INTRODUCTION

The advances in organic chemistry during the last century have led to the discovery and synthesis of numerous novel organic compounds,mostly xenobiotics.Many of these synthetic chemical compounds have found applications as pesticides,solvents,explosives,refrigerants,and dyes in industries and urban and agricultural sectors(Duong et al.,1997).Although pesticides have an important role in agriculture to solve the problem of feeding the world’s over-growing population,they are considered recalcitrant to biodegradation(Villarreal-Chiu et al.,2017).Despite their importance in improving the quality of life of modern society,the extensive usage and inevitable concurrent waste have aggravated the problem of toxic waste release in the environment.Although many of these xenobiotic compounds can degrade rapidly in the soil,some are persistent in nature and can be potentially hazardous as a direct consequence of accidental spills,runofffrom application in agricultural areas,and discharge from containers and waste disposal systems(Cui et al.,2001).Depending on their fate in the air,water,soil,or sediment,xenobiotic compounds may become accessible to microorganisms in different environmental compartments where they are further transformed and degraded(Fig.1).Since earliest times,humans have suffered frustration and losses on a massive scale due to destructive pests.Even today,farmers,herdsmen,and housekeepers wage a constant battle against pests,insects,and plant pathogens(Wright and Boorse,2011).Efforts to control these losses involve the use of synthetic chemicals,such as herbicides and pesticides,with an estimated global annual expenditure of USD$32 billion(Wright and Boorse,2011).

Organophosphate pesticides:current usage and contamination scenarios

Pesticides encompass a wide range of compounds that comprise of insecticides,fungicides,rodenticides,molluscicides,and nematocides.These substances are used to control,destroy,repel,or attract pests to minimize their detrimental effects,and they have applications in various sectors,such as livestock farming,agriculture,horticulture,forestry,homes,and other public and industrial spaces.The population explosion and resulting increased demand for agro-products have resulted in such an increase in the consumption of pesticides that the U.S.Environmental Protection Agency(USEPA)had registered about 23400 different pesticide products by the year 1991.With the objective to feed a global population of 7.16 billion people,it has become necessary to protect agriculture from pests.Hence,farmers are increasingly using these xenobiotics,often without knowing their harmful effects.According to the USEPA,60%of herbicides,90%of fungicides,and 30%of insecticides are identified as potentially carcinogenic(Grube et al.,2011).

The rate of pesticide consumption and usage pattern in different countries depend upon their agricultural land and yield type.The top five highest pesticide-consuming countries globally(kg annually)are Italy(63305000),Turkey(60792400),Colombia(48618470),India(40379240),and Japan(36557000)(Verma et al.,2014).On the other hand,the U.S.currently exports thousands of tons of pesticides(active ingredients)each year,particularly to developing countries(Wright and Boorse,2011).The production of pesticides in India commenced in 1952 with the manufacture of benzene hexachloride(BHC),followed by dichlorodiphenyltrichloroethane(DDT).Since then,the production and consumption of pesticides have increased tremendously due to the diverse applications.Presently,India is the second largest manufacturer of general pesticides in Asia and ranks the twelfth globally(Gupta,2004).

胸部创伤容易引起肋骨骨折,常规X线检查对肋骨骨折的诊断效果不及CT效果好[1]。当前多层CT使用了多层面重组技术,对患者的骨折部位进行显示,因此可以对胸部创伤给予较好的诊断[2]。CT会产生X线辐射,对患者带来损伤,该种诊断方式在临床中使用比较多。因此低剂量CT扫描的临床研究能够为社会效益带来帮助[3-5]。现在的肺部低剂量CT研究有了很大的进步,可是对胸外科肋骨骨折的应用评价不多。此次就低剂量和常规剂量CT进行胸外科肋骨骨折的临床诊断情况进行研究分析,有以下报道。

The introduction and extensive application of these xenobiotics to ecosystems have caused many environmental problems worldwide.These include decreased soil fertility,soil acidification,nitrate leaching,increased floral and faunal resistance,groundwater and surface water pollution,and contamination of agricultural soils(Kumari et al.,2008;Bishnu et al.,2009;Pujeri et al.,2010;Lari et al.,2014;Verma et al.,2014).For example,organochlorides were used extensively in the 1950s to control pests and different diseases,such as malaria and typhus.However,due to their toxicity and persistence,along with the introduction of organophosphates in the 1960s,carbamates in the 1970s,and pyrethroids in the 1980s,organochlorine insecticides have been banned or restricted in most parts of the world(Aktar et al.,2009).

Fig.1 Possible environmental fate of a xenobiotic compound(modified from Fetzner,2002).

Chlorpyrifos(O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate)is one of the most widely used insecticides.It is effective against a broad spectrum of insect pests of economically important crops.In 2007,the U.S.used chlorpyrifos of almost 11 million pounds,more than any other organophosphate pesticides(Grube et al.,2011).It is used for the control of mosquitoes(larvae and adults), flies,and various soil,foliar crop,and household pests.It is also used for ectoparasite control on cattle and sheep.It has a low solubility in water(1.18 mg L−1),but is readily soluble in most organic solvents.According to WHO(2009),it is a moderately toxic compound(class II)with a mammalian median lethal dose(LD50)of approximately 32–1000 mg kg−1.The environmental fate of chlorpyrifos has been extensively studied(Kim and Ahn,2009).Degradation in soil involves both microbial activity and chemical hydrolysis.The half-life of chlorpyrifos in soil is approximately 38 d,whereas its half-life in water(hydrolysis half-life)is 2118 d(Kegley et al.,2014).

