DEGRADATION OF PESTICIDES

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DEGRADATION OF PESTICIDES AND OTHER TOXIC
CHEMICALS BY MICROORGANISMS.
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Definition

• Biodegradation or biological degradation is the
  phenomenon of biological transformation of
  organic compounds by living organisms,
  particularly, microorganisms
• It involves the conversion of complex, toxic
  organic molecules to simpler, non toxic ones.
• Biodegradation provides an effective and economic
  means of disposing toxic chemicals.

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Xenobiotics

• These are unnatural, foreign, synthetic chemicals
  such as pesticides, herbicides, refrigerants,
  solvents and other organic compounds.

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List of mo
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Pseudomonas

• Pseudomonas- most prominent in biodegradation of
  more than 100 toxic compds.
• Can degrade hydrocarbons, phenols,
  organophosphates, polychlorinated biphenyls
  (PCBs), polycyclic aromatics and naphthalene

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Other microbes

• Mycobacterium
• Alcaligenes
• Nocardia

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Consortia of microorganisms

• To degrade a particular compound, it usually
  requires a synergetic action of a host of
  Microorganisms

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Factors affecting Biodegradation

1. Chemical structure of the compound
2. The capability of the individual microorganisms
3. Nutrient and O2 supply
4. Temperature and pH

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General features of Biodegradation

• Aliphatic compds are more easily degraded than
  aromatic ones
• Presence of ring structures and long chains
  decrease the degradation efficiency
• Water soluble compds are more easily degraded
• The presence of halogens makes compds more
  resistant

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How to increase efficiency??

• Biostimulation microbial activity can be
  enhanced by increased supply of nutrients or by
  addition of certain stimulating agents(e.g.,
  electron acceptors)
• Bioaugmentation increase efficiency through
  manipulation of genes. (GEMs)Can also be achieved
  by the use of a consortium of microorganisms

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Enzymes involved

• The enzymes involved in biodegradation and
  bioremediation are usually coded for by genes
  present in plasmids of the organism
• Sometimes, degradation involves the cooperative
  working of products of chromosomal and plasmid
  genes

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Recalcitrant Xenobiotics

• Some compds do not easily undergo bioremediation
  and persist in the environment for a long period.
  These are known as recalcitrant xenobiotics

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Examples

• Halocarbons- compds containing different number
  of halogens in place of H( used as propellants
  for spray cans in cosmetics, paints, used in
  cooling system of condensers,, and in
  herbicides.)
• PCBs- compds having covalently linked benzene
  rings and having halogen substitution for H(used
  in platicisers, insulator coolants in
  transformers)

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• 3. Synthetic Polymers- produced as plastics,
  e.g., polyethylene, polystyrene, nylon garments,
  wrapping materials
• 4. Oil spills
• 5. Others a number of pesticides are based on
  aliphatic, cyclic ring structures containing
  substitutions for halogens, nitro and sulphonate.
  This makes them recalcitrant

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Existence of Xenobiotics

• This maybe because
• The compds are highly stable
• Lack of enzyme system in the organisms
• Lack of transport system in organisms. Hence they
  cannot enter the organisms
• Compds may be highly toxic resulting in the death
  of the organisms

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Biomagnification

• The phenomenon of progressive increase in the
  concentration of the Xenobiotic compd, as the
  substance is passed through the food chain is
  referred to as biomagnification or
  bioaccumulation
• Example- insecticides (DDT)? plants m.o.?birds
  and fish? animals man

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Types of Bioremediation

• In situ Bioremediation- microbial degradation
  occurs at the site of contamination. Done to
  clean up oil spillages, beaches,soil,ground
  water.
• May be an inherent property/engineered
• cost- effective. Minimal exposure to public
• Sites remain minimally interrupted
• Time consuming process
• Progress depends on environmental and seasonal
  conditions

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• 2. Ex situ Bioremediation- waste or toxic
  material collected from the polluted site and
  bioremediation carried out in designed places
• Better controlled and efficient
• completed in a short period
• Costly process
• Site of pollution highly disturbed
• Maybe a disposal problem when process is complete

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Metabolic Effects of microorganisms on xenobiotics

• Detoxification- toxic? non-toxic
• Activation? non toxic? toxic
• Degradation? complex? simpler
• Conjugation ? forming complexes with other
  compds. Simple?complex. Very rare

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Types of reactions in biodegradation

• 1. Aerobic Biodegradation- monooxygenases act on
  aromatic and aliphatic hydrocarbons. Dioxygenases
  hydrolyse only aliphatic compds
• 2. Anaerobic Biodegradation-degradation process
  very slow. Cost effective since it doesnt
  involve continuous supply of O2.
• 3. Sequential BiodegradationIn this, both
  aerobic and anaerobic processes take place
  sequentially

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Biodegradation of Hydrocarbons

• HC are usually pollutants from oil refineries and
  oil spills.
• Slow process due to their insolubility
• Requires a consortium of microorganisms to
  complete degradation
• Organisms involved Pseudomonas, Corynebacterium,
  Arthrobacter, Mycobacterium

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Reactions
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Biodegradation of pesticides and herbicides

• Used regularly to contain various plant diseases
• Contributed to green revolution
• Examples propanil, propham,, atrazine, picloram,
  dichlorodiphenyltrichloroethane (DDT),
  monochloroacetate (MCA), glyphosate
  (oragnophosphate)
• Most pesticides are toxic and recalcitrant in
  nature

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• Most commonly used herbicides and pesticides are
  aromatic halogenated compounds(usually
  chlorinated)
• The rate of degradation of halogenated compounds
  is inversely related to the number of halogen
  atoms.
• Dehalogenation is the first step for
  detoxification. It is catalysed by the enzyme
  dioxygenase

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• Reactions ?
• Loads of them!!!! D D D

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Biodegradation of PCBs

• Used in pesticides, in electrical conductivity of
  transformers, in paints and adhesives
• Inert, very stable and resistant to corrosion
• They have been implicated in cancer, damage to
  various organs and impaired reproductive function

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• PCBs accumulate in soil sediments due to high
  hydrophobic nature and bioaccumulation potential
• Recently, methods have been developed for aerobic
  and anaerobic oxidation using Pseudomonas,
  Alkaligenes, Corynebacterium

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Limitations of Bioremediation

• Slow
• One m.o. does not work for all xenobiotics
• Growth of m.o. may be inhibited by the xenobiotic
  compound
• Some xenobiotics get adsorbed on to the
  particulate matter of the soil and become
  unavailable for biodegradation