Biodegradation of Carbamate Insecticides

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Slide 1: 

Microbial Degradation of Carbamate Pesticides

Introduction : 

Introduction Pesticides: EPA - any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest. The term pest means any harmful, destructive, or troublesome animals, plants, or microorganisms. Biodegradation refers to the degradation of organic contaminants in soil and/or groundwater by indigenous or transplanted / acclimated microorganisms, primarily bacteria and fungi. Biodegradation recycles pesticide residues to harmless and useful elements like N, P, S, etc.

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Organic contaminants are converted into: Under aerobic conditions →→→ CO2, H2O, and microbial cell mass. Most organics contain reduced carbon and their oxidation provides energy. The rate of aerobic biodegradation is related to: molecular weight water solubility presence of aromatic rings amount of “branching” Under anaerobic conditions →→→ CH4, small quantities of CO2 and H2. For most organic chemicals, breakdown is greatly decreased with the lack of oxygen (with few exception).

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Percent conversiona of several carbamate pesticides to CH4 in anaerobic salt marsh sediments mole excess CH4 above controls/mole carbamate added) x 100.

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A pesticide's environmental persistence largely depends on its chemical structure and on the presence of functional groups. The chemical structure determine its water solubility and consequently, its bioavailability, since microbes more readily assimilate water-soluble compounds. Soil microorganisms are capable of metabolizing carbamates and can easily adapt themselves to metabolize the different types of carbamates.

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Occasionally intermediate species, which may be less, equally, or more hazardous than the metabolized compound, are produced.

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A great deal of interest in the mechanisms of biodegradation of carbamate because: ِِAn efficient mineralization of the pesticides could eliminate the problems of environmental pollution. A balance between degradation and efficacy of pesticides could result in safer application and effective insect control. knowledge about the mechanisms of biodegradation could help of bioremediation of polluted environments.

Global Insecticide Sales by value (2006) : 

Global Insecticide Sales by value (2006)

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Leaching Degradation Adsorption Volatilization AIR Runoff Soil Microbial Chemical Physical Hydrolysis Photolysis Pyrolysis Pesticides in the Environment Residue removed in harvest GROUNDWATER Bioaccumulation & Biomagnifications

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Rate of movement through soil into groundwater depends on: Solubility. High solubility means high chance for groundwater contamination. The water solubility of carbamates is rather low. Half life (persistence of material) →→0.02-0.1 year (≈ 1-5 weeks) Soil depth (shallow vs. deep groundwater). Soil type and other soil properties.

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One carbamate may be easily decomposed, while another may be strongly adsorbed on soil. Some leach out easily and may reach groundwater. In these processes, the soil type and water solubility are of great importance.

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Biodegradation Categories

Abundance of soil organisms : 

Abundance of soil organisms Number Organism per gram soil (~1 tsp) Earthworms – Mites 1-10 Nematodes 10 – 100 Protozoa up to 100 thousand Algae up to 100 thousand Fungi up to 1 million Actinomycetes up to 100 million Bacteria up to 1 billion

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Carbamates are derivatives of carbamic acid, containing the esters of N-methyl (or occasionally N,N-dimethyl) carbamic acid. The use of carbamates as insecticides began in the 1960s as a biodegradable alternative to highly stable organochlorines such as dichlorodiphenyl- trichloroethane (DDT) . Carbamic acid

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Three classes of carbamate pesticides are known: The carbamate ester derivatives →→→ insecticides + nematocides. The carbamate R1 and R2 are aromatic and/or aliphatic moieties →→→ herbicides + sprout inhibitors. Carbamate contain a benzimidazole group →→→ fungicides.

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Carbamic acid Carbamates Lannate Sevin

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ALDICARB (Temik) METHOMYL (Lannate) CARBARYL (Sevin) BAYGON (Propoxur) Chemical structure of some the carbamate pesticides. Note that the only common functionality is the N-methy carbamoyl (-OCONCH3) group.

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carbamates as a class are not generally persistent in the environment. A variety of esters of methyl carbamic acid have been synthesized which inhibit acetylcholinesterase, an enzyme vital to nervous system function. Inhibition of enzyme activity is more rapidly reversed in mammals for carbamates. Carbamates are among the most popular pesticides for home use, both indoors and on gardens and lawns.

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Toxicity: Slightly to highly toxic to birds. Moderately to highly toxic to fish. Highly toxic to bees (most type). Approximately 50 carbamate compounds are in use today as pesticides.