Organophosphate pesticides and their degradation

Despite their relatively low persistence and easy degradation in the environment,organophosphate pesticides are readily soluble in water.This makes them more susceptible to human consumption,which may result in severe health hazards.Therefore,their degradation by either biological or physicochemical processes has been intensively researched.In this sense,the ability of microorganisms to reduce contaminants has been a preferred method to degrade these compounds under laboratory conditions(25°C and pH 7),as it has shown to be one order of magnitude faster than chemical hydrolysis,which in turn is roughly ten times faster than photolysis(physical degradation)(Ragnarsdottir,2000).Most organophosphate pesticides are degraded by microorganisms in the environment as a source of their limiting nutrients,carbon(C)and/or phosphorus(P).The principal reactions involved in the degradation process are oxidation,hydrolysis,alkylation,and dealkylation(Singh and Walker,2006).Biodegradation of organophosphates by microorganisms is generally carried out through the hydrolysis of P–O alkyl and aryl bonds(Fig.2)with the help of enzymes such as hydrolase(Cui et al.,2001;Kapoor and Rajagopal,2011;Gao et al.,2012;Lu et al.,2013),phosphotriesterase,and phosphatase(Ortiz-Hern´andez et al.,2001;Bhadbhade et al.,2002;Sogorb and Vilanova,2002).Carboxylesterases are also effi-cient in detoxifying through hydrolytic processes(Zuo et al.,2015).The first microorganisms reportedly able to degrade organophosphorus compounds were isolated and identified in 1973 as Flavobacterium sp.(Singh and Walker,2006).Subsequently,many other types of microbes capable of this degradation were isolated and identified.These include fungi,algae,and cyanobacteria.Fungi such as Aspergillus niger(Liu et al.,2001;Pandey et al.,2014),Aspergillus fumigatus(Pandey et al.,2014),Cladosporium cladosporioides(Gao et al.,2012),Penicillium raistrickii,and Aspergillus sydowii(Alvarenga et al.,2014)have been isolated from various contaminated sites and con firmed to be capable of degrading different organophosphate pesticides.Similarly,genera of algae such as Scenedesmus,Stichococcus,and Chlorella(Megharaj et al.,1987;C´aceres et al.,2009)and cyanobacteria such as Nostoc(Megharaj et al.,1987;C´aceres et al.,2008;Ibrahim et al.,2014),Anabaena(C´aceres et al.,2008;Ibrahim and Essa,2010),and Oscillatoria(Salman and Abdul-Adel,2015)have proven to be efficient in the transformation of organophosphates.

Fig.2 General formula of organophosphate pesticides and their biotransformation(Singh and Walker,2006).

TABLE I Most commonly used organophosphate insecticides(market estimates)

a)Ranked by the range(Grube et al.,2011).

Organophosphate insecticide 2001 2003 2005 2007 Ranka) Range Rank Range Rank Range Rank Range×103kg ×103kg ×103kg ×103kg Chlorpyrifos 2 4.9–7.2 2 4.0–4.9 2 3.1–4.0 1 3.6–4.9 Malathion 1 10.4–14.5 1 4.9–5.8 1 4.9–5.8 2 2.2–4.0 Acephate 5 0.9–1.3 5 0.9–1.8 3 1.8–2.7 3 1.8–2.7 Naled 7 0.4–0.9 5 0.4–0.9 4 0.4–0.9 Dicrotophos 7 0.4–0.9 5 0.4–0.9 Phosmet 8 0.4–0.9 6 0.4–0.9 4 0.4–0.9 6 0.4–0.9 Phorate 6 0.9–1.3 9 0.4–0.9 6 0.4–0.9 7 0.4–0.9 Diazinon 3 1.8–3.1 3 1.3–2.2 8 ＜ 0.4 8 ＜ 0.4 Dimethoate 10 0.4–0.9 10 ＜0.4 9 ＜0.4 Azinphos-methyl 9 0.4–0.9 8 0.4–0.9 9 ＜ 0.4 10 ＜ 0.4

TABLE II Persistence and toxicity to mammals of different insecticide classes

a)High,moderate,and low persistence are in years,weeks,and days,respectively(Grube et al.,2011).

Insecticide class Examples Persistencea) Toxicity to mammals Organochlorides Dichlorodiphenyltrichloroethane,dieldrin,toxaphene,chlordane,lindane High Relatively low OrganophosphatesParathion,malathion,acephate,phorate,chlorpyrifos Moderate High Carbamates Carbaryl,methomyl,aldicarb,carbofuran Low High to moderate Pyrethroids Permethrin,bifenthrin,esfenvalerate,decamethrin Low Low

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Fig.3 Molecular structure of the organophosphate pesticides that are used worldwide.

The fate of pesticides in the environment is determined by both biotic and abiotic factors(Fig.1).Pesticides are degraded in the environment principally by the action of indigenous microorganisms,a process termed as biodegradation,which is defined as the breakdown of a substance to small inert end products(Aislabie and Lloyd-Jones,1995).Organophosphate pesticides are generally regarded as safe for use on crops and animals due to their relatively fast degradation rates.The degradation rates vary as a function of microbial composition along with different environmental factors,such as pH,temperature,and availability of sunlight(Ragnarsdottir,2000).Some studies have shown that organophosphate pesticides degrade rapidly by hydrolysis on exposure to sunlight and air.This has been experimentally reported in agricultural fields in Bikaner and Rajasthan,India,where a gradual decrease in residues of phosphamidon of up to 97%was observed over foliage and silique of the mustard crop 15 d after its spraying,and phosphamidon residues were non-detectable after 26 d(Dhas and Srivastava,2010).

