Biodegradation of Agriculture Chemicals

CHAPTER: 6

Biodegradation of Agriculture Chemicals

Ø Biodegradable Compounds

Ø Recalcitrant Compounds

Ø Role of Microorganisms in Biodegradation of Pesticides (Insecticides, Herbicides & Fungicides)

Ø Biodegradation of Aliphatic Pesticides

Ø Biodegradation of Aromatic Pesticides

Biodegradable compounds:

•      A “biodegradable” product has the ability to break down, safely and relatively quickly, by biological means, into the raw materials of nature and disappear into the environment.  

•      These products can be solids biodegrading into the soil (which we also refer to as compostable), or liquids biodegrading into water.

•      Biodegradable matter is generally organic materials such as plant and animal matter and other substances originating from living organisms, or artificial materials that are similar enough to plant and animal matter to be put to use by microorganisms.

•      Some microorganisms have a naturally occurring, microbial catabolic diversity to degrade, transform or accumulate a huge range of compounds including hydrocarbons (e.g. oil), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), pharmaceutical substances, radionuclides and metals.

•      Biodegradation is the decomposition of organic material by microorganisms.

•      The term biodegradation is often used in relation to sewage treatment, environmental remediation (bioremediation) and to plastic materials.

•      Biodegradation is the chemical dissolution of materials by bacteria or other biological means. Although often conflated, biodegradable is distinct in meaning from compostable.

Ø  Some examples of Biodegradable compounds:

               1. 2,4-Dichlorophenol (2,4-DCP) (Herbicides)

               2. Prophax  (Herbicides)

               3. Carbarly (Insecticides)

               4. Methoxychlor (Insecticides)

               5. Chloronet (fungicide)

Recalcitrant Compounds:

•      Those organic compounds which are quite stable and nutrients possessing in it are stable and not subject to release into soluble form are known as Recalcitrant Compounds.

•      Naturally occurring

•      Unable to degrade by Microorganisms

•      Persist in environment for  longer periods

•      For Examples: hair, melanin, lignin, nail,

                           Propachlor, aldrin, dexon,

                           2,4,5-Trichlorophenoxyacetic acid(2,4,5-T)

                           Benzene hexachloride (BHC)

                           DDT (dichlorodiphenyltrichloroethane) 

Ø  The following molecular and environmental conditions are responsible for the recalcitrance-

a. Molecular causes: an aliphatic hydro-carbon like n-hexad is recalcitrant in an anaerobic environment but it is an difficult many organisms under aerobic condition. In the opposite some chlorinated hydrocarbons like chloro-benzoates and chlorophet that are quite recalcitrant in aerobic environment and it can be catabolized under anaerobic environment.

b. Environmental condition: unfavorable temperature is a important factor. For the growth of micro-organisms an optimum temperature is required. Lack of favorable temperature microbes cannot degrade the recalcitrant. Another factor pH the enzymatic activities of microbes totally depend on the certain pH. But under extreme pH, microbes cannot act an recalcitrant.  

Role of Microorganisms in Biodegradation of Pesticides:

•      Pesticides are the chemical substances that kill pests and herbicides are the chemicals that kill weeds. In the context of soil, pests are fungi, bacteria insects, worms, and nematodes etc. that cause damage to field crops.

•      Thus, in broad sense pesticides are insecticides, fungicides, bactericides, herbicides and nematicides that are used to control or inhibit plant diseases and insect pests.

•      An ideal pesticide should have the ability to destroy target pest quickly and should be able to degrade non-toxic substances as quickly as possible.

Effects Of Pesticides:

•      Pesticides reaching the soil in significant quantities have direct effect on soil microbiological aspects, which in turn influence plant growth. Some of the most important effects caused by pesticides are :

   (1) alterations in ecological balance of the soil microflora,

   (2) continued application of large quantities of pesticides may cause everlasting changes in the soil microflora,

   (3) adverse effect on soil fertility and crop productivity,

   (4) inhibition of N2 fixing soil microorganisms such as Rhizobium, Azotobacter, Azospirillum etc. and cellulolytic and phosphate solubilizing microorganisms,

   5) suppression of nitrifying bacteria, Nitrosomonas and Nitrobacter by soil fumigants ethylene bromide, Telone, and vapam have also been reported,

   (6) alterations in nitrogen balance of the soil,

   (7) interference with ammonification in soil,

   (8) adverse effect on mycorrhizal symbioses in plants and nodulation in legumes, and

   (9) alterations in the rhizosphere microflora, both quantitatively and qualitatively.

