Nature Is Its Own Best Friend

When it comes to post-oil spill clean up, Mother Nature can take care of herself.

Surface oil will evaporate, rise into the atmosphere, and get broken down by sunlight.  Billions of microbes will eat heavy oil that has been tossed around through wind and wave action.  Communities of microorganisms work in concert to convert the oil into carbon dioxide and water.  These microbes are able to do so because they have certain oil-degrading enzymes. [3] What are the most important microorganisms? Well that depends on the composition of the crude oil that is present in the environment; different microorganisms prefer different carbon sources.

Crude oil is made of four main components: saturated hydrocarbons, aromatic hydrocarbons, resins, and, asphaltenes; the latter two are more polar than the hydrocarbons. [3]  Saturated hydrocarbons make up the largest proportion of crude oil, making their degradation an extremely important part of the removal of crude oil from the environment. [9]  The following bacteria are known to use hydrocarbons exclusively as their carbon source:

• Alcanivorax spp.
•  Cycloclasticus spp.
•  Oleiphilus spp.
•  Oleispira spp.
•  Thalassolituus spp.

These bacteria, particularly the Alcanivorax, are frequently found in oil-impacted environments, with their abundancy decreasing as the amount of saturated hydrocarbons present also decreases.  A similar correlation is typically found between Cycloclasticus and aromatic hydrocarbons. [3]

Because resins and asphaltenes are more toxic and persistent due to their polarity, they are of greater long-term environment importance. [3]  However, it is not known which microbes can effectively remove resins and asphaltenes, which is why dispersants and solvents are typically used.

 There are several things that can work against oil-eating bacteria, but in an "ideal" situation, the oil would be light and well dispersed over a large area, it would be composed primarily of hydrocarbons, there would be plenty of nitrogen, phosphate, and oxygen available to the bacteria, the water would be warm and have a relatively low pressure, the salinity and pH of the water would be stable, and there would be a diverse selection of microbes all working together to degrade the oil. [3]  These conditions would allow for the quickest oil-degradation, but of course, it is unlikely that all of these conditions will be met at the same time. [9]

The effects of bio-degradation on oil composition 

Source: "Marine microorganisms make a meal of oil" By Ian M. Head

 (a) The top graph shows the composition of a light crude oil, which is characterized by its high proportion of hydrocarbons (both saturated and aromatic).  The bottom graph shows the composition of a heavy oil, which has more polar carbon compounds (resins and asphaltenes) and a lower proportion of the hydrocarbons. (b) This panel shows how the abundance of hydrocarbons of any length decreases with increasing bio-degradation.  The large peaks on the right hand side of the graphs that do not appear to decrease in abundance are chemicals that have been added to the oil in order to help distinguish between the individual components of the oil.  Bio-degradation is effective at reducing the amount of dangerous hydrocarbons present in the oil that contaminates the world's oceans.

There are also several other natural processes that are involved in oil spill clean-up, including evaporation, dissolution, dispersion, and photo-oxidation. [3]
  

• Evaporation 

 Hydrocarbons in the oil will evaporate fairly quickly (about 50% of light crude oil will         evaporate in the first 20 hours) once they reach the surface of the water. [3] Not all oil is equal, however, as light oil will evaporate much more rapidly than heavier oil.  Environmental conditions such as wave action and temperature also affect evaporation rates. [3]

• Dissolution

Dissolution is the process of dissolving in water, and some of the most volatile and readily biodegradable components of oil can undergo this process.  There is a price to pay, however, since these compounds are usually the most toxic to marine life. [3]

• Dispersion

The process of dispersion allows the oil to break into small droplets and spread out over a larger surface area.  A larger surface area allow for more of the oil to be degraded by bacteria and other microorganisms.  Another benefit of the process of dispersion is that it will push oil droplets into the water column, which will prevent the oil from forming harmful surface slicks.[3] Of course, there is also a disadvantage: dispersion can potentially cause organisms that are present in the ecosystem to be exposed to the oil.

• Photo-oxidation

Sunlight has the ability to break the aromatic ring structures in some hydrocarbons, which makes it easier for certain bacteria to degrade the oil. [3]

Clearly, nature is capable of cleaning up after an oil spill through many various processes.  However, because of numerous outside factors, including location, duration, and form of a spill, nature cannot do all of the work.  Bacteria will degrade the oil, but that takes time, and in order to prevent substantial ecological damage, human involvement is also required.