3）小麦生长中后期强降雨伴随大风天气往往造成倒伏［8］。 2017年 4月 8～9日，襄阳市襄州区降雨量达到45 mm以上，并伴有大风，最大风力达到12.8 m／s。强降雨伴随大风天气是造成此次小麦严重倒伏的主要外部因素。

Alkaline hydrolysis was found to be the most dominant pathway for malathion by Wang and Hoffman(1991),consistent with an earlier laboratory study Wolfe et al.,1977).Only slow biological and photochemical degradation of malathion has been observed,while biological degradation has been found to be significant for parathion.Alkaline hydrolysis and photolysis are only secondary pathways for parathion degradation.The degradation mechanisms include coppercatalyzed hydrolysis of chlorpyrifos(Fig.6).In the study of Wang and Hoffman(1991),the half-lives for parathion and malathion in the Indian River(salinity of 24 g kg−1and pH of 8.16)were 7.84 and 1.65 d,respectively.Pseudomonas putida can utilize parathionmethyl as a sole source of C and/or P(Wang and Hoffman,1991).This bacterium produces the enzyme organophosphorus acid anhydrase,which hydrolyzes parathion-methyl to p-nitrophenol,which is further degraded to hydroquinone and 1,2,4-benzenetriol,which is in turn cleaved by benzenetriol oxygenase to maleyl acetate(Wang and Hoffman,1991).

Scientists have been exploring the bacterial diversity,particularly of contaminated areas,in the search for indigenous bacteria that can utilize and degrade a wide range of pollutants(Stroud et al.,2007).Biotransformation of organic contaminants in the natural environment has been extensively studied to understand microbial physiology,ecology,and evolution due to their bioremediation potential.Among the organophosphate pesticides,chlorpyrifos,dichlorvos,diazinon,fenami-phos,fenitrothion,isofenophos,parathion,phorate,malathion,parathion-methyl,monocrotophos,profenophos,and others have been used extensively,and their environmental fate and bacterial degradation approaches have been studied extensively.Some of the organophosphate pesticides with bacterial/fungal isolates capable of degrading them are detailed in Table V.

MAJOR MODES AND PATHWAYS FOR MICROBIAL DEGRADATION OF ORGANOPHOSPHATE PESTICIDES

Adsorption

Adsorption and the resulting reduced mobility of organophosphate pesticides in soils are significant factors affecting their behavior in nature.The degree of adsorption and the rate and extent of ultimate degradation are in fluenced by many factors,which include solubility,volatility,charge,polarity,molecular structure,and the size of the pesticides.The process of adsorption by soil particles either may retard the degradation of organophosphate pesticides by separating the pesticide from the enzymes that degrade it or may enhance the process of degradation.Abiotic hydrolytic degradation enhances the process of adsorption(Smolen and Stone,1998).On the contrary,the loss of organophosphate pesticides by volatilization or leaching is diminished after adsorption.Various physicochemical forces for the process of adsorption by soil particles include Van der Waals forces,dipole-dipole interactions,hydrogen bonding,and ion exchange(Smolen and Stone,1998).However,less information is available for ionizable pesticide adsorption,and much research is required to analyze background mechanisms to predict the nature of the interactions of pesticide and soil,because these phenomena affect other processes that determine the final fate of the compound,such as chemical,photochemical,and microbiological decomposition,volatilization,uptake by plants,and diffusion(Beltran et al.,1995).

习近平总书记指出：“只有坚持爱国和爱党、爱社会主义相统一，爱国主义才是鲜活的、真实的，这是当代中国爱国主义精神最重要的体现。”[注]习近平：《习近平在中共中央政治局第二十九次集体学习时强调：大力弘扬爱国主义精神 为实现中国梦提供精神支柱》，《人民日报》2015年12月31日，第1版。爱国主义是一个历史范畴，爱国主义和热爱中国共产党、热爱社会主义制度、热爱中国特色社会主义相统一、相一致，是由没有共产党就没有新中国、只有社会主义才能救中国、只有中国特色社会主义才能发展中国、只有中国共产党才能带领中华民族实现伟大复兴中国梦的历史事实和现实逻辑所决定的。

Photodegradation

Photodegradation of various organophosphate pesticides has been studied(Lacorte and Barcelo,1994;Derbalah et al.,2004).Degradation is rapid,and the products of degradation vary,ranging from the oxidized P=S bond to those resulting from isomerization of the starting organophosphate pesticide.Photolysis may be a very important degradation pathway in the aqueous environment as well as in the gas phase.The degradation of fenitrothion,an organophosphate pesticide,under environmental conditions was studied by Lacorte and Barcelo(1994).They applied fenitrothion in irrigation ditches of the Ebre Delta in Spain at 200 and 20 µg L−1to eliminate American crabs.Fenitrothion levels and its transformation products were recorded for 4 d after its application.The fenitrothion transformation products were 3-methyl-4-nitrophenol,fenitrooxon,and S-methyl isomer.The concentration of fenitrothion declined sharply in 2 h to less than 10%of the initial amount and reached a steady state within 10 h,while the transformation products were at a very low level of 0.01 µg L−1.The half-life of fenitrothion was 13 h,with a degradation rate constant of 0.053 h−1primarily via photolysis.The degradation of fenitrothion and the formation of its transformation products are closely related to environmental factors,such as wind(Lacorte and Barcelo,1994).The overall degradation pathway of fenitrothion is shown in Fig.4(Conant,2005).

2.4.2 药剂防治该病除选用上述技术原则外，在发病初期常用药剂有：50%多菌灵可湿性粉剂用种子量0.3%拌种或500～800倍液喷雾，或用70%甲基硫菌灵可湿性粉剂500倍液～800倍液。

Hydrolysis

Various researchers highlighted the presence of oph gene in the cells of microbes that degrade organophosphorus compounds and hydrolase as the chief enzyme behind the process(Mulbry and Karns,1989;Ohshiro et al.,1997;Cui et al.,2001;Liu et al.,2001;Gao et al.,2012)(Table IV).An enzyme derived from an overproducing strain of Pseudomonas diminuta,called parathion hydrolase,carries out hydrolysis of the phosphate ester bond in the organophosphate pesticide molecule,resulting in an as much as 100-fold decrease in toxicity(Havens and Rase,1991).In one study,partially purified parathion hydrolase was covalently immobilized by several rigid supports,retained a large degree of its activity,and subsequently was used to degrade a variety of organophosphate pesticides(Havens and Rase,1991).The use of hydrolase and associated genes in understanding the complex interaction of microbes with pesticides can greatly enhance the understanding of the biodegradation process and will benefit bioremediation efforts.