Pesticides in Soil and Groundwater : 

Pesticides in Soil and Groundwater General pathway for degradation: Organic compounds Hydrolysis Some intermediate products still toxic and provide control Degradation by soil microflora Movement through soil (into water table) Depends on how long material remains in soil vs moves through water

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Achromobacter Arthrobacter Blastobacter Pseudomonas Xanthomonas Micrococcus Penicillium Aspergillus microbial strains capable of degrading carbamate

Carbamate Biodegradation : 

The first step →→→ hydrolysis by hydrolase: When a pesticide’s functional groups are attached with weak or labile bonds, it can degrade more rapidly. Carbamate pesticides have such bonds designed into them to avoid problems of extended persistence. Carbamate Biodegradation

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hydrolase introduce one or two oxygen atoms, respectively, into the structure of a pesticide →→→ oxidation process often makes the pesticide more amenable to further degradation by increasing its water solubility →→→ increasing its bioavailability. Aldicarb Aldicarb Sulfoxide Aldicarb Sulfone OXIDATION REDUCTION

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The second step (A) →→→ Extracellular Decomposition The large polymeric structure of pesticides prevents their passage into the microorganism for consumption. Many of the same enzymes microorganisms use to break down cellulose, hemicellulose, and lignin may also degrade pesticides. Extracellular enzymes can have very low specificity. They can, therefore, react with many different chemicals. If the enzyme finds a pesticide before reaching its intended substrate, it may react with it, changing the pesticide into a possibly less toxic and less hazardous form →→→ co-metabolism.

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Biological transformations of carbamate pesticide residues are usually non-specific enzyme-catalyzed degradations by microorganisms existing in soil. Especially enzyme-catalyzed oxidation processes are the most significant degradation processes in soil. Metabolites are formed as the products of such co-metabolic reactions. They can undergo further degradation reactions until the mineralization to H2O and CO2. As pathways proceed the enzymes become more specific so it stops at some specific- point

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The second step (B) →→→ Intracellular Decomposition Generally pesticides containing more oxygen, nitrogen, and sulfur tend to be more water soluble due to hydrogen bonding. Pesticide may enter the cell of a microorganism. To pass into a cell efficiently, the pesticide must be dissolved in water. Once inside a cell, a pesticide may undergo varying degrees of degradation. Mineralization reduces the pesticide to CO2, H2O, and other inorganic components. Typically, it accounts for only a small portion of the“disappearance” of a pesticide.

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Proposed pathway of carbaryl degradation by Arthrobacter sp. Ring hydroxylation: OH 2. N-methyl hydroxylation: CH2OH carbaryl 1-naphthol hydrolysis by carbaryl hydrolase.

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carbaryl 1-naphthol 2-hydroxybenzal pyruvic acid 1,2- dihydroxy naphthalene + O2 Proposed pathway for the metabolism of carbaryl in Pseudomonas sp. strains C4, C5, and C6. salicylaldehyde Salicylic acid Gentisic acid maleylpyruvate pyruvic acid hydrolase hydroxylase dioxygenase dioxygenase

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Hydroxylase: Present in the membrane-free cytosolic fraction. Requires NAD(P)H and flavin adenine dinucleotide. Has optimum activity in the pH range 7.5 to 8.0. Carbamate-degrading enzymes are inducible.

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REDUCTION OXIDATION HYDROLYSIS DEHYDRATION Proposed aldicarb transformation pathways under oxic and anoxic conditions (21 DAY).

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Total mineralization is rarely achieved in soil. Due to the persistence of pesticides and/or their metabolites, mineralization of pesticide residues usually only happens to a certain degree. Successive degradation of pesticide residues can lead to the formation of different kind of metabolites. Sometimes these degradation products are even biologically more active than their parent compounds (bioactivation).

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Carbamates and their metabolites can, at high dose levels, affect the microflora and cause changes that may be of importance in soil productivity . Example: Carbaryl (50-100 ppm) and its major degradate, 1-naphthol (25- 100 ppm) in toxicity. 1-naphthol was found to be more toxic than its parent compound since it inhibits nitrogen cycling mediated by microorganisms. Chlorella vulgaris. Nostoc linckia. Synechococcus elongates.

Persistence/Degradation: Process Drivers : 

Persistence/Degradation: Process Drivers Temperature. Relative humidity / Rainfall. pH. Insulation. Soil or water biota Macrophytes Microbial populations Worms and microfauna. High temperatures. low humidity, Amount of irrigation or rainfall. Many carbamate insecticides are particularly susceptible to hydrolysis under alkaline conditions. Amount of sunlight. lower application rates.

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Biodegradation: How do we know it is happening? Loss of parent material. Production of metabolites: CO2 (labeled or not). Intermediates. Increase in biomass: Degraders. Predators (e.g. protozoa).



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