Persistence of Pesticides in Soil:

•      Persistence of pesticides in soil for longer period is undesirable because of the reasons:

a)      accumulation of the chemicals in soil to highly toxic levels,

b)      may be assimilated by the plants and get accumulated in edible plant products,

c)      accumulation in the edible portions of the root crops,

d)     to be get eroded with soil particles and may enter into the water streams, and finally leading to the soil, water and air pollutions.

Ø  The effective persistence of pesticides in soil varies from a week to several years depending upon structure and properties of the constituents in the pesticide and availability of moisture in soil.

Ø  For instance, the highly toxic phosphates do not persist for more than three months while chlorinated hydrocarbon insecticides (eg.aldrin, chlordane etc) are known to persist at least for 4-5 years and some times more than 15 years.

Ø  From the agricultural point of view, longer persistence of pesticides leading to accumulation of residues in soil may result into the increased absorption of such toxic chemicals by plants to the level at which the consumption of plant products may prove deleterious / hazardous to human beings as well as livestock's.

Biodegradation of Pesticides in Soil:

•      Pesticides reaching to the soil are acted upon by several physical, chemical, and biological forces.

•      However, physical and chemical forces are acting upon/degrading the pesticides to some extent, microorganism’s plays major role in the degradation of pesticides.

•      Many soil microorganisms have the ability to act upon pesticides and convert them into simpler non-toxic compounds.

•      This process of degradation of pesticides and conversion into non-toxic compounds by microorganisms is known as “biodegradation”.

•      Not all pesticides reaching to the soil are biodegradable and such chemicals that show complete resistance to biodegradation are called “recalcitrant”.

Ø  The chemical reactions leading to biodegradation of pesticides fall into several broad categories which are discussed in brief in the following paragraphs:

a)      Detoxification:

•      Conversion of the pesticide molecule to a non-toxic compound.

•      Detoxification is not synonymous with degradation.

•      Since a single chance in the side chain of a complex molecule may render the chemical non-toxic.

b) Degradation:

•      The breaking down / transformation of a complex substrate into simpler products leading finally to mineralization. Degradation is often considered to be synonymous with mineralization, e.g. Thirum (fungicide) is degraded by a strain ofPseudomonas and the degradation products are dimethlamine, proteins, sulpholipaids, etc.

C. Conjugation (complex formation or addition reaction): 

•      In which an organism make the substrate more complex or combines the pesticide with cell metabolites.

•      Conjugation or the formation of addition product is accomplished by those organisms catalyzing the reaction of addition of an amino acid, organic acid or methyl crown to the substrate.

•      for e.g., in the microbial metabolism of sodium dimethly dithiocarbamate, the organism combines the fungicide with an amino acid molecule normally present in the cell and thereby inactivate the pesticides/chemical.

d) Activation:

•      It is the conversion of non-toxic substrate into a toxic molecule, for eg. Herbicide, 4-butyric acid (2, 4-D B) and the insecticide Phorate are transformed and activated microbiologically in soil to give metabolites that are toxic to weeds and insects.

e) Changing the spectrum of toxicity: 

•      Some fungicides/pesticides are designed to control one particular group of organisms / pests, but they are metabolized to yield products inhibitory to entirely dissimilar groups of organisms, for e.g. the fungicide PCNB fungicide is converted in soil to chlorinated benzoic acids that kill plants.

Factors Affecting Biodegradation of Pesticides/Herbicides:

•      Soil Moisture,

•      Temperature,

•      pH,

•      Organic matter content

•      Microbial Population

•      Pesticides Solubility

Biodegradation of Aliphatic & Aromatic Pesticides:

•      Aliphatic and aromatic hydrocarbons are rather persistent in anoxic conditions.

•      These compounds are better degraded by aerobic microorganisms that use molecular oxygen for the initial activation. Addition of oxygen in a highly soluble form can help to remediate the soils polluted with such hydrocarbons.

•      Perchlorate and chlorate are highly water soluble compounds that yields molecular oxygen upon microbial reduction.

