Bioremediation of Oil Spills: How Does It Work?
Oil spills threaten the health of both human lifestyle and environmental ecosystems. Currently, there are limited methods for cleaning up oil spill which have reached the shoreline or which have spread over a large area. Some of these methods, like in situ burning or the use of dispersal agents, do a lot of harm themselves. There is only one known non-mechanical method to remove oil from an affected area without harming the area – bioremediation [1].
Bioremediation involves the use of nutrients to enhance the activity of indigenous organisms and/or the addition of naturally-occurring non-indigenous microorganisms.
The basic principle behind bioremediation is biodegradation of oils: that is, using microorganisms to break down oils and to change their structure from large, harmful molecules to smaller, harmless substances such as fatty acids or carbon dioxide. In order to increase the effects of natural biodegradation, certain things are added to an oil spill to encourage the growth of microorganisms and therefore increase the rate of oil breakdown [2].
What is the theory behind bioremediation?
Since petroleum hydrocarbons are naturally occurring in the ocean – albeit in low concentrations – numerous microorganisms have evolved to feed off of them. Since these hydrocarbons are toxic and mutagenic to most aquatic organisms, this ensures a food source for the small number of creatures which are able to digest them. When oil enters an ecosystem, these microorganisms are able to breed rapidly and begin the natural breakdown of oil [3].
What is added and why?
Since all organisms require certain nutrients to multiply and prosper, a lack of these nutrients in the immediate environment usually limits the reproduction of oil degrading microorganisms. In marine environments, it is often nitrates, phosphates, and iron which are lacking, and these can be added as a ‘fertilizer’ by scientists. This allows biodegradation to speed up, as a larger number of microorganisms are able to survive in the immediate area [4].
Occasionally, chemical dispersants can also be added to speed up the rate of biodegradation. Since most petroleum hydrocarbons are highly insoluble, the surface area-volume ratio of the spilled oil is usually quite small – it tends to ‘clump’ together in one large, unbroken mat of oil. Hydrocarbon biodegradation occurs at the oil-water interface, and is therefore significantly impacted by the surface area to volume ratio. Using dispersants can increase the surface area of the spill, and therefore can potentially increase the rates of biodegradation [3].
Effectiveness and safety
According to studies done by the EPA and Exxon following the Exxon Valdez spill in 1989, initial bioremediation rates can be as high as 1.2% per day. However, as hydrocarbon concentrations decreased, bioremediation rates dropped accordingly irrelevant of further fertilizer application, suggesting that it is most effective with high concentrations of oil. Follow-up research in the years after the Exxon Valdez spill suggests that the bioremediation efforts were a huge success, as most of the sites tested had little to no detectable oil despite being heavily oiled after the spill. Safety is not an issue as long as non-toxic fertilizers are used in suitable concentrations, as the process is basically an extension of natural ecosystem functions [3].
Conclusion
Bioremediation has been shown to be a safe and effective way to reduce the effects of oil spills. It involves utilizing naturally occurring microorganisms which are able to ‘eat’ hydrocarbons and use them as an energy source. After reviewing past evidence, it is obvious that bioremediation must be taken into consideration when drafting response plans to any future oil spills or disasters.
References
[1] https://www2.usgs.gov/faq/categories/9816/2841
[2] https://goo.gl/FVTWvh
[3] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3155281/
[4] http://pubs.usgs.gov/fs/1995/0054/report.pdf
