Out of thin air: scientists pursue nitrogen fixers with an aim to harness their secrets—and feed the world

Science News,  April 12, 2008  by Susan Milius

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Air is a big tease. Nothing against oxygen, of course, but air is 78 percent nitrogen. Nitrogen is often the deal-breaker for life on Earth, the nutrient that sets the limit for how much of what grows where. Yet even a bonanza of airborne nitrogen passing through lung or leaf does neither animal nor plant a bit of good: One of life's most precious resources just blows away unused with every breath.

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Nitrogen wafts around in the air as paired atoms ([N.sub.2]) locked together chemically with a robust triple bond. Despite a great need for the element, the bodies of living things complex enough to have cells with a nucleus--paramecia and potatoes and people alike--have no natural way to break that bond. Here's where humanity and their kin are routinely humbled by green slime. A roster of "simple" life forms, such as cyanobaeteria floating in water or the rhizobia group of bacteria lurking in soil, breaks that bond. This feat, called nitrogen fixation, turns [N.sub.2] into user-friendly ammonia.

Since 1920, the Haber-Boseh industrial process has let people sunder nitrogen's triple bond as long as there's energy available to raise temperatures to 400[degrees] to 500[degrees] Celsius and pressures to 200 atmospheres. Your basic pond scum fixes nitrogen at room temperature and everyday atmospheric pressure.

Certain plants have come up with a tidier solution. By themselves, soybeans, peas, alder trees and others can't fix nitrogen any better than a person could. Instead they lure immigrant microbes to move in, and do the job for them.

In a border-crossing as delicate as any in human society, the microbes and the plants exchange signals and test chemical bona tides until the immigrants settle down, often in specialized lumps or pockets within the plant, and start fixing nitrogen. With help from their new friends, those plants get fertilizer out of thin air.

It's enough to turn a person soybean-green with envy. Making fertilizer via the Haber-Bosch process in order to grow crops takes an enormous amount of energy. And energy costs are soaring, not to mention that burning fossil fuels is increasing concentrations of greenhouse gases, and that a growing global population means more and more demand for food. For more than a third of the world's people, more food means more artificial fertilizer. If only food crops could use the nitrogen from [N.sub.2] in the air.

"People are always asking me when we're going to get nitrogen-fixing wheat," says Allan Downie of the John Innes Centre in Norwich, England, who authored a recent article about plant-microbe signaling in the Annual Review of Plant Biology. It's not that easy, notes Downie, who started studying nitrogen fixation during the 1980s and sees a long way left to go.

The good news is that science is picking up the pace. Explorations of both plants and their microbes have found new, unsuspected diversity in nitrogen fixing and given scientists more partnerships to study for clues on how to engineer the process. Researchers are also refining their knowledge of how legumes use a chemical Craigslist to find and negotiate with potential microbe workers. Science is apprenticing itself to the masters, crowding in to watch each nuance of the process. Even if the masters are just dots in the dirt.

FIXERS There's power in those dots, says David Dalton of Reed College in Portland, Ore. Some of them, such as the cyanobacteria, drift in the sea and process so much nitrogen they are now recognized as a major force in ocean chemistry.

Much of the nitrogen in the old-growth Douglas fir forests of the Pacific Northwest could be coming from Nostoc cyanobaeteria, Dalton says. Several Lobaria lichens include Nostoc in their shaggy, green forms, which after some 80 years can establish abundant colonies high in the trees. Dalton, a tree-climber, says, "It's like somebody dumped a trainload of exploding lettuce."

Other nitrogen fixers form loose associations with plants, nestling near the roots or moving into tissues without any obvious specialized accommodations. One of the most famous of these, now called Gluconacetobacter diazotrophicus, turned up inside sugarcane plants in Brazil in 1988. It belongs to a bacterial group known for producing acetic acid, but under the right circumstances, this species makes enough nitrogen to boost sugarcane growth.

The richest partnerships, though, involve more specialized structures, such as separate tissues inside the plant. Cycads, which Dalton describes as looking "like squatty palms," grow little bumps as cyanobacteria condos. And one oddball genus of flowering plants, Gunnera, accepts pockets of cyanobacteria in its stems. Cut a Gunnera stem just below one of its umbrella-sized leaves and look for the green blobs.

School textbooks may feature bean plants in the diagram of nitrogen fixation, but the Frankia genus of bacteria evoke nodules in un-beanish plants, such as alder trees and bayberries. The nitrogen fixers, looking "extremely skinny," live in clusters of nodules on the roots, Dalton says.