•      The oxygen produced may then be used to degrade persistent aliphatic and aromatic hydrocarbons. The metabolites thus formed are easier to degrade further anaerobically.

•      Bacteria are known which can oxidize anaerobically recalcitrant hydrocarbons like benzene and alkanes.

Microbial degradation of aliphatic hydrocarbons:

•      Many types of microorganisms can degrade hydrocarbons. Bacteria, yeasts, and filamentous fungi all have properties to degrade some types of hydrocarbon molecules.

Bacteria –

•      The autotrophic bacteria Thiobacillus and Desulfovibrio can both metabolize the sulfur component of crude oil. Thiobacillus can metabolize S to H₂SO₄ (sulfuric acid) and Desulfovibrio metabolizes this under anaerobic conditions to sulfide. Both sulfide and sulfuric acid can damage metal containers and fuel systems.

•      There are many different reactions involved with the degradation of hydrocarbons by microorganisms.

We will deal only with the more common reaction types:

A). Hydroxylation at C1.

B). Hydro-peroxidation

C). Dehydrogenation reactions

D). Subterminal reactions

Ø  There are some generalizations that can be made concerning the biodegradation of aliphatic hydrocarbons (alkanes, alkenes and  alkyne ):

1. The chain length has a significant effect on biodegradability. It is almost the same for alkanes, alkenes and alkines [alkynes]. Not a simple relationship to number of carbon atoms.

2. Aliphatic compounds are more easily degraded with decreasing saturation and increasing reactivity; Thus;

Rate of Biodegradation => Alkanes => Alkenes => Alkynes

3. The degree and type of branching in a structure has a marked effect on the biodegradability of aliphatic compounds. In general terms, branching decreases biodegradability. In aliphatic compounds with very large numbers of carbons, though, the presence of branching may bring the branch chain length down to the number of carbon atoms which can be degraded; this would promote biodegradability compared to the compound with the same number of carbon atoms but in a non-branched linear chain.

Degradation of Aromatic hydrocarbons:

•      There are many pathways of breakdown for aromatic compounds. Many of them have catechol as a central intermediate.

•      The next stage is ring fission of catechol, followed by incorporation into the normal biochemical pathways of the cell. This can be done by two distinct mechanisms; ORTHO or META cleavage

1. ORTHO cleavage:

•      The aromatic ring is fissioned between the two hydroxyl groups on catechol with the production of muconic acid.

2. META cleavage:

•      The cleavages may also occur between a hydroxylated carbon atom and a non hydroxylated carbon atom which is called meta cleavages.

•      One important mechanism in the breakdown of many compounds is the removal of the halogen atoms which confer much of the resistance to decomposition of such diverse compounds as the insecticide DDT polychlorinated biphenyl (PCB), pentachlorophenol (PCP), halogenated benzenes (e.g. hexachlorobenzene), etc.

•      This is often a process of dehalogenation involving several mechanisms by which halogen atoms are removed from the molecules.

•      These processes can occur under both anaerobic (reductive dehalogenation) and aerobic conditions.

•      Some compounds, such as the chlorinated benzenes, appear to be dehalogenated only under aerobic conditions, whereas other compounds can be dehalogenated under either anaerobic or aerobic conditions.

•      Yet other compounds (some pesticides and halogenated 1 and 2 carbon compounds) undergo only reductive dehalogenation. Another important concept in aromatic biodegradation (and with other compounds) is that of co-metabolism.

•      In this process, a compound is transformed by a microorganism, even though the organism is unable to gain energy by the transformation.

•      It occurs "by accident", in that an enzyme produced by an organism is able to metabolize a compound but the product of the reaction is not metabolized.

•      It is obviously important in mixed cultures or natural situations more than is indicated by pure culture work since different microorganisms may be able to metabolize these different products of previous transformations.

•      The degradation of aromatic compounds are favored by specific enzymes oxygenase which is secreted by many microorganisms.

•      In some cases, one portion of the pesticides molecule is biodegradable while another portion may be recalcitrant.

•      For example: Acyl anilide herbicides are cleaved by microbial enzymes acylamidases and the aliphatic moiety is mineralized but aromatic portion which is stabilized by chlorine substitutes resists mineralization. In such cases various other enzyme like oxidases, peroxidases are required to degrade the compound.