Methane on the menu in the Gulf of Mexico?

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Methane on the menu in the Gulf of Mexico?

When Deepwater Horizon blew up and melted down in April, the wound it tore in the Earth’s crust released a gusher of crude oil, estimated at 4.2 million barrels, into the Gulf of Mexico.

The massive microbial munching of methane during the BP spill may be the only good news from the Deepwater Horizon disaster.

The blowout also released about 160,000 tons of methane. If you counted molecules in BP’s blowout, methane (CH4), the simple hydrocarbon that fuels stoves, furnaces and electric generators, was the single most abundant one.

But a report published in today’s Science shows that BP’s methane was totally devoured by microbes in the Gulf of Mexico, leaving less than .01 percent of the methane to enter the atmosphere. “We measured the sea-to-air flux of methane and found it was completely negligible,” says first author John Kessler, an assistant professor of oceanography at Texas A&M University.

Within four months of the April 20, 2010, blowout, a population explosion among methane-eating bacteria native to the Gulf decomposed virtually all of the methane, mainly in deep water, says Kessler.

On a ship, man looking at tube attached to tank valve, man behind him bent over checking tubes

Study author John Kessler extracts a water sample from a device that detects changes in water conductivity and temperature with depth.
NOAA Pisces.

The study offered three lines of evidence that bacteria were “eating” the released methane:

  • Methane levels in the Gulf fell up to 10,000 times between June and October.
  • Methane-munching microorganisms became extremely abundant downstream of the blowout. “Over the summer, the methane degraders were higher than we have ever seen at any other place in the world,” says Kessler.
  • Dissolved oxygen in the water dropped as methane and oxygen reacted to form carbon dioxide and water, Kessler says. “Once we summed up all the lost oxygen in the area of the methane plume, we saw that it could only be explained by a complete [microbial] consumption of this methane.”


Although oxygen depletion is already a concern in the Gulf’s “Dead Zone,” the average loss was only 3 percent, Kessler says.

In a previous study, ethane and propane, two other natural gases that BP also released, decomposed even faster than methane, and were no higher than background levels by early fall. In both studies, Kessler collaborated with David Valentine of the University of California at Santa Barbara.

Cool news for your atmosphere

In the short term, spilled methane is less environmentally dangerous than crude oil, but it can pose a global warming problem in the long term, since a molecule of methane stores much more heat than a molecule of carbon dioxide.
Methane seeps are frequently found at ocean floors, where methane from decomposition enters the ocean. And unfathomable quantities of frozen methane are stored beneath the seabed.

So inquiring minds want to know: If and when this methane enters the ocean, could it reach the atmosphere and accelerate global warming?

Large multi-level ship, top festooned with scientific instruments, at dock; with a smaller boat docked alongside.

Photo: John D. Kessler/TAMU
Pisces, a research ship of the National Oceanic and Atmospheric Administration, was a floating laboratory to study Deepwater Horizon’s aftershocks.

The giant Deepwater spill contained too little methane to affect atmospheric levels, says Kessler, “but it does simulate a very energetic release from a seep or a methane hydrate, and so we were interested in using it as an analog for understanding how a massive submarine release of methane might behave.”

Although the microbes-eat-methane story provides a rare bright spot in BP’s ecological disaster, it’s not clear what would happen in shallow water, and in places lacking natural methane and a ready supply of methane eaters.

“The Gulf of Mexico has many natural methane seeps,” says Kessler, “that probably account for why Gulf waters are populated with these microorganisms, which are ready to degrade methane once there is a massive restocking of their ‘buffet.’ How this may play out at another place, without the natural seeps, I’m not sure.”

Within four months, bacteria had spawned enough offspring to devour essentially all of the added methane in the Gulf. “But if the bacteria are at lower abundance, would this take five months or two years? We don’t know.”

– David J. Tenenbaum

Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. A Persistent Oxygen Anomaly Reveals the Fate of Spilled Methane in the Deep Gulf of Mexico, J.D. Kessler et al, Science, 7 Jan. 2011.