Biodegradation Processes Notes and PYQs MCQs

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Definition: Biodegradation is the biological transformation or breakdown of organic compounds by microorganisms such as bacteria, fungi and, in some cases, other microorganisms. Under suitable conditions, complete biodegradation (mineralization) can convert organic compounds into simpler inorganic products, biomass and other end products.

1. Aerobic vs Anaerobic Biodegradation

Condition	Major Electron Acceptor	Typical End Products
Aerobic	O2	CO2, H2O, biomass
Nitrate reduction	NO3−	N2, N2O and other reduced products
Sulfate reduction	SO42−	H2S / HS−
Methanogenesis	CO2 in hydrogenotrophic methanogenesis	CH4

Exam Point: Anaerobic biodegradation does not necessarily produce only CO₂ and H₂O. Depending on the pathway, products such as CH₄, H₂S and reduced nitrogen compounds may be formed.

2. Biodegradation vs Bioremediation

Parameter	Biodegradation	Bioremediation
Meaning	Biological transformation or breakdown of compounds.	Application of biological processes to remove, transform or detoxify contaminants.
Occurrence	Can occur naturally or under engineered conditions.	Usually deliberately applied or managed for pollution control.
Purpose	May be part of natural nutrient/carbon cycling.	Primarily environmental cleanup or contaminant control.
Examples	Microbial degradation of hydrocarbons.	Biostimulation, bioaugmentation and treatment of contaminated soil/water.

3. Electron-Acceptor Sequence

During progressive anaerobic respiration, microorganisms generally use electron acceptors according to their energetic favourability and availability. A commonly taught sequence is:

O₂ → NO₃⁻ → Mn(IV) → Fe(III) → SO₄²⁻ → CO₂

Caution: The exact sequence and energetic yield can vary with environmental conditions, concentrations, pH and microbial community.

4. Recalcitrant Compounds

Recalcitrant compounds are substances that resist biological degradation because of their chemical structure, low bioavailability, toxicity, strong chemical bonds or other environmental factors.

Practice MCQ Recalcitrant compounds are those that:

(a) Resist microbial degradation
(b) Are always easily degraded
(c) Are exclusively synthetic
(d) Always act as nutrients

Correct Answer: (a) Resist microbial degradation

5. Co-metabolism

Co-metabolism occurs when a microorganism transforms a compound that does not provide sufficient energy or carbon for growth, while the microorganism is growing on another primary substrate.

Practice MCQ Co-metabolism refers to:

(a) Transformation of a secondary compound during growth on a primary substrate, without significant growth benefit from the secondary compound
(b) Symbiotic degradation only
(c) Sequential degradation only
(d) Competitive inhibition

Correct Answer: (a)

6. Aerobic Biodegradation of Aromatic Compounds

Aerobic degradation of aromatic compounds such as benzene commonly begins with oxygenation of the aromatic ring. Depending on the pathway, monooxygenases or dioxygenases may participate in the initial oxidation.

Practice MCQ The enzymes commonly involved in the initial oxygenation of benzene during aerobic biodegradation are:

(a) Monooxygenases or dioxygenases
(b) Reductases only
(c) Hydrolases only
(d) Lyases only

Correct Answer: (a) Monooxygenases or dioxygenases

7. Lignin Biodegradation

Lignin is a complex aromatic biopolymer and is particularly resistant to degradation. White-rot fungi are especially important lignin degraders.

Major ligninolytic enzymes include:

• Lignin peroxidase (LiP)
• Manganese peroxidase (MnP)
• Laccase

Practice MCQ Lignin is degraded efficiently by:

(a) White-rot fungi
(b) Algae only
(c) Viruses
(d) Protozoa only

Correct Answer: (a) White-rot fungi

8. Reductive Dechlorination

Under anaerobic conditions, chlorinated ethenes can undergo reductive dechlorination, in which chlorine atoms are removed and replaced by hydrogen.

PCE → TCE → DCE → VC → Ethene

where:

• PCE = Tetrachloroethene
• TCE = Trichloroethene
• DCE = Dichloroethene
• VC = Vinyl chloride

Practice MCQ The transformation PCE → TCE → DCE → VC → ethene under suitable anaerobic conditions is an example of:

(a) Reductive dechlorination
(b) Oxidative hydroxylation
(c) Hydrolysis only
(d) Polymerization

Correct Answer: (a) Reductive dechlorination

9. Petroleum Hydrocarbon Biodegradation

Petroleum contains several classes of hydrocarbons and non-hydrocarbon fractions. In general, saturated hydrocarbons are often more readily biodegraded than many high-molecular-weight aromatic and polar fractions.

Resins and asphaltenes are generally among the more recalcitrant petroleum fractions.

Practice MCQ Which petroleum fractions are generally among the most recalcitrant?

