Microbes Metabolize Hydrocarbons
Mary Anderson's presentation on June 4, encompassing three main points:
1. Microbial Degradation of Oil in the Gulf of Mexico and Oxygen Depletion
2. Dispersants
3. Looming Questions
a. Existence of Oil Plumes Disputed
b. How Will Oil Spill Affect Hypoxic Zone at Mouth of Mississippi River?
c. Potential of Hurricanes to Disperse Oil Plumes
1. Microbial Degradation of Oil in the Gulf of Mexico and Oxygen Depletion
The BP oil leak may be pouring as much as 19,000 barrels of oil a day into the Gulf of Mexico.
Every ocean in the world contains communities of naturally occurring bacteria that degrade oil.
Petroleum has thousands of compounds. It is complex. The petroleum-degrading bacteria and fungi in the sea are also complex. They use hydrocarbons as fuel. As a result, they emit carbon dioxide (CO2). The microbes break down the ring structures of the hydrocarbons in seaborne oil using enzymes and oxygen in the seawater.
The Gulf of Mexico has an indigenous population of microbes well adapted to oil, because of the heavy volume of marine traffic, daily oil spills and natural seeps. In the right conditions, microbial populations can grow exponentially in days. They will keep metabolizing the hydrocarbons as long as they have oxygen. When oxygen is depleted, an anaerobic environment is created, and they do not survive.
That explains why oxygen depleted, anaerobic environments can develop locally in the sea. In deepwater, where there's less mixing with the surface waters to provide fresh supplies of oxygen, a dead zone can result . It's not exchanging with the atmosphere. Once the oxygen is gone you can’t replace it.
Measurements of oxygen depletion of as much as 30 percent in water near the oil in the Gulf of Mexico suggest that microbes are degrading the oil.
It takes weeks or months or years for the microbes to break down hydrocarbons, depending on the possibility looms that oxygen levels could fall so low as to create large dead zones, especially at the seafloor. That's the big worry,
Scientists say that they need to know the size of the oil release. They say that they can’t make any kind of calculation of potential oxygen demand or understand how the microbes will work and how quickly, until they know the amount of oil being spilled in the sea.
Microbial degradation of hydrocarbons happens most efficiently near the surface where warm-water bacteria thrive. Microbial growth is inhibited in colder, deeper waters. Scientists estimate that metabolism slows by a factor of two or three for every 10 degree[s] Celsius drop in temperature. The deeper the oil, the more slowly oil will degrade.
The BP oil is deep in the sea, as we know.
The Amoco Cadiz spill off the coast of France in 1978 is an interesting example regarding microbial degradation of the oil. What occurred in 10 days aerobically, took 365 days to occur anaerobically.
In sediment, whether it is sediment in the Louisiana marshland or sediment on the deep ocean floor, there is a lack of oxygen. In this case, anaerobic microbes—organisms that live via sulfate rather than oxygen—metabolize the hydrocarbons
2. Dispersants
The reason BP released chemical dispersants on the oil slick above and below surface of the sea was to break the oil into smaller droplets that bacteria could consume more easily. When oil is in very small droplets, degradation takes place much more quickly.
Dispersants have never been used in an oil leak of this size a mile under the ocean, and their effects at such depth are largely unknown. The dispersants may kill the microbes that will degrade the oil, or they may be promoting microbial growth and thus speeding the depletion of oxygen to a dangerous extent.
The dispersant BP is using is Corexit 9527A. It contains the solvent 2-butoxyethanol, which is a known human carcinogen and toxic to animals and other life.
EPA has just told BP that it can no longer spray dispersants on surface water and it can inject only 15,000 gallons per day into the subsurface.
3. Looming Questions:
a. Existence of Oil Plumes Disputed
BP questions the existence of the plumes.
Scientists are finding enormous oil plumes in the deep waters of the Gulf of Mexico, including one as large as 10 miles long, 3 miles wide and 300 feet thick in spots.
The research vessel Pelican reported subsurface plumes of oil at 2,300 feet to 4,200 feet in depth.
There is a tremendous amount of oil in multiple layers, three or four or five layers deep in the water column, according to scientists. They are worried that the oxygen level could eventually fall so low as to kill off much of the sea life near the plumes. If the 30 percent drop in oxygen levels near some of the plumes-if that rate continues, in several months, the levels could be dangerous to animals.
Much of the oil emerging from the well could be lying far below the sea surface.
The scientists on the Pelican mission suspect that the heavy use of chemical dispersants may have broken the oil up into droplets too small to rise rapidly.
NOAA has sent a survey vessel to answer the question: Are there huge, miles-long oil plumes spreading underwater from the Deepwater Horizon leak?
b. How Will Oil Spill Affect Hypoxic Zone at Mouth of Mississippi River?
The largest hypoxic (less than 2 ppm, or less than 2 milligrams per liter, dissolved oxygen) zone currently affecting the United States, and the second largest hypoxic zone worldwide, is the northern Gulf of Mexico adjacent to the Mississippi River.
The dead zone has seasonally-depleted oxygen levels (< 2mg/l). Most aquatic species cannot survive at such low oxygen levels. The oxygen depletion, or hypoxia, begins in late spring, reaches a maximum in midsummer, and disappears in the fall.
c. Potential of Hurricanes to Disperse Oil Plumes
We’ve already entered the hurricane season in the U.S. and it will last until the end of November. Hurricanes could disperse oil in various directions, depending upon where they hit. Hurricanes swirl in a counter-clockwise direction and could therefore push the oil toward the shore or back out into the Gulf.