What Is Biodegradation?
Biodegradation is the chemical breakdown of organic compounds by microorganisms — primarily bacteria and fungi — into simpler molecules, ultimately CO₂, H₂O, and inorganic salts (a process called mineralization). It is the planet's primary mechanism for recycling organic matter and is exploited in wastewater treatment, composting, and pollution cleanup.
Bioremediation is the deliberate application of biodegradation to clean up contaminated sites — using naturally occurring or engineered microorganisms, plants (phytoremediation), or their enzymes (enzymatic remediation) to destroy or immobilize pollutants.
The Biochemistry of Biodegradation
At the molecular level, biodegradation proceeds through sequences of enzymatic reactions that progressively simplify complex organic molecules. Three key processes are:
Aerobic Biodegradation (with Oxygen)
In the presence of O₂, microorganisms use molecular oxygen as the terminal electron acceptor in respiration, oxidizing organic compounds to CO₂ and H₂O and extracting energy:
Organic compound (CₓHᵧOᵤ) + O₂ → CO₂ + H₂O + energy
For hydrocarbons, the initial step often involves oxygenase enzymes that incorporate oxygen directly into the molecule. For example, a monooxygenase converts an alkane to a primary alcohol:
R-CH₃ + O₂ + NADH + H⁺ → R-CH₂OH + H₂O + NAD⁺
This alcohol is then progressively oxidized through aldehyde and carboxylic acid intermediates, eventually entering the citric acid cycle for complete oxidation. Most common environmental contaminants — petroleum hydrocarbons, many solvents — degrade relatively rapidly under aerobic conditions.
Anaerobic Biodegradation (without Oxygen)
In the absence of O₂ (subsurface aquifers, wetland sediments, landfills), microorganisms use alternative electron acceptors: NO₃⁻, SO₄²⁻, Fe³⁺, Mn⁴⁺, or CO₂. This generates different products:
- Denitrification: organic compound + NO₃⁻ → CO₂ + N₂
- Sulfate reduction: organic compound + SO₄²⁻ → CO₂ + H₂S (causes "rotten egg" odor in anaerobic sediments)
- Methanogenesis: CO₂ + 4 H₂ → CH₄ + 2 H₂O (final step in anaerobic decomposition, carried out by archaea)
Anaerobic degradation is slower than aerobic but is critical in waterlogged environments and is the basis of anaerobic digestion — the conversion of organic waste to biogas (methane + CO₂).
Reductive Dehalogenation
Chlorinated solvents — trichloroethylene (TCE, CHCl=CCl₂), perchloroethylene (PCE, CCl₂=CCl₂), PCBs, and DDT — were among the most widespread groundwater contaminants of the late 20th century. Aerobic bacteria have difficulty attacking highly chlorinated compounds. However, specialized anaerobic bacteria such as Dehalococcoides mccartyi can perform reductive dehalogenation, using the chlorinated compound as an electron acceptor:
PCE + H₂ → TCE + HCl → DCE + HCl → vinyl chloride (VC) + HCl → ethylene + HCl
Each step removes one chlorine atom. Complete dechlorination to harmless ethylene requires a complete consortium of organisms and sufficient hydrogen donors. Unfortunately, the intermediate vinyl chloride (VC) is itself a potent human carcinogen, so incomplete reductive dehalogenation can worsen health risks.
Factors Affecting Biodegradation Rate
| Factor | Effect |
|---|---|
| Oxygen availability | Aerobic >> anaerobic for most organics |
| Temperature | Rate roughly doubles per 10°C (Q₁₀ ≈ 2) up to ~35°C |
| pH | Optimal 6–8 for most bacteria |
| Nutrient availability | N and P often limiting; amendment needed |
| Bioavailability | Contaminants sorbed to soil or in NAPL phase degrade slowly |
| Microbial community | Presence of specific degraders is critical |
| Redox conditions | Determines which electron acceptors are available |
Bioavailability is often the rate-limiting factor. Hydrophobic compounds (PAHs, PCBs) partition strongly into soil organic matter or non-aqueous phase liquids (NAPLs), making them physically inaccessible to bacteria in the aqueous phase. Surfactants can enhance bioavailability by increasing apparent solubility.
Bioremediation Strategies
In-Situ Bioremediation
Treatment occurs in place, without excavating soil or extracting groundwater — minimizing disruption and cost.
Bioventing: Air is injected into the unsaturated zone to supply O₂ to aerobic degraders of petroleum hydrocarbons. Very cost-effective for gasoline and diesel spills.
Biosparging: Air is injected below the water table to oxygenate contaminated groundwater.
Enhanced reductive dechlorination (ERD): Fermentable carbon sources (molasses, vegetable oil) are injected into chlorinated-solvent-contaminated aquifers to stimulate growth of Dehalococcoides and provide electron donors for complete dechlorination.
Monitored Natural Attenuation (MNA): Documentation that natural biodegradation is reducing contamination at an acceptable rate, with monitoring to verify progress.
Ex-Situ Bioremediation
Contaminated soil or groundwater is excavated/extracted and treated above ground.
Biopiles and land farming: Excavated contaminated soil is piled and aerated (biopiles) or spread in thin layers on lined areas (land farming) to promote aerobic degradation by indigenous microbes.
Bioreactors: Contaminated water or slurried soil is treated in engineered reactors with optimal conditions (aeration, mixing, nutrient addition, temperature control).
Notable Bioremediation Applications
Exxon Valdez oil spill (1989): After the tanker ran aground in Prince William Sound, Alaska, application of oleophilic fertilizers (nitrogen and phosphorus in a form that adheres to oily surfaces) stimulated naturally occurring hydrocarbon-degrading bacteria. Bioremediation-treated shorelines showed dramatically faster recovery than untreated areas — a landmark demonstration.
Aromatic compound degradation: The TOL plasmid in Pseudomonas putida encodes the complete pathway for degrading toluene, xylene, and related aromatic compounds. Pseudomonas species are among the most metabolically versatile organisms known and are widely used in industrial and environmental biotechnology.
Mycoremediation: Fungi, particularly white-rot fungi (Phanerochaete chrysosporium), produce powerful extracellular oxidative enzymes (lignin peroxidase, manganese peroxidase, laccase) capable of breaking down extraordinarily recalcitrant molecules — including PAHs, TNT, PCBs, and even some plastics. The enzyme system is non-specific, which is an advantage for complex contaminated matrices.
Limitations and Emerging Approaches
Bioremediation is highly effective for many contaminants but has limitations: - Very high contaminant concentrations can be toxic to bacteria - "Forever chemicals" (PFAS) — perfluoroalkyl substances — are highly resistant to biodegradation because C-F bonds are among the strongest in organic chemistry - Inorganic contaminants (heavy metals) cannot be biodegraded, only immobilized or transformed
Synthetic biology offers new possibilities: engineering microorganisms with expanded metabolic capabilities to degrade previously recalcitrant compounds, including plastic polymers and PFAS.