Crude oil is a complex mixture of hydrocarbon compounds released into the environment through both natural seeps and accidental spills. These hydrocarbons, composed of hydrogen and carbon atoms, are inherently stable, leading to their persistence in water and soil. The natural environment possesses various mechanisms that transform and reduce pollutant concentration over time. The overall process of oil decay is collectively known as weathering, involving a combination of physical, chemical, and biological actions.
Non-Biological Degradation Processes
The initial breakdown of oil after a spill is often dominated by physical and chemical processes that do not involve living organisms. Evaporation is the first and most rapid process, quickly removing lighter, volatile hydrocarbon components into the atmosphere. This loss of low-molecular-weight compounds can account for a significant portion of the spilled oil mass within the first few days.
Dissolution occurs when water-soluble components separate and mix into the surrounding water column, primarily affecting smaller aromatic molecules. Oil slicks can also undergo emulsification, forming a stable mixture of oil and water, often described as “mousse” due to its thick, tar-like consistency. This physical change traps water within the oil matrix, significantly increasing the volume and making the residue more difficult to disperse or degrade.
Another chemical transformation is photo-oxidation, where exposure to sunlight, particularly ultraviolet radiation, introduces oxygen into the hydrocarbon molecules. This process changes the chemical structure of the oil, sometimes making the compounds more susceptible to dissolution or subsequent microbial attack. These abiotic mechanisms primarily alter the physical state and remove only the most easily handled components, leaving behind the denser, more persistent residues.
The Science of Microbial Oil Consumption
The ultimate mechanism for removing oil from the environment is biodegradation, carried out by specialized microorganisms. Bacteria and fungi that naturally occur in soil and water possess the metabolic machinery to use hydrocarbons as a source of carbon and energy. These organisms are universally present and their populations rapidly multiply when a substantial carbon source, such as spilled oil, becomes available. Specific bacteria, including species from the genera Alcanivorax, Pseudomonas, and Rhodococcus, are well-known for their ability to metabolize various components of crude oil.
The core of microbial oil consumption involves enzymes, primarily oxygenases, which initiate the breakdown of the hydrocarbon chains. These enzymes incorporate molecular oxygen into the hydrocarbon structure, creating intermediate compounds like alcohols and fatty acids. This oxidative attack is why most effective oil-degrading microbes are aerobic, requiring a steady supply of oxygen to sustain their metabolism. The microbes then further process these simpler molecules through standard metabolic pathways, ultimately converting the carbon and hydrogen atoms into carbon dioxide, water, and new cellular biomass.
The speed and efficiency of this biological process depend heavily on the type of hydrocarbon molecule present. Microbes show a distinct preference for simpler, straight-chain linear alkanes, which are degraded much faster than complex, ring-structured aromatic compounds. The heaviest fractions of crude oil, such as resins and asphaltenes, are the most resistant to enzymatic breakdown and can persist for decades. Environmental factors also place constraints on the rate of degradation, with microbial activity slowing considerably in cold temperatures. The rapid growth of oil-eating microbes can quickly deplete local reservoirs of nitrogen and phosphorus, necessary nutrients for cell growth.
Accelerating Natural Breakdown
Human strategies to enhance the natural microbial process are collectively known as bioremediation, which optimizes the limiting factors for microbial activity. One common technique is biostimulation, which involves adding limiting nutrients, specifically nitrogen and phosphorus, to the contaminated area. Supplying these compounds overcomes the nutrient deficit that typically halts microbial growth after the initial bloom, allowing the native, oil-degrading populations to continue metabolizing the hydrocarbons. This approach was demonstrably effective in cleaning up shorelines following major oil spills.
Another strategy is bioaugmentation, which involves introducing commercially grown, specialized oil-degrading microbial strains. This technique is typically reserved for situations where the native microbial community is sparse or lacks the metabolic capability to break down certain recalcitrant oil components. However, this method is less frequently employed than biostimulation because environments usually contain sufficient indigenous oil-eating microbes that simply need a boost of nutrients.
Physical manipulation of contaminated material is also used to ensure adequate oxygen supply, particularly in soil or sediment. Techniques like tilling or aeration mechanically introduce more oxygen, a prerequisite for the fast, aerobic degradation pathways utilized by the most active microbes. These efforts can be carried out on-site (in-situ treatment), or the contaminated material can be excavated and moved to a specialized facility (ex-situ). Regardless of the location, the goal is to create optimal conditions of oxygen, temperature, and nutrient availability to maximize the rate of microbial consumption.
Environmental Impact and Limitations
Despite the existence of robust natural breakdown mechanisms, spilled oil remains a significant environmental concern because the process is slow and often incomplete. The effectiveness of microbial degradation is severely hampered in certain environmental settings. For example, deep-sea environments are characterized by high pressure, low temperatures, and often limited oxygen, all of which substantially slow the metabolism of oil-consuming bacteria.
Oil degradation also ceases when oxygen is completely absent, forcing microbes to switch to anaerobic pathways that are orders of magnitude slower. The most resistant components, such as asphaltenes and resins, ensure long-term persistence as tar balls or residual sludge. Furthermore, the initial breakdown of some hydrocarbons can produce intermediate compounds that are sometimes more water-soluble and potentially more toxic than the original oil, complicating the immediate ecological risk assessment.