Biodegradation of selected
organic compounds
• Hydrocarbons
• Halogenated aliphatic compounds
• Halogenated aromatic compounds
Hydrocarbons
• Over 2 billion metric tons of petroleum
are produced every year worldwide and
large amounts of these products end up
polluting the environments.
• These include: Low-level routine
discharges (urban runoff, effluents oil
treatment of roads, etc) account for over
90% of the total petroleum
hydrocarbon discharges.
• Accidents such as tanker disasters,
pipeline breaks, and well blowouts
account for less than 10% of these
discharges.
• Hydrocarbon in crude petroleum are
classified as alkanes, cycloalkanes,
aromatics, polycyclic aromatics,
asphatines, and resins.
• Alkenes are generally not encountered
in crude oil but may be present in small
quantity in refined petroleum products.
Alkanes
 The most biodegradable of the petroleum
hydrocarbons.
 However, in the C5 to C10 range, alkanes
may be inhibitory to many hydrocarbon
degraders because as solvents they
disrupt lipid membranes. In the C20 to
C40 range (waxes), their low solubility
interferes with their biodegradation.
Alkanes--Degradation pathway:
The monooxygenase enzyme attacks the
terminal methyl group to form an alcohol.
The alcohol is oxidized further to an
aldehyde and then to a fatty acid. The fatty
acid is degraded further by b-oxidation of
the aliphatic chain.
Alkanes
In general, the degradation products are less
volatile than the parent compounds. In some
cases, the parent alkanes are highly volatile
and may be removed from soil by stripping
under aerobic conditions.
Alkenes
 Location of the unsaturated linkage is a
factor. 1-alkenes are more degradable
than alkenes with an internal double
bond.
 Two general pathways: either the double
bond is oxidized or the saturated chain
end is oxidized.
Cycloalkanes
 Less degradable than the straight chain
alkanes but more degradable than the
polycyclic aromatics. Part of the decreased
degradability is due to decreased solubility.
 Alkyl-substituted cycloalkanes are more
readily degraded than nonsubstituted
hydrocarbons, and cyloalkanes with longchain side groups are more easily degraded
than those with methyl or ethyl groups.
Cycloalkanes
 Cycloalkanes are usually degraded by
oxidase attack to produce a cyclic
alcohol which is dehydrogenated to a
ketone.
Aromatics
Aromatics
 Biodegradation involves two steps:
activation of the ring, and ring cleavage
 Activation is achieved by enzymes
known as oxygenases.
 Acids produced are readily utilized by
microorganisms for cell synthesis and
energy.
Polycyclic aromatic hydrocarbons
 Produced during high temperature
industrial operations such as petroleum
refining, coke production, and wood
preservation.
 In general, PAHs with increase
molecular weight and number of ring
structures have decreased solubility and
volatility.
 PAHs are degraded one ring at a time
Asphaltines and Resins
 High molecular weight compounds
containing N, S, and O.
 They are usually recalcitrant to
biodegradation.
 Cometabolism may be significant in
degrading these compounds.
Halogenated aliphatic compounds
• Industrially important ones include: chlorinated
and brominated alkanes and alkenes in C1 to C3
range.
• Chlorinated ethanes and ethenes are commonly
used as cleaning solvents and in dry-cleaning
operations and semiconductor manufacturing.
• Brominated compounds are used as pesticides
(e.g. ethylene dibromide or EDB,
dibromochloropropane or DBCP) and
halogenated methanes (CHCl3, CHCl2Br,
CHClBr2, and CHBr3) are formed during the
disinfection of water.
• The greater the number of halogens in
the molecule, the less biodegradable
the compound will be in aerobic
systems and the more degradable it
will be in anaerobic systems.
• The biodegradation rate is also
dependent on the type of halogen in
the compound. In general, Br>Cl>F.
Degradation pathways
Substitutions
Oxidations
Reductions.
Halogenated aromatic compounds
• Include: solvent, lubricants, pesticide,
plasticizers, PCBs (insulators in
electrical transformers and capacitors),
and pantachlorophenol (a wood
preservative)
• The more halogen substituents the
compound has, the more likely it is to
undergo reductive dehalogenation in
reducing environments.
Degradation pathways
Oxidations and ring cleavage
Reductive dehalogenagtion