Science & the Public

Janet Raloff
Science & the Public

Deep-sea plumes: A rush to judgment?

Studies yield data — but a paucity of information

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A new study released on Aug. 24 reported the discovery of a novel bacterium in diffuse, deep-sea clouds of oil that had been spewed by BP’s damaged Deepwater Horizon well. And according to the paper's lead author, a microbial ecologist at Lawrence Berkeley National Laboratory, the bacterium's appetite for oil is so voracious that hoards of it swarmed to a plume his team had been studying — and then gobbled it up. Completely.

That’s what Terry Hazen told me and many other reporters.

And it may be true. But he doesn’t “know” that. He can’t — yet.

Moreover, he didn’t say (at least to me) that he had any comparable data on the biodegradation of plumes other than the one he was studying. Yet many news accounts now report as fact the claim that all plumes are gone.

There are, perhaps not surprisingly, plenty of skeptics — including researchers now aboard ships who claim to still be monitoring plumes in the Gulf.

Which suggests that in an attempt to make sense of emerging data, some people are rushing to judgment. And in so doing, they risk muddling our understanding of the ecological ramifications of the massive BP accident.

There is reason to suspect that what happens in one subsea plume would occur in others roaming at comparable depths. Like urban lunch carts, they would all advertise meals to carbon-hungry microbes adapted to the Gulf’s oh-so-chilly (~5 °Celsius) waters. And, presumably, microbes mix throughout the Gulf, at least within a particular ecosystem, like its cold, dark bottom.

But “science” doesn’t assume all plumes will behave similarly. It tests hypotheses that they might. And until confirmation that such hypotheses are correct, we’re left with only educated guesses as to how broadly new data should be extrapolated.

John Kessler of Texas A&M University put it succinctly when he noted to me that: “There’s a difference between data and information. And everybody’s trying to make information out of their [Gulf plume] data as quickly as they can” — even when they may not have many data to work from.

Researchers with the Woods Hole Oceanographic Institution reported August 19 on ring-based molecules, such as benzene, within deep plumes of BP crude. This family of hydrocarbons appeared to be degrading slowly.

Five days later, Hazen’s team reported measurements of different, straight-chain hydrocarbons — alkanes — that were quickly disappearing from the water, both at sea and, later, in lab experiments. The consortium of researchers that Hazen works with found a residual cloud of microbes where a plume had been — along with detritus of the bugs’ feeding: a floc of wastes (including exopolysaccharides), dead cells, oxidation products, and changes in the chemistry of the water.

However, Kessler observes, most constituents of crude oil that entered the Gulf from the BP accident have not been surveyed. For instance, he says, “In no way, shape or form did he [Hazen] measure all of the thousands of different hydrocarbons that were emitted from this [BP] disaster.” The alkanes that his team focused on may have been degrading rapidly as bacteria dined on them, he acknowledges. But some hydrocarbons have much longer half-lives — “on the order of years, sometimes even decades,” he notes. “And one of those that we study a lot in our lab is methane.”

About that methane
A one-carbon molecule that is the principle component of natural gas, methane “was by far the largest quantity of any single hydrocarbon emitted from this disaster,” Kessler says. The relatively small compound has proven especially resistant to breakdown and elimination by microbes, he says. Perhaps, he argues, it’s because bugs derive more energy per bite when dining on somewhat bigger, more complex molecules.

But there’s another reason why methane may be especially long-lived at the depth of the wandering Gulf plumes, posits Hazen. Bugs that chow down on these molecules are known as methanotrophs, “and we did look for these,” he says. (He’s knows these bacteria well and even has patents, through the Department of Energy, for use of methanotrophs to clean up polluted groundwater.)

In the Gulf-plume samples, Hazen notes, “We did not find any methane monooxygenase — the enzyme that is principally associated with methanotrophs — or any significant signature of methanotrophs.” But that makes sense, he adds, because in the cold, high-pressure environment at which these deep plumes formed, methane doesn’t really exist as methane but as hydrates, aka methane ice. And methane “is not biodegradable as ice,” he says.

Methane hydrates are slightly buoyant, though, and once they ascend to around 400 meters below the surface, they’ll start to melt. By 200 meters depth, he says, methanotrophs should be munching away at the small hydrocarbon. And that’s where researchers interested in biodegradation of this pollutant should set their sights, he says.

For now, Kessler and others argue, it’s too early to say any plume — much less all plumes — have been thoroughly removed from the water. Such an assurance must wait, they say, until scientists confirm that something more than the hydrocarbon equivalents of cookies and steak have disappeared from the Gulf's crude-oil lunch carts

Up next: ‘Bug traps’ in Gulf to use BP oil as bait

Citations

Hazen, T.C., et al. Deep-sea oil plume enriches indigenous oil-degrading bacteria. Science, Published online August 24, 2010.
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