11.3 Biodegradation of Organic Pollutants

Microbes are nature's cleanup crew, breaking down pollutants in various ways. They use oxygen, alternative electron acceptors, or co-metabolism to degrade contaminants. Enzymes like oxygenases and dehalogenases are their tools, each specializing in different types of pollutants.

Environmental factors like nutrients, oxygen, and pH greatly influence how well microbes can do their job. To boost their performance, we can either feed the native microbes (biostimulation) or bring in specialized degraders (bioaugmentation). These strategies help microbes work faster and more efficiently.

Microbial Processes and Enzymes in Biodegradation

Microbial processes in biodegradation

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• Aerobic degradation oxidizes pollutants using oxygen as electron acceptor completely mineralizing them to CO2 and water (petroleum hydrocarbons)
• Anaerobic degradation breaks down pollutants without oxygen using alternative electron acceptors (nitrate, sulfate, iron) (chlorinated solvents)
• Co-metabolism degrades pollutants as secondary substrates requiring presence of primary growth substrate (trichloroethylene)
• Biosorption adsorbs pollutants onto microbial cell surfaces concentrating contaminants (heavy metals)
• Biotransformation partially modifies pollutant structure resulting in less toxic or more biodegradable compounds (pesticides)

Enzymes for organic contaminant degradation

• Oxygenases introduce oxygen atoms into substrates
  • Monooxygenases add one oxygen atom (methane monooxygenase)
  • Dioxygenases add two oxygen atoms (catechol dioxygenase)
• Dehalogenases remove halogen atoms from halogenated compounds (reductive dehalogenase)
• Hydrolases cleave chemical bonds using water (esterases)
• Transferases move functional groups between molecules (glutathione S-transferase)
• Reductases catalyze reduction reactions in anaerobic conditions (nitrate reductase)
• Specific pathways break down different contaminant classes
  1. Ortho-cleavage pathway degrades aromatic compounds
  2. Meta-cleavage pathway breaks down methylated aromatics
  3. Beta-oxidation metabolizes aliphatic hydrocarbons

Environmental Factors and Enhancement Strategies

Factors influencing biodegradation rates

• Nutrient availability affects microbial growth and enzyme production
  • ###carbon:nitrogen:phosphorus_ratio_0### balances essential elements
  • Micronutrients support enzymatic functions (iron, zinc)
• Oxygen levels determine degradation pathways
  • Aerobic vs anaerobic conditions shape microbial communities
  • Redox potential influences electron flow
• Temperature impacts microbial growth rates and enzyme activity (mesophilic, thermophilic)
• pH affects microbial community composition and enzyme function (acidophiles, alkaliphiles)
• Bioavailability determines contaminant accessibility
  • Sorption to soil particles limits degradation
  • Dissolution in water phase increases accessibility
• Co-contaminants interact with primary pollutants
  • Synergistic effects enhance overall degradation
  • Antagonistic effects inhibit degradation processes
• Microbial community structure shapes degradation potential
  • Presence of degrader populations determines efficiency
  • Diversity and functional redundancy ensure resilience

Biostimulation vs bioaugmentation strategies

• Biostimulation enhances native microbial activity
  • Adding limiting nutrients boosts degrader populations (nitrogen, phosphorus)
  • Oxygen injection stimulates aerobic processes (bioventing)
  • Electron donor addition supports anaerobic processes (lactate, hydrogen)
  • pH adjustment optimizes enzymatic activity (lime, sulfur)
• Bioaugmentation introduces specific degrader microorganisms
  • Cultured native strains adapt to site conditions
  • Genetically engineered strains target recalcitrant compounds
• Surfactant addition increases bioavailability of hydrophobic contaminants (biosurfactants)
• Phytoremediation uses plants to enhance microbial activity in rhizosphere (poplar trees)
• Composting accelerates degradation through temperature and nutrient optimization (yard waste)
• Bioventing and biosparging deliver oxygen to stimulate aerobic degradation in soil and groundwater
• Permeable reactive barriers treat groundwater contaminants in situ using reactive materials (zero-valent iron)