Nine months after the Deepwater Horizon oil spill, researchers John Kessler of Texas A&M University and David Valentine of UC Santa Barbara finally uncovered optimistic data in the haze of tar balls and oil-slicked water.
Though experts thought it could take anywhere from years to decades for the natural gases released in the spill to retreat to their normal levels, since the spill, the amount of methane in the Gulf went from 100,000 times the normal level to virtually no extra methane in the ocean.
All thanks to bacteria – specifically, methane-consuming bacteria called methanotrophs.
Methanotrophs convert methane into energy by “taking in methane and attacking it with oxygen,” Valentine said.
Methanotrophs typically grow at low abundance on the warm shores of continents because most natural methane releases occur at these spots.
When the Deepwater Horizon oilrig exploded in April and released 200,000 metric tons of methane into the water, the northern Gulf of Mexico was already home to a small community of methanotrophic bacteria. Kessler, Valentine and other researchers ventured to the Gulf in June of 2010 to sample the methane content of the water and atmosphere.
“On a molecule by molecule basis, methane was approximately half of all the molecules that were emitted,” Kessler said.
The researchers then put together three hypotheses of where that methane would go and how long it would take to be consumed by bacteria.
The first hypothesis was that the huge amount of methane in the water would “be both dissolved in the water column and emitted to the atmosphere.”
The first hypothesis was proven only partially correct, as none of the methane was actually emitted into the atmosphere. The methane stayed dissolved in plumes of natural gas that were 200 meters thick and about a mile deep into the water.
“Methane dissolves in water like sugar dissolves in tea,” Kessler said. “But had it happened in more shallow waters, it would have been released more easily to the atmosphere.”
Their second hypothesis was that microbial consumption of the methane would “contribute to low oxygen levels in the northern Gulf of Mexico.”
“We found that starting back in June we were seeing significant dips in oxygen … when we came back in September we were using dips and sags in oxygen to track where the plumes had gone,” Valentine said. “We found that there was a significant depression of oxygen.”
Their third hypothesis was that if methane stayed in those deep plume layers and was consumed slowly by the bacteria at those depths, the methane would remain in the water for years.
“When we were there in June, we measured their metabolic rates and it was pretty slow,” Valentine said. “That indicated to me that it [methane depletion] would be very slow, and the methane would be around for a long time.”
However, this hypothesis was also shown to be incorrect. The researchers found that the metabolism rates were very low in June, and then had a sharp peak at the beginning of August before dropping back to normal levels by the middle and end of August.
Their return expedition to the Gulf in September revealed “not a time point, but an end point,” said Kessler. The methane from the oil spill was gone.
This data allowed the researchers to make a startling conclusion.
“The methane must have been consumed between those few months,” Valentine said.
The next goal for researchers involves tracking the relaxation of the microbial community back to its baseline. According to their report, the population of methanotrophic bacteria decreased back to normal as the amount of methane was rapidly consumed. When the ocean encounters and recovers from the release of tons of methane, does it retain the ability to respond should it happen again?
Methane releases are common in the natural world; the 83 days of the Deepwater Horizon oil spill is actually short in comparison to natural plumes that emit methane for years.
“Releases of methane, natural or man-made, have similar characteristics and will be met with a similar bacterial response,” Kessler said. “Our research gives hints into how this behaves, but there are many more variables into how this system will react, especially in the future.”
AMY STEWART can be reached at firstname.lastname@example.org.