10^T for bioremediation of CAAHs residue-contaminated environment.

Anaerobic biodegradation of acetochlor by acclimated sludge and its anaerobic catabolic pathway

Acetochlor is a chloroacetamide herbicide that has been widely used for weed control in recent decades. The contamination from its residue in the environment has raised major serious concerns. The aerobic degradation of acetochlor has been well studied; however, little is known regarding its anaerobic degradation. In the study, anaerobic sludge with high acetochlor degradation efficiency was obtained by pressure acclimation in a continuous flow anaerobic reactor. The acetochlor degradation dynamics followed a first-order kinetic reaction equation. The acclimated sludge could degrade six chloroacetamide herbicides with the degradation efficiencies observed as alachlor > acetochlor > propisochlor > butachlor > pretilachlor > metolachlor, and the N-alkoxyalkyl structure of these herbicides significantly affected their biodegradability. Five metabolites, 2-ethyl-6-methyl-N-(ethoxymethyl)-acetanilide, N-(2-methyl-6-ethylphenyl) acetamide, N-2-ethylphenyl acetamide, N-2-ethylphenyl formamide and 2-ethyl-N-carboxyl aniline were identified, and a putative anaerobic acetochlor degradation pathway, initiated by dechlorination, was subsequently proposed. During acclimation, the community diversity of both eubacteria and archaea in the anaerobic sludge decreased, while the abundance of microbes belonging to genera Sporomusa, Sporobacterium, Dechloromonas, Azotobacter and Methanobacterium were significantly increased and dominated the acclimated sludge, and showing a positive correlation with the acetochlor degradation capacity. These findings should be valuable to elucidate the mechanisms associated with the anaerobic catabolism of acetochlor and facilitate the engineering application of anaerobic treatment for removing acetochlor from wastewater.

Using compound-specific isotope analysis to assess the degradation of chloroacetanilide herbicides in lab-scale wetlands

Compound-specific isotope analysis (CSIA) is a promising tool to study the environmental fate of a wide range of contaminants including pesticides. In this study, a novel CSIA method was developed to analyse the stable carbon isotope signatures of widely used chloroacetanilide herbicides. The developed method was applied in combination with herbicide concentration and hydrochemical analyses to investigate in situ biodegradation of metolachlor, acetochlor and alachlor during their transport in lab-scale wetlands. Two distinct redox zones were identified in the wetlands. Oxic conditions prevailed close to the inlet of the four wetlands (oxygen concentration of 212±24μM), and anoxic conditions (oxygen concentrations of 28±41μM) prevailed towards the outlet, where dissipation of herbicides mainly occurred. Removal of acetochlor and alachlor from inlet to outlet of wetlands was 56% and 51%, whereas metolachlor was more persistent (23% of load dissipation). CSIA of chloroacetanilides at the inlet and outlet of the wetlands revealed carbon isotope fractionation of alachlor (εbulk=-2.0±0.3‰) and acetochlor (εbulk=-3.4±0.5‰), indicating that biodegradation contributes to the dissipation of both herbicides. This study is a first step towards the application of CSIA to evaluate the transport and degradation of chloroacetanilide herbicides in the environment.

Removal of alachlor in anoxic soil slurries and related alteration of the active communities

Despite the implication of anaerobic soil communities in important functions related to C and N biogeochemical cycles, their responses to pesticides are rarely assessed. This study focused on the impact of alachlor, a chloroacetanilide herbicide, on two agricultural soils differing in their land use (fallow and corn-cultivated) in order to investigate the potential adaptation of anaerobic or facultative anaerobic soil microorganisms from fields with long history of herbicide use. The experiment was performed by developing slurries in anoxic conditions over 47 days. Changes in the community structure assessed through terminal restriction fragment length polymorphism analysis of 16S rRNA genes clearly showed a shift in the bacterial community of the cultivated soil, whereas the modification of the microbial community of the fallow soil was delayed. In addition, the analysis of alachlor degradation capacities of the two anaerobic communities indicated that 99 % of alachlor was removed in anoxic slurries of cultivated soil. Both these results suggested the preexistence of microorganisms in the cultivated soil able to respond promptly to the pesticide exposure. The composition of the anaerobic active community determined by 16S rRNA transcript analysis was mainly composed of strictly anaerobic Clostridia and the facultative anaerobe Bacilli classes. Some genera, described for their role in herbicide biodegradation were active in alachlor-treated slurries, whereas others were no longer detected. Finally, this study highlights, when triggered, the important diversity of the anaerobic community in soil.

