16.2 THIS IS HOW WE DO IT: When 200,000 tons of methane disappears, how do you find it?

Even in the midst of an ecological catastrophe, there can be opportunities for scientific thinking and problem solving. Such situations, however, can require fast thinking about how to approach a problem, particularly when a real understanding of the issues is necessary to reduce further impact of the catastrophe and to guide management strategies.

An example of just such a catastrophe occurred in April 2010. An oil rig in the Gulf of Mexico was drilling a well about a mile under the surface of the water when it exploded and caused a massive leak of crude oil. It took nearly three months to cap the well, during which time more than 200 million gallons of oil poured into the Gulf.

The oil spill caused extensive damage to the coastal and marine environments and was responsible for killing thousands of animals, including birds, dolphins, sea turtles, mollusks, and crustaceans. The longer-term consequences of the oil spill for the region—including the eight U.S. national parks threatened—are not yet clear.

Methane was the most abundant hydrocarbon dumped into the Gulf—with almost 200,000 tons released by the spill. As scientists worried about the impact of the gas and oil on the ecosystem, they needed to monitor the area in order to map where the methane was and where it was headed.

Researchers set out to build a three-dimensional map of the Gulf in the regions above and around the site where the drilling had been. At 207 locations, they lowered sensors into the water and collected water samples at a large number of depths, from the surface down to the Gulf floor. This enabled them to identify where in the water column the methane was and how concentrated it was.

Writing in the journal Science, the team reported something quite unexpected. Just a couple of months after the leak had been sealed, they discovered that a huge proportion of the methane released in the spill was gone.

Two explanations for the methane disappearance seemed possible. Perhaps, as the researchers tried to track the flowing gas and oil underwater, they had simply lost it. Or, they wondered, could populations of bacteria be consuming the methane?

The researchers made a testable prediction. If bacteria were consuming the methane, there would be a chemical trace of their metabolic activities. Specifically, the amount of dissolved oxygen in the water should have decreased significantly. One consequence of such metabolic activity—particularly on such a huge scale—would be that the bacteria would consume large amounts of oxygen as they burned the methane for fuel. If, on the other hand, the methane had simply flowed somewhere else in the Gulf, there would be no drop in the dissolved oxygen.

Sure enough, as the researchers sampled the water and evaluated oxygen concentrations, they noted an unprecedented, significant drop in oxygen saturation—down from 67% to 59%. This suggested that bacterial populations were indeed proliferating rapidly and “mopping up” the oil.

Making some calculations, the researchers found that the amount of missing oxygen was in fact almost exactly equal to the amount required by bacteria to consume the amount of methane that had leaked from the well. “The math worked out scary good,” is how one researcher put it.

Twenty years ago, researchers would have had to culture any bacteria they found in order to identify them. But scientists of today were able to bypass this difficult step. Using DNA sequencing methods, they were able to test the microbes (and pieces of microbes) in the Gulf. In these sequences, they found genetic fragments previously identified in methane-eating microbes.

Moreover, the bacterial DNA they found was related to several previously observed oil-eating bacterial species from the Gulf of Mexico. These bacteria can live in the Gulf because natural oil—equivalent to as much as a million barrels of oil per year—seeps out of the ground. The researchers hypothesized that because the bacteria were already present in the Gulf, the populations of microbes were able to respond extremely quickly to the spill.

By virtue of having methane-consuming microbes already in place, the Gulf of Mexico was particularly suited to a rapid, effective response to an oil spill. In other places, however, such as the numerous Arctic drilling locations, there is not the same sort of natural oil seepage. And with no similar populations of methane-eating bacteria already in place, the impact of an oil spill on these habitats might be quite different.