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

By Janet Raloff

Web edition: August 3, 2011

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.