在与郑斌董事长的沟通中，其多次谈及“简单就是快乐”，这是其对于人生的感悟，确也是作为企业掌舵人引领企业发展的精神所在。精密达，精益求精，极简前行，一直在路上！

Fig.4 Photodegradation pathway of fenitrothion(Conant,2005).

Fig.5 Phorate hydrolysis pathways yielding formaldehyde(Conant,2005).

Fig.6 Copper-catalyzed chlorpyrifos hydrolysis pathways(Conant,2005).

Enzymatic degradation(enzymatic hydrolysis)

Enzymatic application for degradation of pesticides has gained a lot of interest.Enzymes that are capable of hydrolyzing many organophosphate pesticides are known to be produced by a large number of aquatic species.These enzymes are termed organophosphorus acid anhydrases,although they have also been referred to as paraoxonase,esterase,phosphotriesterase,diisopropyl fluorophosphatase,somanase,and parathion hydrolase(Liu et al.,2001).The natural substrates for the organophosphorus acid anhydrases are not known.However,these enzymes can hydrolyze a wide variety of organophosphorus acetylcholinesterase inhibitors(Liu et al.,2001).An organophosphorus esterhydrolyzing enzyme was also reported by Munnecke and Hsieh(1975);it was isolated from a mixed microbial culture and can hydrolyze chlorpyrifos.In aquatic species,the enzymes have been identified and partially characterized from squid, fish,invertebrates such as Rangia cuneata,the protist Tetrahymena thermophila,and various thermophilic and other bacteria(Liu et al.,2001).These enzymes have evolved in response to the metabolism of naturally occurring organophosphates and halogenated organic compounds(Ohshiro et al.,1997).

Hydrolysis is the most thoroughly studied degradation pathway of organophosphate pesticides.For this reason,this section focuses on some of the intriguing findings regarding the hydrolysis mechanisms of organophosphate pesticides(Fig.5).The hydrolysis products of organophosphate pesticides can be numerous and usually involve the cleavage of the P–S bond(in the case of phosphorodithioates and phosphorothioates)or the P–O bond(in the case of phosphorothioates)that results in the optimal product(Lai et al.,1995).A good example of P–O bond cleavage can be found in the hydrolysis of diazinon(a phosphorothioate),where the oxygen attached to the pyrimidine ring can stabilize the negative charge most effectively(i.e.,pyrimidinol is a better product compared with ethanol).Similar behavior can be found for the other phosphorothioates.The same argument regarding the products can also be made for phosphates.Thus,during the hydrolysis of dichlorvos,the likely initial hydrolytic cleavage is between the P and O atoms attached to the C atom of the double bond.

Bacterial/fungal degradation

With the knowledge that microbes can transform and degrade xenobiotics such as pesticides,researchers are now focusing on microbial diversity,particularly at contaminated sites.Among all the microbes,bacterial degradation approaches have been extensively studied worldwide.This review summarizes the modes and pathways for microbial degradation of organophosphate pesticides and bacteria degradation of the most commonly used organophosphate pesticides.

TABLE IV Major enzymes involved in degradation of organophosphate pesticides

Enzyme Host microbial strain Strain type Organophosphate pesticide(s) Source Hydrolase Cladosporium cladosporioides Fungal Chlorpyrifos Gao et al.,2012 Flavobacterium sp. Bacterial Parathion Mulbry and Karns,1989 Aspergillus niger Fungal Dimethoate Liu et al.,2001 Arthrobacter sp. Bacterial Parathion,EPN,diazinon,isofenphos Ohshiro et al.,1997 Plesiomonas sp. Bacterial Parathion-methyl Cui et al.,2001 Phosphotriesterase Flavobacterium sp. Bacterial Parathion-methyl Ortiz-Hern´andez et al.,2001

TABLE V Organophosphate pesticides with bacterial/fungal isolates capable of degrading them

(to be continued)

