Biodegradation of oil spills in the Arctic
As nations across the globe race to strengthen their industrial and technological sectors, it is inevitable that the environmental responsibilities become neglected, evident in the largest liquid diesel spill occurring just last May 2020 in the Arctic waters of northern Russia. While the local ecosystem has played a large role in the recovery process of the ecosystem, the Arctic’s unique biosystem presents difficulty in using the microbial population for oil biodegradation.
In the recovery of oil spill disasters, microorganisms have played a central role in reducing the impact of the catastrophe and functioned as key roles in the clean-up process of oil spills. Bioremediation focuses on controlling the environmental factors that affect how microorganisms grow. There are many species of bacteria, archaea, fungi, and other microorganisms that can degrade hydrocarbons in the spilled oil because these petroleum hydrocarbons are found naturally in all marine environments. Since petroleum hydrocarbons found in oil are a common compound rather than something unfamiliar, the ecosystem knows how to decompose and make use of it. External intervention, for example, burning of the oil spill, can have harmful side effects on the environment, such as injuring the marine animals. A notable example of effective bioremediation would be in the Exxon Valdez Spill of 1989 in Prince William Sound Alaska. Environmentalists added nitrogen fertilizer to the contaminated area to enhance the growth of toxic hydrocarbon-degrading bacteria and microorganisms naturally present in the area. Nitrogen increases the release of the hydrocarbons from the whole compound, making it easier for the microorganisms to excrete molecules responsible for biodegradation. Field tests showed that the fertilizer addition demonstrated by the increased rates of oil breakdown. The hydrocarbon losses, a way to measure biodegradation, increased 1.2% per day1. Polycyclic-aromatic hydrocarbons (PAH) are hydrocarbon molecules commonly found in oils. They take an extremely long time to degrade and are extremely toxic to both the environment and human health. The bioremediation method resulted in a doubled PAH reduction.
The efficacy of natural biodegradation depends on the environment and observing the environmental influences on the process provides insight on areas that can be targeted to best increase the rate of biodegradation. The types of microorganisms and the marine conditions are different for every environment, which is why the plans for each oil spill has to be adjusted specifically for those circumstances. In light of the largest oil spill occurring recently in the Arctic, the potential for biodegradation of the Arctic waters in Greenland was examined. The potential was measured using current knowledge of three major environmental constraints characteristic of the Arctic environment, which are temperature, hydrodynamic conditions, and the production of sediment plumes, key components affecting oil biodegradation. It is important to consider these factors when reviewing the natural capabilities of the ecosystem when recovering from a man-made disaster.
Looking back at the Exxon Valdez oil spill, the biodegradation process focused mostly on the shorelines and given that this was the first oil spill where the efficacy of biodegradation was tested, the recovery process was not complete even to this day. Instead, the spill still provides incredibly informative lessons in current and future bioremediation effects. While the ecosystem has relatively recovered on the surface, with 19 of the 24 species considered injured by the spill having recovered2, the lingering effects of the oil spill still pervades around the area, on land in the breeding habitats of seabirds to the oil deeply embedded under the beaches. While biodegradation played a great deal with reducing the effects of the Exxon Valdez oil spill, that tragedy served more as an examination grounds for the capabilities of biodegradation.
Temperature
A characteristic of the Arctic’s waters is its low temperature, with the surface temperatures being at approximately -1.8 °C, lower than other marine environments3. As temperature decreases, the oil viscosity increases exponentially. This makes it more difficult for the oil to deform and decreases its ability to flow. This temperature is reached by the oil present in the Arctic waters, resulting in the oils becoming close to a solid-state where it cannot break up into smaller oil droplets. This has a heavy impact on the biodegradation potential as smaller oil droplets make it easier for chemicals to move from the water and into contact with the oil droplets. The bacteria can transfer their compounds which allows them to biodegrade the oil through different processes like diffusion.
