An Appetite for Oil: Oceans Rebound from Oil Spills with the Aid of Microbes
In the aftermath of the Deepwater Horizon oil spill, which sent an estimated 4.9 million barrels of crude oil into the Gulf of Mexico during three months in 2010, nature continued to do what it has long done - slowly but surely break down the oil into its constituent parts including carbon dioxide and water.
Oil in the environment is nothing new. Petroleum seeps - leaks of natural gas, crude oil and bitumen into the Earth's atmosphere or onto its surface, including the ocean floor - have been recorded as far back as the Paleolithic Era. Recent studies have documented their occurrence around the world in places such as the Arctic, the Gulf of Mexico and on the west coast of California.
Scientific studies have verified that naturally occurring microorganisms in the environment, primarily bacteria and fungi, have a huge capacity for breaking down, or degrading, oil. "Taking into account the volume of oil released from natural seeps, if it weren't for microbial degradation we'd probably be knee deep in oil," says Dr. Kenneth Lee, Executive Director of the Centre for Offshore Oil, Gas and Energy Research (COOGER), a Fisheries and Oceans Canada centre of expertise headquartered at the Bedford Institute of Oceanography in Dartmouth, Nova Scotia. COOGER coordinates research efforts into the environmental and oceanographic impacts of offshore petroleum exploration, production and transportation. The work conducted by this research group has established an international reputation for Fisheries and Oceans Canada in oil spill response research.
To address the public's interest in the fate and effect of the oil released into the Gulf of Mexico, the American Society for Microbiology recently coordinated a meeting of world experts in marine microbiology to produce the publication Microbes and Oil Spills. The participants in this workshop, including Dr. Lee, discussed the importance of microbes in the degradation of oil and methods that can be used to speed up the rate of "natural attenuation" - recovery by natural processes alone.
"Even where the background levels of oil are low, a few microbes with the capability of degrading oil always seem to be present," says the report, which estimates that half of the oil entering the world's oceans today comes from natural seeps and the rest from human activities.
According to Microbes and Oil Spills:
- Crude oils vary from source to source, containing different proportions of hydrocarbons ranging from methane (natural gas) to light materials similar to gasoline and heavy materials that resemble asphalt. Differences in the chemical composition of crude oil and/or refined petroleum products influence their environmental fate following their release into the environment.
- The tens of thousands of different compounds that make up oil can only be biodegraded by communities of microorganisms acting together.
- Microbes can biodegrade up to 90% of some light crude oil, but the largest and most complex molecules - similar to the ones that make up road asphalt - are not very biodegradable.
While microbes can be "counted on to biodegrade oil over time," the report says the process may not be fast enough to prevent ecological damage. For this reason, immediate containment or physical removal of the oil is an important first line of defence.
"The presence of oil-degrading microbes does not necessarily mean that environmental conditions are ideal for oil biodegradation. Just as crops grow faster with the right amount of light, water and fertilizer, microbes that dine on oil do their job much more quickly when environmental conditions are optimal," says the report.
Important factors that can affect the rate of natural attenuation include:
- the physical and chemical nature of the oil;
- availability of nutrients and oxygen (or other electron acceptors);
- water temperature and pressure;
- pH and salinity of the water; and
- the composition of the microbial community
Bioremediation - helping nature along
Dr. Lee has conducted pioneering research in the field of bioremediation - the enhancement of microbial degradation rates for the removal of contaminants. By determining the factors that may limit optimal rates oil degradation such as nutrient limitation and providing strategies to overcome them, a number of oil spill countermeasures have been developed and validated by Fisheries and Oceans Canada research. For example, this research has resulted in operational guidelines for marine oil spill bioremediation sanctioned by the International Maritime Organization. In addition to bioremediation by nutrient enrichment, other remedial techniques such as phyto-remediation, enhanced dispersion by chemical oil dispersants, and oil-mineral aggregate formation, are being continually developed, refined and tested for use in intertidal regions including wetlands, cold temperate waters, and the Arctic. The aim is to accelerate natural recovery processes to protect our marine environment and its living resources.
Gulf of Mexico - Deepwater Horizon Oil Spill
The COOGER research team was requested to provide scientific advice and technical field support for response operations under the Unified Command following the Deepwater Horizon Oil Spill. Thousands of gallons of chemical dispersants - similar to dish detergent - were used in the Gulf of Mexico to break the oil down into smaller droplets that would be diluted to less toxic concentrations within the water column. Besides reducing the potential for the oil to reach sensitive coastal wetland habitats, this process enhanced the availability of the oil to oil-degrading microbes. The DFO science team provided support to the spill response operations by monitoring oil droplet size in the water column following the addition of chemical dispersants from both aircraft and subsurface injection.
"The consensus now is that microbial degradation was a major process responsible for cleaning up the Deepwater Horizon spill…much of the oil spilled has already been broken down by this process," says Dr. Lee. There is evidence of recovery within the Gulf of Mexico ecosystem. As part of the food web, the oil-degrading bacteria are consumed by other micro-organisms that are, in turn, eaten by larger predators. When the oil is gone, the oil-degrading microbes have less food and eventually return to their lower, pre-spill abundance.
The ongoing research following the aftermath of the Deepwater Horizon spill is also providing answers to some of our concerns in Canada. The subsurface blowout at 1,500 metres depth and the addition of dispersants resulted in the formation of a subsurface oil plume at a depth greater than a thousand metres, where the water temperature is approximately four degrees Celsius, similar to that found in our northern waters. According to Dr. Lee, there is now "conclusive evidence" that low temperatures may not be as significant as previously thought in retarding the degradation of oil in the marine environment.
Resilience and preparedness
Research is improving our understanding of how efficient the natural environment is at self-preservation. However, the impacts of man-made spills can sometimes be too much for nature to handle, and for this reason it is important to be prepared to lend a hand. "The ocean is more resilient than we think. In almost all cases following a major oil spill, although it may be slow, there is evidence of significant habitat recovery over time," says Dr. Lee. "Understanding how fast oil is naturally biodegraded and studying what cleanup techniques do or don't work under varying conditions is helping us inform science-based decisions and operational guidelines to assist those responsible for oil spill response."
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