AEROBIC AND ANAEROBIC BIODEGRADATION

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• AEROBIC AND ANAEROBIC BIODEGRADATION

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Bioavailability
Not accessible
Accessible
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Biodegradation and Mineralization
Biodegradation a biological process of reducing
a compound complexity. Mineralization a
degradation process of organic compounds into
inorganic one.
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Biodegradation and Biotransformation

• Conversion of contaminants to mineralized (e.g.
  CO2, H2O, and salts) end-products via biological
  mechanisms
• Biotransformation refers to a biological process
  where the end-products are not minerals (e.g.,
  transforming TCE to DCE)
• Involves the process of extracting energy from
  organic chemicals via oxidation of the organic
  chemicals

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Biodegradation

• Aerobic and anaerobic degradation
• Reduces aqueous concentrations of contaminant
• Reduction of contaminant mass
• Most significant process resulting in reduction
  of contaminant mass in a system

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Fundamentals of Biodegradation

• All organics are biodegradable, BUT
  biodegradation requires specific conditions
• There is no Superbug - not Volkswagon
• Contaminants must be bioavailable
• Biodegradation rate and extent is controlled by a
  limiting factor

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Biodegradation
GROWTH - CELL DIVISION INCREASE IN BIOMASS
O2 consumption
ORGANIC POLLUTANT AND NUTRIENTS (C,P,N,O,Fe,S)
CO2 evolved
Controlled release of energy Slow Burning!
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Microbes Role on Biodegradation
Biodegradation uses naturally occurring
microbes. Microbes use the organic compounds for
their carbon and sometimes energy sources to grow.
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What do Microbes Need to Grow?

• Like all living things, microorganisms need
• Food
• Supply carbon
• Supply energy
• Respiratory substrate
• Something to breathe
• Some use oxygen as the electron acceptor,
  others can use alternatives including chlorinated
  solvents. Otherwise there should be donor
  electron as energy source

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Oxygen and Electron Acceptors Crucial for
biodegradation reactions in the environment
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Terminology and Definitions

• Electron donor
• A compound that donates electrons during its
  oxidation
• Simple organic compounds such as sugars,
  alcohols, or methane can be oxidized to carbon
  dioxide (CO2)
• Electron acceptor
• A compound that accepts electrons during its
  reduction
• Inorganic compounds like oxygen, nitrate,
  sulfate, oxidized metals, or CO2 can be reduced
  to water, dinitrogen gas, hydrogen sulfide,
  dissolved metals, or methane, respectively

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Electron Acceptor Use Preferred Order
High
O2 NO3- Fe (III) Mn (IV) SO42- CO2, Organics
Energy Yield
Low
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Role of electron acceptors rate of biodegradation
O2
NO3-
SO42-
Fe3
NO2- N2
H2O
H2S
Fe2
0.814V
-0.214V
-0.185V
0.741V
FAST GROWTH
SLOW GROWTH
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Aerobic vs Anaerobic Biodegradation (A matter of
terminal electron acceptor)

• If oxygen is the terminal electron acceptor, the
  process is called aerobic biodegradation
• All other biological degradation processes are
  classified as anaerobic biodegradation
• In most cases, bacteria can only use one terminal
  electron acceptor
• Facultative aerobes use oxygen, but can switch to
  nitrate in the absence of oxygen

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Electron Acceptor Zone Formation
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Microbes

• Obligate aerobes - Microbes for which the
  presence of oxygen is essential. Oxygen is the
  only electron acceptor that these species can
  employ.
• Facultative anaerobes - Can use oxygen if it is
  available but are able to switch to alternate
  electron acceptors when oxygen is depleted.
• Obligate anaerobes - Use alternate electron
  acceptors exclusively. Oxygen is toxic.

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Electron Exchange
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Metabolism and Oxidation

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Biotransformation of Organic Substances

• If carbon is in oxidized form (positive
  valencies), biotranformation by reduction is more
  important
• If carbon is reduced (negative valencies),
  biotranformation by oxidation is more efficient

perchloroethene (PCE) C(II)
carbon tetrachloride (CT) C(IV)
vinyl chloride C(-I)
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Aerobic Oxidation
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Oxidation and Extraction of Energy

• Oxidation of organic matter provides energy for
  living organisms because such reactions are
  thermodynamically favored
• ¼ CH2O ½ O2 ? ¼ CO2 ¼ H2O
• ?G? -119.98 kJ/electron equivalent
• Microbes employ catalyzing enzymes to surmount
  kinetic barriers.
• Enzymes function by forming a complex with the
  reactants, bringing them in close contact.