Organophosphate pesticide Bacterial/fungal strain(s)isolated Bacterial/fungal isolate matrix(location) Source Acephate Exiguobacterium sp.,Rhodococcus sp. Agricultural soil(Maharashtra,India) Phugare et al.,2012 Cadusafos Pseudomonas putida Farm soil(Saudi Arabia) Abo-Amer,2012 Sphingomonas sp.,Flavobacterium sp. Potato field soil(Greece) Karpouzas et al.,2005 Chlorfenvinphos Arthrobacter sp.,Mycobacterium sp. Petroleum-contaminated soil(Hilo,Hawaii,USA)Seo et al.,2007 Chlorpyrifos Bacillus aryabhattai Agricultural soil(West Bengal,India) Pailan et al.,2015 Stenotrophomonas sp. Industrial sludge(China) Deng et al.,2015 Lactobacillus brevis,Lactobacillus plantarum Not mentioned Zhang et al.,2014 Cupriavidus sp. Industrial sludge(Jiangsu,China) Lu et al.,2013 Serratia marcescensAgricultural soil(Poland)Cyco´n et al.,2013 Sphingobacterium sp. Soil from paddy field(Tamil Nadu,India)Abraham and Silambarasan,2013 Pseudomonas putida,Klebsiella sp.,Pseudomonas stutzeri,Pseudomonas aeruginosa Soil from paddy field(Tamil Nadu,India)Sasikala et al.,2012 Streptomyces chattanoogensis,Streptomyces olivochromogenes Soil from blueberry field(Southern Chile)Brice˜no et al.,2012 Pseudomonas stuzeri,Enterobacter aerogenes,Pseudomonas pseudoalcaligenes,Pseudomonas maltophilia,Pseudomonas vesicularis Agricultural soil(Cairo and Giza,Egypt)Awad et al.,2011 Pseudomonas spp.,Agrobacterium spp.,Bacillus spp.Agricultural farm soil(Varanasi,India) Maya et al.,2011 Cladosporium cladosporioidesa) Industrial soil(USA) Gao et al.,2012 Pseudomonas aeruginosa,Pseudomonas nitroreducens,P.putida Effluent storage ponds and moist soil(Iran)Lati fiet al.,2012 Bacillus sp.,Pseudomonas sp. Soil from groundnut fields(Andhra Pradesh,India)Madhuri and Rangaswamy,2009 Leuconostoc mesenteroides,L.brevis,L.plantarum,Lactobacillus sakei Kimchi during fermentation(Korea) Cho et al.,2009 Sphingomonas sp.,Stenotrophomonas sp.,Bacillus sp.,Brevundimonas sp.,Pseudomonas sp.Soil and industrial water(Jiangsu,China)Li et al.,2008 Pseudomonas sp. Wastewater irrigated agricultural soil(Uttar Pradesh,India)Bhagobaty and Malik,2008 Pseudomonas fluorescence,Brucella melitensis,Bacillus subtilis,Bacillus cereus,Klebsiella sp.,S.marcescens,P.aeruginosa Field soil(Punjab,India) Vidya Lakshmi et al.,2008 Serratia sp.,Trichosporon sp. Activated sludge(Shandong,China) Xu et al.,2007 Sphingomonas sp. Industrial effluent(Nantong,China) Li et al.,2007 Providencia stuartii Agricultural soil(Andhra Pradesh,India) Rani et al.,2008 Flavobacterium sp.,Arthrobacter sp. Agricultural soil(India) Mallick et al.,1999 Arthrobacter sp. Turf green soil(Japan) Ohshiro et al.,1997 Chlorpyrifosmethyl Burkholderia cepacia Soil from paddy field(Korea) Kim and Ahn,2009 Coumaphos Leuconostoc mesenteroides,L.brevis,L.plantarum,L.sakei Kimchi during fermentation(Korea) Cho et al.,2009 Dichlorvos Proteus vulgaris,Vibrio sp.,Serratia Agricultural farm soil(Nigeria) Agarry et al.,2013

TABLE V(continued)

(to be continued)

Organophosphate pesticide Bacterial/fungal strain(s)isolated Bacterial/fungal isolate matrix(location) Source sp.,Acinetobacter sp.Bacillus sp.,Pseudomonas sp. Soil from groundnut field(Andhra Pradesh,India)Madhuri and Rangaswamy,2009 Trichoderma atroviride Not mentioned Tang et al.,2009 Bacillus sp. Soil from grape wine yard(Maharashtra,India)Pawar and Mali,2014 Ochrobactrum sp. Sludge from wastewater(Jining,China) Zhang X H et al.,2006 Diazinon Stenotrophomonas sp. Industrial sludge(China) Deng et al.,2015 Lactobacillus brevis Not mentioned Zhang et al.,2014 Serratia marcescens Agricultural soil(Saudi Arabia) Abo-Amer,2011 Leuconostoc mesenteroides,L.brevis,L.plantarum,L.sakei Kimchi during fermentation(Korea) Cho et al.,2009 Serratia liquefaciens,S.marcescens,Pseudomonas sp.Seo et al.,2007 Arthrobacter sp. Turf green soil(Japan) Ohshiro et al.,1997 Dimethoate Pseudomonas pseudoalcaligenes,Exiguobacterium aurantiacum,Bacillus sp.,Micrococcus luteus Agricultural soil(Poland)Cyco´n et al.,2009 Arthrobacter sp.,Mycobacterium sp. Petroleum-contaminated soil(Hilo,Hawaii,USA)Natural lake water(Antequera,Spain) L´opez et al.,2005 Aspergillus nigera) Sewage and soil from cotton fields(China) Liu et al.,2001 Ethoprophos Sphingomonas sp.,Flavobacterium sp. Potato field soil(Greece) Karpouzas et al.,2005 Pseudomonas putida,Enterobacter sp. Soil(Northern Greece) Karpouzas et al.,2000 Fenamiphos Pseudomonas putida,Acinetobacter rhizosphaerae Soil farm banana field(Eastern Crete,Greece)Chanika et al.,2011 Soil(UK and Australia) Singh et al.,2003 Brevibacterium sp. Soil(Adelaide Hills,Australia) Megharaj et al.,2003 Fenitrothion Lactobacillus brevis Not mentioned Zhang et al.,2014 Serratia marcescensAgricultural soil(Poland)Cyco´n et al.,2013 Burkholderia sp. Wastewater sludge(China) Zhang Z H et al.,2006 Microbacterium esteraromaticum Turf green soil(South Australia) C´aceres et al.,2008 Pseudomonas spp.,Flavobacterium sp.,Caulobacter crescentus Burkholderia spp.,Pseudomonas spp.,Sphingomonas spp.,Cupriavidus sp.,Corynebacterium sp.,Arthrobacter sp.Soil from agricultural field and golf course(Korea)Kim et al.,2009 Soil(Japan) Tago et al.,2006 Parathion Stenotrophomonas sp. Industrial sludge(China) Deng et al.,2015 Serratia marcescensAgricultural soil(Poland)Cyco´n et al.,2013 Leuconostoc mesenteroides,L.brevis,L.plantarum,L.sakei Bartonella spp.,Rhizobium sp.,Burkholderia spp.,Cupriavidus sp.,P.putida Kimchi during fermentation(Korea) Cho et al.,2009 Flavobacterium sp. Not mentioned Mulbry and Karns,1989 Phorate Ralstonia eutropha,P.aeruginosa,E.cloacae Agricultural soil(Maharashtra,India) Rani and Juwarkar,2012 Bacillus sp.,Pseudomonas sp. Soil from groundnut fields(Andhra Pradesh,India)Agricultural soil(Aligarh,India) Bano and Musarrat,2003 Tetrachlorvinphos Madhuri and Rangaswamy,2009 Rhizobium sp.,Pseudomonas sp.,Proteus sp.Stenotrophomonas maltophilia,P.vulgaris,Vibrio metschinkouii,Serratia ficaria,Serratia sp.,Yersinia enterocolitica Corn field soil(Mexico) Ortiz-Hern´andez and S´anchez-Salinas,2010 Triazophos Stenotrophomonas sp. Industrial sludge(China) Deng et al.,2015 Malathion Bacillus amyloliquefaciens,Pseudomonas sp.Agricultural soil(Assam,India) Baishya and Sharma,2014 Lactobacillus brevis Not mentioned Zhang et al.,2014 Pseudomonas sp. Agricultural soil(Pakistan) Jilani,2013