Another reason why smaller oil droplets are more effective is that it gives a larger surface for biofilm to develop. When the droplets are smaller, there is more surface area on the oil available for degradation. A larger droplet means that most of the molecules are shielded by other molecules and hidden inside the mass, making it inaccessible for degradation. Biofilm is made of microbial cells that stick to each other and the surface. It facilitates the movement of bacteria and other compounds from the aqueous environment into the oil phase by making the distance between the oil and the bacterial cell smaller4. Biofilms are made of varying molecules and can provide channels to allow for materials to exchange and also for microorganisms to adhere to it. The material needed to degrade the oil needs to travel a smaller distance, and so is more effective and efficient in doing its job. Biofilm is also important in supporting the survival of the organisms as they can withstand changing conditions and harsher environments within the biofilm, including nutrient deprivation, shear forces, pH changes, and antibiotics, advantageous in the Arctic environment. As more bacteria can survive and have greater access to the oil, the degradation of the oil increases.
Another effect of low temperatures is that the oil does not evaporate as much as it would at higher temperatures, much like how water turns into water vapour much easier at higher temperatures. Since the temperature is low, there are fewer molecules that have enough energy to escape from the substance. The combination of high oil viscosity and the presence of sea ice makes the oil thicker and reduces spreading, greatly stopping the ability of volatile substances in the oil to evaporate. Volatile compounds found in crude oils, such as BTEX, are inhibitory compounds that would remain in the oil phase because of the reduced evaporation and most likely will dissolve in the ocean water instead3. These compounds have a toxic effect on bacteria by interacting with the cell membrane and blocking its degradation ability, thus reducing the amount of oil being degraded. The harmful compounds remain longer in the oil phase and are more likely to dissolve into the water, just driving up the difficulty for the microorganisms to degrade the oil. This further delays biodegradation and reduces the hydrocarbon degradation rates even after the oil spill clean-up due to the presence of the toxic hydrocarbons dissolved in the water. The temperature of the environment is an important factor that influences how effective biodegradation of oil is going to be.
Hydrodynamic Conditions
Arctic water bodies form layers because of the salt dissolved in the body of water. A horizontal separation of fluid is then caused by the vertical difference in the saltiness (salinity) of the water, making it a β-ocean3. This strong vertical gradient is strengthened by the presence of sea ice as it reduces the ability of the wind to mix the two different water properties. The weak mixing of the water makes it difficult for biodegradation to occur as the hydrocarbons in the water are not as well dispersed and dissolved into the water.
Although mixing ordinarily causes the oil to increase the impact area it also allows for the hydrocarbons moving in the water to be exposed to water containing hydrocarbon-degrading microorganisms. Furthermore, biodegradation microorganisms previously exposed to hydrocarbons degrade at a significantly faster rate. Since these microorganisms have been using hydrocarbons for a long time, they already have established mechanisms that can efficiently break them down. This is most likely because of natural selection, where microorganisms that can effectively breakdown and use hydrocarbons at a faster rate are more likely to survive5. Allowing those previously exposed microorganisms to interact with more of the spread out oil makes a great combination for increased biodegradation rates. Vertical mixing of the water also allows for the upwelling of deeper, nutrient-permeating water onto the surface layer of water, propagating the microorganism population.
Sediment Plumes
Sediment plumes are masses of suspended clumps of rocks and minerals that spread across a body of water. Sediment plumes can cover large areas of the ocean, occurring when a mixture of sediment clumps and water are disturbed from the seabed. This causes the rougher sediment to sink immediately while the finer sediments are suspended, forming a plume.