(a) Resins and asphaltenes
(b) Low-molecular-weight alkanes
(c) Simple volatile hydrocarbons
(d) Methane

Correct Answer: (a) Resins and asphaltenes

10. Plastic Biodegradation

Important Correction: Ideonella sakaiensis is famous for its ability to degrade PET (polyethylene terephthalate), not polyethylene (PE). Therefore, it should not be presented as the standard organism for polyethylene degradation.

Practice MCQ Which statement is correct regarding microbial plastic degradation?

(a) Some microorganisms can degrade or modify certain polymers under appropriate conditions, but biodegradability varies greatly among plastics.
(b) All plastics are rapidly degraded by ordinary soil bacteria.
(c) Ideonella sakaiensis is primarily known for polyethylene degradation.
(d) Plastic degradation never involves enzymes.

Correct Answer: (a)

11. Mineralization

Mineralization refers to the conversion of organic compounds into simpler inorganic products through biological processes. The exact products depend on whether the environment is aerobic or anaerobic.

Condition	Representative Products
Aerobic	CO2 + H2O + inorganic ions + biomass
Anaerobic	CO2, CH4, H2S and other reduced products, depending on the pathway

Practice MCQ Mineralization is best described as:

(a) Biological conversion of organic compounds into simpler inorganic products and associated end products
(b) Adsorption of pollutants onto soil
(c) Physical evaporation of pollutants
(d) Polymerization of organic molecules

Correct Answer: (a)

12. Methanogenesis

In hydrogenotrophic methanogenesis, carbon dioxide acts as an electron acceptor and is reduced to methane:

CO₂ + 4H₂ → CH₄ + 2H₂O

Practice MCQ In hydrogenotrophic methanogenesis, the terminal electron acceptor is:

(a) CO₂
(b) O₂
(c) NO₃⁻
(d) SO₄²⁻

Correct Answer: (a) CO₂

13. Co-metabolic Degradation of TCE

Methanotrophs can transform trichloroethylene (TCE) co-metabolically. Methane serves as the primary growth substrate, while TCE transformation is associated with enzymes such as methane monooxygenase.

Practice MCQ A primary substrate supporting co-metabolic transformation of TCE by methanotrophs is:

(a) Methane
(b) Glucose
(c) Lactate
(d) Ethanol

Correct Answer: (a) Methane

14. Biodegradation and Half-Life

For a first-order degradation process:

t_1/2 = 0.693 / k

Therefore, a shorter half-life indicates a faster disappearance rate under the specified conditions.

Practice Numerical A pollutant has a half-life of 30 days under aerobic conditions and 300 days under anaerobic conditions. Under the stated experimental conditions, the pollutant disappears more rapidly aerobically.

(a) More rapidly biodegradable aerobically
(b) More rapidly biodegradable anaerobically
(c) Equally rapidly degraded
(d) Necessarily non-biodegradable

Correct Answer: (a) More rapidly biodegradable aerobically

15. Important Exam Facts

Topic	Key Fact
Biodegradation	Biological transformation/breakdown of compounds.
Bioremediation	Application of biological processes for contaminant treatment.
Aerobic mineralization	Commonly produces CO₂ and H₂O plus biomass/inorganic products.
Methanogenesis	Hydrogenotrophic pathway reduces CO₂ to CH₄.
Recalcitrance	Resistance to biological degradation.
Co-metabolism	Transformation of a secondary compound while growing on a primary substrate.
Benzene oxidation	Oxygenases such as monooxygenases/dioxygenases may initiate degradation.
Lignin	White-rot fungi are major degraders; LiP, MnP and laccase are important enzymes.
PCE degradation	PCE → TCE → DCE → VC → ethene can occur through reductive dechlorination.
PET	Ideonella sakaiensis is associated with PET degradation.
Petroleum recalcitrance	Resins and asphaltenes are among the more resistant fractions.
Half-life	For first-order degradation, shorter t1/2 means faster disappearance.

16. Golden Revision Points

1. Biodegradation ≠ always complete mineralization. Partial transformation may occur.
2. Mineralization means conversion toward inorganic end products; the exact products depend on environmental conditions.
3. O₂ is generally the most energetically favourable terminal electron acceptor among the common respiratory acceptors.
4. CO₂ → CH₄ is characteristic of hydrogenotrophic methanogenesis.
5. White-rot fungi are especially important in lignin degradation.
6. LiP + MnP + laccase are important ligninolytic enzymes.
7. Co-metabolism means transformation of a compound that does not significantly support growth while another substrate supports growth.
8. Ideonella sakaiensis → PET, not polyethylene.
9. PCE → TCE → DCE → VC → ethene is associated with reductive dechlorination under suitable anaerobic conditions.
10. Shorter half-life → faster disappearance under comparable conditions.