TABLE V(continued)

a)Fungus.

Organophosphate pesticide Bacterial/fungal strain(s)isolated Bacterial/fungal isolate matrix(location) Source Pseudomonas sp.,P.putida,Micrococcus lylae,Pseudomonas aureofaciens,Acetobacter liquefaciens Soil from agricultural field(Cairo,Egypt)Goda et al.,2010 Agricultural wastewater(Egypt) Mohamed et al.,2010 Acinetobacter johnsonii Soil(suburbs of Beijing,China) Xie et al.,2009 Isofenphos Arthrobacter sp. Corn field soil(USA) Racke and Coats,1988 Arthrobacter sp. Turf green soil(Japan) Ohshiro et al.,1997 Monocrotophos Arthrobacter atrocyaneus,Bacillus megaterium Enterobacter aerogenes,Bacillus thuringiensis Bhadbhade et al.,2002 Aspergillus fumigatusa),A.nigera) Not mentioned Pandey et al.,2014 Paracoccus sp. Wastewater sludge(China) Jia et al.,2006 Parathion-methylStenotrophomonas sp. Industrial sludge(China) Deng et al.,2015 Agrobacterium sp. Activated sludge(Shandong,China) Wang et al.,2012 Bacillus sp.,Pseudomonas sp. Soil from groundnut fields(Andhra Pradesh,India)Vegetable farm soil(Maharashtra,India)Madhuri and Rangaswamy,2009 Leuconostoc mesenteroides,L.brevis,L.plantarum,L.sakei Kimchi during fermentation(Korea) Cho et al.,2009 Pseudomonas pseudoalcaligenes,M.luteus,Bacillus sp.,E.aurantiacum Foster et al.,2004 Ethion Stenotrophomonas sp. Industrial sludge(China) Deng et al.,2015 Profenofos Pseudomonas putida,Burkholderia gladioli Natural lake water(Antequera,Spain) L´opez et al.,2005 Plesiomonas sp. Not mentioned Cui et al.,2001 Flavobacterium sp. Agricultural soil(Mexico) Ortiz-Hern´andez et al.,2001 Pseudomonas sp.,Azospirillum sp. Contaminated soil surrounding disused cattle dip site(Australia)Soil(Hanchuan,China) Malghani et al.,2009 Quinalphos Staphylococcus sp.,Bacillus licheniformis Agricultural soil(Assam,India) Baishya and Sharma,2014

BACTERIAL DEGRADATION OF THE MOST WIDELY USED ORGANOPHOSPHATE PESTICIDES

Chlorpyrifos

Currently,organophosphate pesticides are the most commercially favored group of pesticides,with large application areas all over the world.According to Grube et al.(2011),from 2001 to 2007,334 million pounds of organophosphate insecticides were used in USA alone,with chlorpyrifos,malathion,acephate,dicrotophos,diazinon,naled,and phosmet leading the list of those most consumed(Table I).Organophosphate pesticides(organophosphorus compounds)can be defined as degradable organic compounds and derivatives of phosphoric,phosphonic,phosphinic,or thiophosphoric acids that are usually in the form of esters,amides,or thiols.They are commonly coupled with two organic groups and an additional side chain consisting of cyanide,thiocyanate,or phenoxy groups(Balali-Mood and Abdollahi,2014)(Fig.2).A general comparison of the persistence and toxicity to mammals of different insecticide classes is provided in Table II.Some of the organophosphate pesticides used worldwide and their related physicochemical properties with their respective toxicity classes are tabulated in Table III and Fig.3.

