Bioremediation and oil spills
What are the oil spills?
Oil spills occur when large quantities of petroleum hydrocarbons leak into the environment from storage tanks, pipelines, non-suitable waste disposal practices, and leaching from landfills. Mainly they occur during shipping, offshore and onshore exploration and production and transportation. However, more than 90% of the oil pollution incidents are triggered by either natural causes such as runoff from land-based sources, or anthropogenic sources (not necessarily accidents) such as normal ship operation and tank washing.
Effects of oil spills
The costs of these environmental disasters are incalculable and can influence wildlife and human health. Marine oil spills prevent light diffusion and oxygen penetration while the oily layer on the water surface threatens marine flora and fauna and their food webs.
History of oil spills
The first oil spill occurred in 1907 where 7400 tons of paraffin oil contaminated the sea surrounding the United Kingdom. Since then, about 140 large spills have occurred, and in total seven million tons of oil-bulk has been discharged to the environment.
In 1989, the Exxon Valdez oil spill occurred when a tanker crashed a reef in Alaska, spilling thousands of tons of oil into the sea. It was reported that this incident has cost the lives of more than 250 000 seabirds. However, bioremediation was applied effectively as a clean-up strategy.
One of the most famous oil spills was the Deepwater Horizon accident which occurred in 2010 in the Gulf of Mexico during the drilling rig explosion, releasing more than 700 thousand tons of crude oil. This accident decreased the biodiversity of vertebrates and metazoan meiofauna in the particular geographical area. The cleaning cost of that spill was estimated to be 10 billion USD and was deemed as the worst oil accident in history.
Week-long blaze of an oil tanker in Sri Lankan waters
A massive oil tanker -the New Diamond- carrying about two million barrels of crude oil caught fire on the 3rd of September 2020, off the eastern coast of Sri Lanka. Sri Lanka’s navy and India’s coast guard attempted to douse the blaze, with water cannons and helicopter drops. Since a 2 m crack in the vessel’s hull, about 10 m above the water line was observed, authorities expressed concern over a potential spill. The vessel is owned by a Liberia-based Porto Emporios Shipping Inc. Regardless of possible diesel leakage during the week-long blaze, no crude oil escaped the cargo as the leak was plugged by divers.
However, Sri Lankan environmental authorities express their concern over a marine disaster according to the discussions on the transfer of oil to another vessel in Sri Lanka’s water.
Existing oil spill cleaning mechanisms
Pollution with crude oil and its products has become a major environmental concern globally. The faster the response to the spill, the higher the chances of preventing contamination. The first step of response is controlling the source to prevent the oil spread which can be carried out using mechanical equipment such as booms, barriers, and sorbents.
Skimmers, dispersants and controlled in situ burning are among the clean-up response strategies. Solvents and dispersants can be applied to reduce the size of oil droplets to make it consumable by microorganisms more easily.
The conventional physical and chemical treatment methods are proved to be effective for the removal of oil spills, but they produce several hazardous compounds that are still immunotoxicant and carcinogenic and they are mostly neither cost-effective nor environmentally friendly. For example, incineration leads to air pollution since the spilled oil is simply burned, with the consequences of raising the atmospheric carbon dioxide levels. Nitrogen and sulfur oxides released during burning are responsible for acid rain. Surely, those approaches lead to the removal of substantial proportions of the spilled oil. However, the unpredictable hazards associated with their use are certainly serious limitations for their implementation.
What is bioremediation?
Oil-bioremediation is globally recognized as a cost-effective and environmentally safe approach. It comprises two distinct operations, bioaugmentation and biostimulation. Bioaugmentation implies the addition of gene pools of microorganisms into the polluted site. Biostimulation relies on the already existing (native) microorganisms which may be enhanced in their activities via specific managements. Most frequent among those managements is the fertilization with N- and P- compounds. Applying fertilizers has an equally high chance of aiding the growth of unwanted algae, which consume much of the available oxygen and cut off sunlight to the deeper water which can negatively impact marine life and prove to be counter-productive.
Pros and cons of oil-bioremediation
Bioremediation is a slow process that requires weeks or months for effective clean-up. The prevention of significant adverse effects such as the production of secondary contaminants, minimal physical disruption of the site, effectiveness in removing toxic compounds, and simpler mechanical technologies are other advantages of this process. The necessity of a specific approach for each polluted site and each spill type is a disadvantage of this process. Bioremediation is a less effective treatment strategy in the sea, and the available knowledge is still inadequate.
Microorganisms for oil-bioremediation
Microorganisms use enzymes and oxygen to break down the structure of hydrocarbons. They use petroleum hydrocarbons as substrates to produce biomass and decompose pollutants into water, carbon dioxide, and fatty acids. Bioremediation is considered as a complementary treatment after conventional clean-up. Auxiliary treatments such as aeration and temperature adjustment can improve the bioremediation process.
It was reported that more than 200 different species; to classify, 79 bacterial, 9 cyanobacterial genera, 103 fungi, 14 algae, and 56 yeasts can degrade the hydrocarbon pollutants. These bacteria include Pseudomonas strains isolated from soil and aquifers, which bear the capability of degrading polycyclic aromatic hydrocarbons (PAHs). Other microorganisms with the ability to degrade petroleum hydrocarbons are Sphingomonas sp., Micrococcus sp., Corynebacterium sp., Flavobacter sp., Streptococcus sp., Sphingobacterium sp., Bacillus sp., Enterobacter s.
To increase the efficiency of bioremediation, strain improvement can be executed to generate improved versions of the original oil-degrading microorganisms. One such approach is protein engineering, aimed at generating new or modified versions of proteins. Directed evolution and selection pressure are the methods used to obtain the most elite microbe under these approaches.
References
Ali, N., Dashti, N., Khanafer, M., Al-Awadhi, H. and Radwan, S. (2020) ‘Bioremediation of soils saturated with spilled crude oil’, Scientific Reports, 10, 1116. DOI: 10.1038/s41598–019–57224-x
Aljazeera (2020) Sri Lanka: Salvage team struggles to plug supertanker fuel leak. Available at: https://www.aljazeera.com/amp/news/2020/9/11/sri-lanka-salvage-team-struggles-to-plug-supertanker-fuel-leak
Baniasadi, M. and Mousavi, S. M. (2018) ‘A comprehensive review on the bioremediation of oil spills’, Microbial Action on Hydrocarbons, pp. 223–254. DOI: 10.1007/978–981–13–1840–5_10
British Broadcasting Corporation (2020) Oil tanker towed from Sri Lanka shoreline amid spill fears. Available at: https://www.bbc.com/news/world-australia-54026289
Office of Response and Restoration (2015) How do oil spills out at sea typically get cleaned up? Available at: https://response.restoration.noaa.gov/about/media/how-do-oil-spills-out-sea-typically-get-cleaned.html
