Hottest Fixer: Undersea-vent microbe sets nitrogen record

A spherical microbe from the weird world of hot-water ocean vents has trumped the nitrogen-processing powers of all organisms previously studied.

Like some soil microbes and bacteria living in pea plants and their relatives, the microbe known as FS406-22 turns plain nitrogen (N2) into a form that other living creatures can use, explains Mausmi P. Mehta of the University of Washington in Seattle. However, FS406-22 does the chemistry at 92°C (198°F). That’s 28°C above the record set by the previous champ, a microbe collected from sea sediments near Naples, Italy.

FS406-22 is also the first nitrogen fixer identified in an undersea vent, say Mehta and her Seattle colleague John A. Baross. Vents differ in what forms of nitrogen are available in the water they release, says microbial oceanographer David M. Karl of the University of Hawaii in Honolulu. But if most of the nitrogen is the virtually inert N2 form, the activity of FS406-22 “would be of great ecological significance,” he says. Living creatures use nitrogen in complex molecules, such as DNA and proteins, and nitrogen shortages limit growth in some habitats.

Microbes with enzymes that work at the high water temperatures near vents might have “biotechnological potential,” says nitrogen-cycle specialist Douglas Capone of the Oceanographic Laboratory at Villefranche in France. “There is a fair amount of research interest on thermally stable enzymes,” he says.

With the limited availability of input from the sunlit world, seafloor vents tend to harbor creatures that have novel chemistry. Mehta set out to find any vent organism that fixes nitrogen under those extreme conditions.

She and Baross pursued this quest for several years during a series of research cruises to the underwater Axial volcano in the northeast Pacific. Scientists on board sent down robotic collection vehicles with arms that poked into gashes in the seafloor. They brought back samples that “look like water,” says Mehta, but have a “nasty, rotten-egg smell.”

Mehta put vent-water samples into containers of various nutrient soups that only a nitrogen-fixing microbe could love. In hundreds of containers where the only nitrogen was in the N2 form, she ended up with nothing. In 2004, however, one batch of containers turned cloudy with a thriving microorganism that Mehta designated FS406-22.

Under a microscope, FS406-22 looks “mundane,” Mehta admits. It clumps into groups of two or three cells. When Mehta and Baross sequenced a bit of DNA, they found that FS406-22 is an anaerobic archaean, a one-celled organism that looks like a bacterium but has chemical similarities to multicelled organisms. Furthermore, FS406-22 thrived in nutrient broth that Mehta had dosed with antibiotics that kill bacteria.

The new microbe may give clues to the distinguished history of nitrogen fixing, which probably evolved before photosynthesis. Mehta and Baross analyzed the three genes that encode the proteins in the enzyme complex that drives nitrogen capture. They appear to be the most ancient ones yet seen and may resemble the nitrogen-fixing genes in the common ancestor of archaea, bacteria, and multicellular life, the researchers say in the Dec. 15 Science.