Marine microbes prove potent greenhouse gas emitters

The unsung climate warmers are single-celled organisms once considered a type of bacteria.

Earth’s oceans emit an estimated 30 percent of the nitrous oxide, or N2O, entering the atmosphere. Yet the source of this potent greenhouse gas has puzzled scientists for years. Bacteria — long the leading candidate — can generate N2O, although the seas don’t seem to contain enough to account for all of the N2O that the marine world has been coughing up. Now researchers at the Woods Hole Oceanographic Institution and MIT offer a more likely candidate: archaea.

The seas contain more than enough of these single-celled microbes (once thought to be a type of bacteria). Moreover, the Massachusetts-based researchers’ new data show, these microbes have a penchant for transforming ammonia into N2O. And there’s no shortage of ammonia in the oceans.

“Basically, we’ve figured out that the most abundant organism on the planet makes a very potent greenhouse gas,” observes microbial oceanographer Alyson Santoro of Woods Hole, who led the new investigation. And not only are archaea the most abundant microbes in general, she says, but they’re also the most numerous residents of the marine world. Her team’s findings appeared online July 28 in Science.

Despite their vast numbers, the presence of marine archaea only came to light in 1992, she says. A little more than a dozen years later, a pivotal paper emerged suggesting that these microbes might break down ammonia. “What we’ve now shown,” Santoro explains, “is that archaea can make N2O” — and plenty of it, at least in the lab.

So what? Apportioning oceanic N2O production to archaea doesn’t change the quantities of this gas available to help warm Earth’s climate. But Santoro observes that knowing its likely source is “an important first step in predicting how N2O emissions could change as the oceans warm, or acidify — or low oxygen areas expand.” All of these conditions are underway and likely to increase as global warming continues.

Indeed, her group is curious about how archaea might respond to growing dead zones, like the one that forms in the Gulf of Mexico, each year (and at hundreds more coastal sites around the world). The researchers’ concern: As oxygen concentrations in seawater fall, ammonia-oxidizing (or degrading) bacteria ramp up their production of N2O.

Janet Raloff

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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