The mineral particles tend to interact with the spilled oil to form oil-mineral aggregates (OMA). The clusters result from the oil binding with the fine mineral particles in the aqueous environment. This is beneficial for the biodegradation process as it prevents oil droplets from reforming together, placing the dispersed oil droplets in the oil-mineral aggregates in a more stabilized state compared to oil-only or mineral-only aggregates. The OM aggregates in the stable state makes the oil more available for degradation as they are in suspension for a longer period of time and can spread further than unoiled sediment. The ratio with a good amount of oil allows the sediment to float as oil is less dense than water and has the property to float. Too much sediment would cause the entire aggregate to sink. Too much oil, on the other hand, causes the aggregates to merge together instead of remaining as separate droplets6. As mentioned before, this means there is less surface area for the microorganisms to degrade the oil. The OM aggregates remain at the surface of the water, increasing the rate of biodegradation by the microorganisms thriving at the surface level. If the OM aggregates have too little oil, the sediments would sink to the ocean floor. This is where oxygen levels are low and microorganisms that degrade oil are not present at high levels, slowing down biodegradation. OM aggregate formation is integral to the biodegradation rate because it increases the surface to volume ratio of the spilled oil, optimizing the natural weathering process on the oils such as erosion by wind or evaporation by heat/light from the sun.
The potential for biodegradation in Arctic seawater was reviewed by investigating the environmental conditions that influence the oil spill recovery process by local microorganisms. At low temperatures, the oil tends to remain in a viscous, concentrated phase, reducing the ability for microorganisms to produce an effect on the degradation of the oils and for the weathering processes to disperse and dissolve the oils. Keeping this in mind when creating a strategy for the oil clean up in the Arctic is important and other processes that can counter this issue should be investigated, such as taking advantage of the high light concentration to emulsify the oil instead. The spatial gradient of the Arctic waters caused by salinity can also be challenged by integrating methods that would enhance the vertical mixing of the water phases to upwell nutrient-rich water. The formation of OM aggregates is also significant in the biodegradation rates of hydrocarbons where the aggregates keep the oil in suspension and droplet formation for a longer time, enhancing the natural degradation process. The clean up of oil spills depends on the environment as different factors such as temperature, hydrodynamic conditions and the structure of oil influences how well the local microorganisms can degrade the oil. Controlling those factors can allow implementation of an effective recovery plan for future oil spills.
The investigation of the self-cleaning capacity of the Arctic marine environment suggests that the unique challenges presented by the Arctic-specific environmental conditions must be taken into consideration to further understand and improve the interventions required to optimize the biodegradation of oil spills in Arctic waters.
Bibliography
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2 Tim Lydon. “Wounded Wilderness: The Exxon Valdez Oil Spill 30 Years Later | Hakai Magazine.” Hakai Magazine, Hakai Magazine, 22 Mar. 2019, www.hakaimagazine.com/news/wounded-wilderness-the-exxon-valdez-oil-spill-30-years-later/. Accessed 30 Apr. 2021.
3 Vergeynst, Leendert, et al. “Biodegradation of Marine Oil Spills in the Arctic with a Greenland Perspective.” Science of the Total Environment, vol. 626, 1 June 2018, pp. 1243–1258, www.sciencedirect.com/science/article/pii/S0048969718302110, 10.1016/j.scitotenv.2018.01.173.
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6 Stoffyn-Egli, Patricia, and Kenneth Lee. “Formation and Characterization of Oil–Mineral Aggregates.” Spill Science & Technology Bulletin, vol. 8, no. 1, 1 Feb. 2002, pp. 31–44, www.sciencedirect.com/science/article/pii/S1353256102001287, 10.1016/S1353-2561(02)00128-7. Accessed 3 Mar. 2021.
Image Sources:
Oil droplet: https://www.sciencedirect.com/science/article/pii/B978012804595400002X
Arctic halocline: https://www.whoi.edu/multimedia/arctic-halocline/
Sediment plumes: https://earthobservatory.nasa.gov/images/79677/sediment-plume-along-the-coast-of-spain
Oil-mineral aggregate: https://www.sciencedirect.com/science/article/pii/S1353256102001287
Surface area on diffusion: https://medium.com/@linzel/osmosis-and-surface-area-volume-relationships-43cdd4fe2d7b
Ivy is a first-year student studying medical sciences at Western University, Canada. She loves reading about anything and learning about the history of everything. She also really likes exploring anatomy and the little details about how the body works.
