Microbial Hydrocarbon Cleanup: Bioremediation Techniques
Microbial Biodegradation of Hydrocarbons
Hydrocarbons are major constituents of crude oil and petroleum. They can be biodegraded by naturally occurring microorganisms in freshwater and marine environments under a variety of aerobic and anaerobic conditions.
The Role of Microbes in Bioremediation
The ability of microorganisms—including bacteria, archaea, fungi, and algae—to break down hydrocarbons is the fundamental basis for both natural attenuation and enhanced bioremediation efforts.
Enhancing Hydrocarbon Biodegradation (Biostimulation)
To promote effective biodegradation, specific amendments are often added to stimulate microbial growth and metabolism. These biostimulation techniques include:
- Nutrient Addition: Adding nitrogen and phosphorus fertilizers.
- Electron Acceptor Addition: Supplying oxygen (O₂), nitrate, or sulfates to enhance biodegradation rates, especially in oxygen-limited environments.
The practice of bioaugmentation (adding specialized hydrocarbon-degrading microbial cultures to contaminated sites) is generally uncommon for hydrocarbons owing to the natural abundance of indigenous hydrocarbon-degrading microbes.
Common Remediation Strategies
Aside from excavation, various forms of bioremediation are common approaches for hydrocarbon contamination cleanup:
- Simple tilling of soils (aeration)
- Landfarming
- In situ biostimulation
Advantages and Limitations of Bioremediation
Compared to other technologies, bioremediation offers several advantages:
- Lower Cost
- Environmentally Friendly
- Often Non-Invasive [1]
However, there are limitations:
- The time needed for full site cleanup can be longer and less predictable than for other methods.
- Successful outcomes may not be guaranteed due to variability in site conditions.
Nonetheless, bioremediation remains the most common approach for cleaning up hydrocarbon-contaminated environments.
Introduction to Hydrocarbons and Microbial Metabolism
Hydrocarbons are organic compounds composed entirely of carbon and hydrogen. They encompass a wide range of chemical structures, including:
- Aromatic hydrocarbons
- Alkanes
- Alkenes
- Cycloalkanes
- Alkynes
Complex mixtures of these compounds occur naturally in crude oil and gasoline. Most hydrocarbons can be utilized as substrates (energy sources or electron donors) for microbial metabolism by bacteria, archaea, fungi, and algae [2][3].
Aerobic vs. Anaerobic Degradation Pathways
Microbes employ different pathways depending on oxygen availability:
- Fungi and Algae: Typically degrade hydrocarbons aerobically (requiring oxygen).
- Bacteria and Archaea: Capable of both aerobic and anaerobic degradation [4][5][6][7].
Owing to the relatively reduced nature of most hydrocarbons, their oxidation must be coupled to the reduction of a suitable electron acceptor.
Aerobic Degradation
Aerobic degradation uses molecular oxygen (O₂) as both a reactant to oxidize the substrate and the final electron acceptor for microbial respiration.
Anaerobic Degradation and Syntrophy
Anaerobic degradation utilizes different biotransformation pathways that do not depend on oxygen. Instead, it is coupled to the microbial respiration of alternative electron acceptors. Under anaerobic conditions, biodegradation often results from the stepwise concerted action of many different microbes in a process called syntrophy.
Monitoring Natural Attenuation and Biostimulation
Biodegradation occurs naturally because hydrocarbons have always been present in the environment, released from seeps and reservoirs through various geologic processes.
Analytical Methods for Monitoring
Natural attenuation of hydrocarbon contamination can be monitored using several analytical methods, including:
- Gas chromatography
- Infrared spectroscopy [8]
Absolute proof of degradation (versus dilution or other non-degradative processes) is often obtained using Compound Specific Isotope Analysis (CSIA). CSIA confirms biotransformation by detecting an enrichment in the natural abundance of heavier isotopes of carbon and hydrogen in the parent hydrocarbons.
Biostimulation—the strategic addition of limiting nutrients—is often implemented in sites where natural attenuation occurs slowly or not at all.