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Microbiology of Aerobic Oxidation

• A wide variety of microorganisms can carry out
  these oxidation processes
• Activity is believed to be ubiquitous
  bioaugmentation is not likely to be required
• Activity can be stimulated by oxygen addition
• Oxygen solubility is limited so the treatable
  concentrations are low

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Oxygen Utilization of Substrates

• Benzene C6H6 7.5O2 gt 6CO2 3H2O
• Stoichiometric ratio (F) of oxygen to benzene
• Each mg/L of benzene consumes 3.07 mg/L of O2

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Fixation of oxygen as a first step in
biodegradation
Cell membrane
Cell Biomass
Further degradation
CO2
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Cl
Cl
Cl
BphD
BphC

BphA
BphB
OH
O
COOH
COOH
OH
OH
Degradation pathway of biphenyl/PCBs by
Pseudomonas KF707
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Biodegradation of Pentachlorophenol
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Aerobic Oxidation
Cl-
R-Cl
CO2 H2O
O2
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Cometabolism (Aerobic)

• Fortuitous transformation of a compound by a
  microbe relying on some other primary substrate
• Generally a slow process - Chlorinated solvents
  dont provide much energy to the microbe
• Most oxidation is of primary substrate, with only
  a few percent of the electron donor consumption
  going toward dechlorination of the contaminant
• Not all chlorinated solvents susceptible to
  cometabolism (e.g., PCE and carbon tetrachloride)

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Methanotrophs, example of cometabolism

• Use methane as the primary substrate, but
  cometabolize chlorinated solvent compounds.
• They oxidize methane to methanol using methane
  monooxygenase (MMO).
• MMO is non-specific, and cometabolizes
  trichloroethene (TCE) to TCE epoxide
• This eventually degrades to CO2, Cl- and H2O.

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Cometabolic Transformations ofChlorinated
Aliphatic Hydrocarbons (CAHs)
NADH, O2
Primary Reaction
CO2 , H2O
CH4
MMO
O

• CCl2CHCl Cl2C CHCl CO2,
  Cl , H2O

-
Secondary Reaction
NADH, O
2
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Aerobic Co-metabolism
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Anaerobic Co-metabolism
Cl-
R-Cl
O2
Mn4 NO3- Fe3 SO42- CO2 H2
Mn2 N2 Fe2 S2- CH4
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Dehalogenation

• Dehalogenation refers to the process of stripping
  halogens (generally Chlorine) from an organic
  molecule
• Dehalogenation is generally an anaerobic process,
  and is often referred to as reductive
  dechlorination
• RCl 2e H gt RH Cl
• Can occur via dehalorespiration or cometabolism
• Some rare cases show cometabolic dechlorination
  in an aerobic environment

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Dehalogenation of PCE

• PCE (perchloroethylene or tetrachloroethylene)
• TCE (trichloroethylene)
• DCE (cis-, trans-, and 1,1-dichloroethylene
• VC (vinyl chloride)

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Anaerobic Transformation

• Reductive dechlorination - Most common anaerobic
  process. A cometabolic process in which the
  solvent is reduced by the replacement of a
  chlorine atom with a hydrogen atom.
• Compounds that contain more than 1 Cl atom
  dechlorinate in a series of steps, each involving
  loss of a single Cl atom.
• Carbon tetrachloride degrades to chloroform via
  reductive dechlorination. The latter is more
  resistant and accumulates.
• PCE and TCE degrade with cis-1,2-DCE and vinyl
  chloride as intermediates.
• VC is highly mobile and toxic.

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Anaerobic biotransformation pathways of
chlorinated aliphatic hydrocarbons (From Vogel et
al., 1987). a abiotic pathway.
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Reductive dechlorination of TCE under anaerobic
conditions (from Vogel, 1994).
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Dehalorespiration (Anaerobic)

• Certain chlorinated organics can serve as a
  terminal electron acceptor, rather than as a
  donor
• Confirmed only for chlorinated ethenes
• Rapid, compared to cometabolism
• High percentage of electron donor goes toward
  dechlorination
• Dehalorespiring bacteria depend on
  hydrogen-producing bacteria to produce H2, which
  is the preferred primary substrate

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Dependence on Redox Condition













1. Highly biodegradable
2. Moderately biodegradable
3. Slow biodegradation
4. Not biodegraded
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Biodegradation of Chlorinated Organics

• More resistant to biodegradation than aromatic
  hydrocarbons.
• Bacteria cannot use most of these compounds as a
  substrate.
• Most biodegradation occurs via cometabolism.
• Cometabolism is slower than heterotrophic
  metabolism and requires the presence of suitable
  primary substrates.

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Stoichiometry

• Electron Donor to Electron acceptor ratios
• Hydrocarbon requirements for electron acceptor
  are well defined
• Electron donor requirements for dechlorination
  are poorly defined
• Cometabolic processes are not predictable
• Each Electron Donor/Electron Acceptor pair has a
  unique stoichiometric ratio