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| Fig. 1: Breakdown of global energy consumption in 2013. [1] |
As of 2013, as much as a third of the annual global energy needs (Fig. 1), corresponding to approximately 4.2 billion tons of oil equivalent of energy (TOE), are met by oil. [1] Oil reserves are not equitably distributed across our planet, thereby necessitating the transport of large quantities of crude oil as well as refined products over long distances from one part of the world to another in order to meet the energetic needs of regions bereft of oil resources of their own. [2,2] Currently, oil tankers serve as the primary means of transporting crude oil and oil-derived refined fuels: nearly 2 billion tons of crude oil are annually ferried in tankers across maritime routes worldwide. [2]
The reliance on tankers for the transport of oil carries with it the risk of occasional oil spills en route, where millions of liters of oil may be inadvertently released into the surrounding marine environment (Fig. 2). [3,4] For instance, the Exxon Valdez incident in 1989, which represents one of the most well-known oil spill events, resulted in the release of nearly 42 million liters of crude oil into Prince William Sound on the Alaskan coast at great cost to the local flora and fauna. [3,4] Interestingly, the Exxon Valdez spill was also the first instance of an extensive use of bioremediation approaches to remove the oil contamination, in this case from the surface of Prince William Sound and, more importantly, from the affected coastlines of Prince William Sound and the Gulf of Alaska (with affected areas amounting to approximately 15% of the total Alaskan coastline). [3,4]
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| Fig. 2: Shown here are birds that were killed due to oil spilled by the Exxon Valdez in 1989. (Source: Wikimedia Commons) |
In the context of ameliorating the adverse environmental impact of an oil spill, bioremediation entails taking advantage of the oil-degrading capabilities of a number of bacterial species with the aim of diminishing and/or eliminating oil- and other hydrocarbon- based environmental contamination. [3-6] Bacteria capable of consuming and degrading oil are extremely diverse, with different species often specialized for degrading a specific subset of the chemical compounds that comprise what we refer to as oil. In essence, oil is a mixture of various forms of energy-rich hydrocarbons, including numerous kinds of aliphatic hydrocarbons and various functionalized monocyclic and polycyclic aromatic hydrocarbon compounds. [4-7] Interestingly, the diversity of hydrocarbons that comprise oil coupled with the specialized hydrocarbon-degrading capabilities of bacteria means that the effective decontamination of areas affected by an oil spill via bacterial bioremediation necessitates the cooperative activity of multiple bacterial species. [5,7]
Since the Exxon Valdez spill of 1989, the biodegrading abilities of bacterial species have been harnessed, to great effect, in numerous bioremediation efforts. However, the use of bacteria in oil spill bioremediation efforts are constrained by the following two factors:
The presence of low quantities of nitrogen, phosphorus, and iron in marine environment, which are necessary for the bioremediating bacteria to multiply to quantities sufficient for appreciably degrading the oil contamination, makes these factors rate-limiting in the biodegradation process. [3,4]
Direct, physical contact of the biodegrading bacteria with oil is necessary such that the bacteria are able to access a large surface area of the contamination relative to its volume. [3,4] The larger the surface area of the oil that is accessible to bioremediating bacteria, the faster the oil spill can be degraded.
The addition of nitrogen- and phosphorus-rich fertilizers to areas contaminated by oil has proven to be effective in overcoming the rate-limiting effect of the inherent nutrient-deficient nature of marine environments. [3-6] However, current fertilizers implemented in bioremediation efforts are toxic and not carbon-neutral. [3-6] Moreover, since such fertilizers are often also water-soluble, they get diluted rapidly upon contact with water and are therefore soon unavailable for use by bacteria acting to degrade oil contaminants at the surface. [3-6] Separately, the development of various emulsifiers (referred to as "dispersant" compounds) to break up surface oil contamination and thereby increase the surface area of oil available to bacteria for degradation have also facilitated some improvement in bioremediation efforts. However, such compounds are not only toxic, but also increase the overall density of the oil such that the contamination spreads below the surface to deeper layers of the marine environment. [3-6] Overall, although much progress has been made in developing bioremediation practices, more work is needed to enhance the efficiency of oil biodegradation in bioremediation efforts as well as limit the introduction of toxic compounds to the environment as an undesired side-effect of bioremediation.
A series of improvements in bioremediation techniques are currently being explored to address the weaknesses of existing bioremediation practices mentioned above. The development of novel synthetic dispersant compounds with lower toxicities and densities is currently under way, which will hopefully facilitate the use of larger quantities of such compounds in oil spill situations. [3-6] Additionally, improved fertilizers which are carbon- neutral, less soluble, and less toxic than existing options are currently under development. [3-6] In fact, the use of uric acid as a fertilizer has already shown promising results in bioremediation tests due to its low solubility, affinity for hydrocarbon compounds, low toxicity, and abundance as a common waste product of various birds and animals. [4] On a longer time scale, the emergence of synthetic biology coupled with strides in computational biology and whole-cell simulations offer the promise of moving beyond simply enhancing biodegradation activity of existing bacteria via chemical additives to the surroundings to engineering the bacteria themselves to more effectively degrade hydrocarbon contaminants. [8]
© Bojan Milic. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "BP Statistical Review of World Energy," British Petroleum, June 2014.
[2] F. Henning et al., "Maritime Crude Oil Transportation - A Split Ppickup and Split Delivery Problem," Eur. J. Oper. Res. 218, 764 (2012).
[3] R. M. Atlas and T. C. Hazen, "Oil Biodegradation and Bioremediation: A Tale of the Two Worst Spills in US History," Environ. Sci. Technol. 45, 6709 (2011).
[4] E. Z. Ron and E. Rosenberg, "Enhanced Bioremediation of Oil Spills in the Sea," Curr. Opin. Biotechnol. 27, 191 (2014).
[5] L. M. Gieg, S. J. Fowler, and C. Berdugo-Clavijo, "Syntrophic Biodegradation of Hydrocarbon Contaminants," Curr. Opin. Biotechnol. 27, 21 (2014).
[6] B. A. Kolvenbach et al., "Emerging Chemicals and the Evolution of Biodegradation Capacities and Pathways in Bacteria," Curr. Opin. Biotechnol. 27, 8 (2014).
[7] B. Z. Fathepure, "Recent Studies in Microbial Degradation of Petroleum Hydrocarbons in Hypersaline Environments," Front. Microbiol. 5, 173 (2014).
[8] B. Milic, "Synthetic Biology and Whole-Cell Simulations," Physics 240, Stanford University, Fall 2014.