Chlorpyrifos is degraded co-metabolically and catabolically by bacteria isolated from different kinds of matrices such as agricultural soil,industrial sludge,activated sludge,and effluents(Li et al.,2007,2008;Kim and Ahn,2009;Chishti et al.,2013).Different species of Pseudomonas,which include P.putida,Pseudomonas stutzeri,Pseudomonas aeruginosa,Pseudomonas nitroreducens,and Pseudomonas fluorescence,isolated from agricultural soils and contaminated effluents throughout different regions have proven to be very efficient in the biodegradation of chlorpyrifos(Bhagobaty and Malik,2008;Vidya Lakshmi et al.,2008;Maya et al.,2011;Lati fiet al.,2012;Sasikala et al.,2012).Other bacterial species such as Bacillus aryabhattai isolated by Pailan et al.(2015)from agricultural soil of West Bengal,India,can efficiently degrade chlorpyrifos as well as parathion at an optimal concentration of 200 mg mL−1.Similarly,Stenotrophomonas sp.isolated from an industrial sludge in China was fo-und to be able to degrade 63%of chlorpyrifos within 24 h with an initial concentration of 50 mg mL−1.Some strains of lactic acid bacteria(LAB)such as Leuconostoc mesenteroides,Lactobacillus brevis,Lactobacillus plantarum,and Lactobacillus sakei isolated from kimchi during fermentation in the presence of 200 mg L−1 of chlorpyrifos can utilize this pesticide as a sole source of C and P(Cho et al.,2009).Some bacterial strains such as Streptomyces chattanoogensis,Streptomyces olivochromogenes isolated from the soil of blueberry field in Southern Chile and Pseudomonas sp.and Enterobacter sp.isolated from agricultural soils of Cairo and Giza,Egypt,respectively,degraded chlorpyrifos when it was provided as a sole source of C and P(Awad et al.,2011;Bricen˜o et al.,2012).Five species of aerobic bacteria capable of degrading chlorpyrifos as a sole C source showed degradation in the range of 46%–72%after 20 d.However,Vidya Lakshmi et al.(2008)detected 75%–87%degradation of chlorpyrifos by P. fluorescence,Brucella melitensis,Bacillus subtilis,Bacillus cereus,Klebsiella sp.,Serratia marcescens,and P.aeruginosa.During bioremediation studies,3,5,6-trichloro-2-pyridinol(TCP)was detected as a metabolite of chlorpyrifos degradation by P.aeruginosa(Vidya Lakshmi et al.,2008).The specific pathway for degradation of chlorpyrifos and its different intermediate metabolites is illustrated in Figs.7 and 8.Huang et al.(2000)studied the degradation of chlorpyrifos in poultry and cow-derived effluents and reported that chlorpyrifos was degraded by aerobic microbial processes in animal-derived lagoon effluents.Singh et al.(2003)studied the biodegradation of chlorpyrifos with different pH ranges in the UK and Australia and concluded that the dissipation of chlorpyrifos in UK soils varying in pH from 4.7 to 8.4 was mediated by the co-metabolic activities of the soil organisms.A bacterial population that utilized chlorpyrifos as a source of C is detected in an Australian soil,and the enhanced ability to degrade chlorpyrifos was successfully transferred to UK soils(Singh et al.,2003).Similar studies were also conducted on Indian agricultural soils by Mallick et al.(1999),who isolated Arthrobacter sp.,and by Ohshiro et al.(1997),who reported the tolerance towards chlorpyrifos with a significant ability to degrade it from Turf green soil in Japan.Chlorpyrifos-methyl is relatively less toxic and has 10–100 times less half-life in soil as well as hydrolysis half-life than chlorpyrifos(Table III).Regardless of it being more user friendly and less persistent,chlorpyrifos is still being used more preferably over chlorpyrifos-methyl.Thus,degradation approaches for chlorpyrifos-methyl have been less studied.Kim and Ahn(2009)isolated Burkholderia cepacia from paddy field soil in Korea and found that the bacterium could hydrolyze chlorpyrifos-methyl to TCP and utilized it as a sole source of C for its growth.The isolate was also found to be able to degrade chlorpyrifos,diazinon,dicrotophos,dimethoate,fenitrothion,malathion,monocrotophos,parathion,and parathionmethyl(Fig.9)(Singh and Walker,2006).

Diazinon

Diazinon(O,O-diethyl O-[6-methyl-2-(1-methylethyl-4-pyrimidinyl)]ester)is an insecticide used to control cockroaches,silver fish,ants,and fleas in residential and non-food-preparation buildings.It is also commonly used in home gardens and on farms to control a wide variety of leaf-eating insects(Grube et al.,2011).It is usually present in dusts,granules,seed dressings,wettable powders,and emulsifiable solution formulations(Eisler,1986).It has a high solubility in water(60 mg L−1).According to the classification of WHO(2009),it is hazardous∶it is a moderately toxic compound(class II)with a mammalian oral LD50of approximately 26–300 mg kg−1and a mammalian dermal LD50of 379 mg kg−1.The half-life of diazinon in soil is approximately 40 d,whereas its hydrolysis half-life is 138 d(Kegley et al.,2014).

为确保水轮发电机组运行的安全与稳定，最大程度降低水轮发电机组和电力系统发生故障的损失，水轮发电机组配置保护需要完成以下基本任务：

Some LAB such as L.brevis,L.plantarum,and L.sakei are very efficient in the degradation of diazinon when provided as a sole source of C and P(Cho et al.,2009;Zhang et al.,2014).The Enterobacteriaceae member S.marcescens was isolated from agricultural soil of Saudi Arabia using an enrichment technique and was found to completely degraded 50 mg L−1diazinon in a minimal salt medium within 11 d(Abo-Amer,2011).This strain was subsequently found to be able to degrade diethyl thiophosphate-containing organophosphates,such as chlorpyrifos,coumaphos,parathion,and isazofos,provided as a source of C and P(Abo-Amer,2011).Seo et al.(2007)isolated strains of Arthrobacter sp.and Mycobacterium sp.from petroleum-contaminated soil of Hawaii,USA and found them to be very efficient in the degradation of diazinon.Similar results were also obtained by Ohshiro et al.(1997),who reported that Arthrobacter sp.was able to hydrolyze diazinon along with other organophosphate pesticides such as chlorpyrifos,ethoprophos,fenitrothion,isofenphos,and parathion.Biodegradation of diazinon was also studied by Cyco´n et al.(2009)using Serratia liquefaciens,S.marcescens,and Pseudomonas sp.isolated from contaminated agricultural soil of Poland.Subsequently,the ability of diazinon-degrading S.marcescens to degrade other organophosphate pesticides such as chlorpyrifos,fenitrothion,and parathion was tested in three soils of different characteristics.It was found that S.marcescens was able to utilize all these insecticides as a sole C source at a concentration of 50 mg L−1and degrade them within 14 d(Cyco´n et al.,2013).A recent study also reported that the Stenotrophomonas sp.isolated from sludge collected at a drain outlet of a chlorpyrifos-manufacturing plant in China might be an excellent candidate for applications in remediating the pollution of diazinon and other organophosphate pesticides,with a removal effi-ciency of nearly 100%(Deng et al.,2015).