Enzymatic vegetable organic extracts as soil biochemical biostimulants and atrazine extenders

The purpose of this study was to gather information on the potential effects of organic biostimulants on soil activity and atrazine biodegradation. Carob germ enzymatic extract (CGEE) and wheat condensed distiller solubles enzymatic extract (WCDS-EE) have been obtained using an enzymatic process; their main organic components are soluble carbohydrates and proteins in the form of peptides and free amino acids. Their application to soil results in high biostimulation, rapidly increased dehydrogenase, phosphatase and glucosidase activities, and an observed atrazine extender capacity due to inhibition of its mineralization. The extender capacity of both extracts is proportional to the protein/carbohydrate ratio content. As a result, these enzymatic extracts are highly microbially available, leading to two independent phenomena, fertility and an atrazine persistence that is linked to increased soil activity.

Anaerobic degradation of atrazine and metolachlor and metabolite formation in wetland soil and water microcosms

The half-lives, degradation rates, and metabolite formation patterns of atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] were determined in an anaerobic wetland soil incubated at 24 degrees C for 112 d. At 0, 7, 14, 28, 42, 56, and 112 d, the soil and water were analyzed for atrazine and metolachlor, and their major metabolites. The soil oxidation-reduction potential reached -200 mV after 14 d. Degradation reaction rates were first-order for atrazine in anaerobic soil and for metolachlor in the aqueous phase. Zero-order reaction rates were best fit for atrazine in the aqueous phase and metolachlor in anaerobic soil. In anaerobic soil, the half-life was 38 d for atrazine and 62 d for metolachlor. In the aqueous phase above the soil, the half-life was 86 d for atrazine and 40 d for metolachlor. Metabolites detected in the anaerobic soil were hydroxyatrazine and deethylatrazine for atrazine, and relatively small amounts of ethanesulfonic acid and oxanilic acid for metolachlor. Metabolites detected in the aqueous phase above the soil were hydroxyatrazine, deethylatrazine, and deisopropylatrazine for atrazine, and ethanesulfonic acid and oxanilic acid for metolachlor. Concentrations of metabolites in the aqueous phase generally peaked within the first 25 d and then declined. Results indicate that atrazine and metolachlor can degrade under strongly reducing conditions found in wetland soils. Metolachlor metabolites, ethanesulfonic acid, and oxanilic acid are not significantly formed under anaerobic conditions.

Biodegradation of the acetanilide herbicides alachlor, metolachlor, and propachlor

Alachlor, metolachlor, and propachlor are detoxified in biological systems by the formation of glutathione-acetanilide conjugates. This conjugation is mediated by glutathione-S-transferase, which is present in microorganisms, plants, and mammals. Other organic sulfides and inorganic sulfide also react through a nucleophilic attack on the 2-chloro group of acetanilide herbicides, but the products are only partially characterized. Sorption in soils and sediments is an important factor controlling the migration and bioavailability of these herbicides, while microbial degradation is the most important factor in determining their overall fate in the environment. The biodegradation of alachlor and metolachlor is proposed to be only partial and primarily cometabolic, and the ring cleavage seems to be slow or insignificant. Propachlor biodegradation has been reported to proceed to substantial (> 50%) mineralization of the ring structure. Reductive dechlorination may be one of the initial breakdown mechanisms under anaerobic conditions. Aerobic and anaerobic transformation products vary in their polarity and therefore in soil binding coefficient. A catabolic pathway for chloroacetanilide herbicides has not been presented in the literature because of the lack of mineralization data under defined cultural conditions.