Dichlorvos

Fig.7 Proposed pathways of bacterial degradation of chlorpyrifos(Singh and Walker,2006).

Dichlorvos(2,2-dichlorovinyl dimethyl phosphate)has been reported to be toxic to animals and humans(Johnson,1981).It is a synthetic colorless insecticide used against aphids,caterpillars, flies,and spiders as an active control in household and stored products,as a fumigant,and in making pet collars and pest strips.It has a relatively high solubility in water(1×104mg L−1).According to WHO(2009),it is a moderately toxic compound(class II)with a mammalian oral LD50 of approximately 56–108 mg kg−1and mammalian dermal LD50of 75–210 mg kg−1.

The bacterial degradation approaches of dichlorvos have been studied extensively.In an agricultural farm in Nigeria,bacterial strains such as Proteus vulgaris,Vibrio sp.,Serratia sp.,and Acinetobacter sp.were identified as potential degraders of dichlorvos(Agarry et al.,2013).Zhang X H et al.(2006)successfully isolated a bacterial species from wastewater sludge from Jining,China,and identified it as Ochrobactrum sp.This strain was found to be able to utilize dichlorvos as a sole C source at optimal pH and temperature of 7.0 and 30°C,respectively.Complete degradation of dichlorvos could be achieved in just 24 h.Similarly,Bacillus sp.isolated from different yard fields of India were reported to be efficient in degrading dichlorvos(Madhuri and Rangaswamy,2009;Pawar and Mali,2014).

Fig.8 Proposed pathways of bacterial degradation of parathion/parathion-methyl(Singh and Walker,2006).

Fig.9 Proposed pathways for bacterial degradation of dicrotophos and monocrotophos(Singh and Walker,2006).

Fenitrothion

Fenitrothion (O,O-dimethylO-(4-nitro-m-tolyl)phosphorothioate)is the major pesticide currently used in the agricultural industry and on golf courses.This pesticide is a potent inhibitor of acetylcholinesterase.Extensive use of this xenobiotic compound results in major pollution of soil and water systems,and constitutes potential environmental and human health hazards due to its major hydrolysis metabolite,3-methyl-4-nitrophenol(Tago et al.,2006;Kim et al.,2009).Its first introduction was in 1959(Singh and Walker,2006).It is a moderately toxic compound(class II)according to WHO(2009),with a mammalian oral LD50of approximately 500–1416 mg kg−1and mammalian dermal LD50of 1416 mg kg−1(Kumar et al.,2016).It is less persistent than chlorpyrifos in nature with a half-life in soil of 2.7 d and a hydrolysis halflife of about 183 d(Kegley et al.,2014).Unlike other pesticides,very little is known about its biodegradation;however,species of Burkholderia,Pseudomonas,Sphingomonas,Cupriavidus,Corynebacterium,and A-rthrobacter have been isolated from different matrices around Asia and found to be very efficient in degradation of fenitrothion(Tago et al.,2006;Zhang Z H et al.,2006;Kim et al.,2009).Kim et al.(2009)also first reported the involvement of Gram-positive strains(Corynebacterium and Arthrobacter species)in the degradation of fenitrothion.

CONCLUSIONS

Pesticide production and consumption has become inevitable with urbanization and the increasing global population.Among the various pesticide groups,organophosphate pesticides are the major and most widely used group.They are naturally degradable by various microorganisms,but enhanced degradation strategies are still needed so that their chances of accumulation and related health hazards can be minimized in the environment.Microbial degradation involves not only the presence of microbes,but also suitable environmental parameters and different degradation approaches such as hydrolysis and adsorption.Enzymatic application for degradation of pesticides has gained a lot of interest,and engineered microbes have been designed to increase the potentialities of these microor-ganisms and enhance the rates of biodegradation.Various researchers highlighted the presence of oph gene in the cells of microbes that degrade organophosphorus compounds and hydrolase as the chief enzyme behind the process.The use of this enzyme and associated genes in understanding the complex interaction of microbes with pesticides can greatly enhance the understanding of the biodegradation process and will benefit bioremediation efforts.In conclusion,the most widely used organophosphate pesticides chlorpyrifos,diazinon,dichlorvos,fenitrothion and others can be eliminated effectively,ecofriendly,and efficiently from the environmental matrices by bacterial degradation.However,further studies on the degradative enzymes,genes,and other biochemical aspects are still required for improved biodegradation and will facilitate us to expound the exact degradative pathways implicated in the bacterial biodegradation.

数学本身就是理性精神的一种，强调并追求真理，推动人们对世界的认知与发展.用批判与反思的方式来探究知识的真正含义.数学核心素养就是要让学生有探究事物根本、追求真理、构建知识、找到规律并进行创新的意识能力，塑造学生的理性精神，让学生能冲破束缚，用全新的视角看待现实事物，并且用有效的证据来解放人们的思想，发展学生个体的思维能力与学习能力.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the generous financial support for mobility grant provided by the National Council for Science and Technology of Mexico-Department of Science and Technology(CONACYTDST)under the Mexico-India Bilateral Cooperation Project of Mexico(No.266482)and India(No.INT/MexicoP-04/2016).

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