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Out of Thin Air
Natural History, Sept, 2001 by David W. Wolfe
A very special group of microbes tap into the atmosphere's huge storehouse of nitrogen and make it available to the rest of us.
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Take a deep breath. Most of what is filling your lungs is not oxygen but nitrogen, in the form [N.sub.2] gas. Every cubic yard of the air around us, in fact, is about 80 percent [N.sub.2]. On the face of it, this abundance would appear to be an unqualified blessing, for nitrogen is an essential ingredient of life--a key component of amino and nucleic acids, the basic building blocks of proteins and our genes. There is a catch, however. This immense supply might as well be on Jupiter or Mars as far as most living things are concerned. No animal, plant, fungus, or protist has mastered the chemical art of converting the abundant gaseous form of nitrogen into a biologically useful one. In fact, if [N.sub.2] were the only nitrogen available, most species would quickly go extinct--like sailors dying of thirst while adrift on a life raft at sea. Fortunately, within the dark and hidden realm that is the ground beneath our feet--a realm generally ignored by us oversized, sunlight-and oxygen-dependent "surface chauvinists"--live a handful of bacterial species that can metabolize [N.sub.2] and thereby bring this nitrogen into the food chain.
Earth's first microbes, which emerged about 3.5 billion years ago, had no such ability. [N.sub.2] became available to them only when it was transformed into nitrate ([NO.sub.3], nitrogen combined with oxygen) during lightning strikes and meteor impacts. At that time, the energy and heat generated by these random events were the only forces in nature capable of breaking the powerful chemical bond that holds the two atoms of [N.sub.2] together. This sprinkling of nitrate from the heavens had some inherent limitations with regard to supporting life on earth, however. For one thing, nitrate is easily washed away by rain and often winds up in groundwater, rivers, and ultimately the oceans before any terrestrial organisms can make use of it. For another, once photosynthesis evolved (perhaps 3 billion years ago), the number of living things on earth expanded rapidly, creating a demand for usable nitrogen that exceeded this meager supply.
Enter a unique group of bacteria that "invented" a way to convert, or fix, [N.sub.2] gas into ammonia ([NH.sub.3], another usable form of nitrogen, this time in combination with hydrogen) without the intense heat and pandemonium that come with lightning bolts and meteor bombardment. When these bacteria die and decompose, or are consumed by other organisms, the nitrogen they managed to incorporate into their cells becomes available to the rest of us. Essentially all the nitrogen contained within the proteins and genes of plants, animals, and humans has, at one time or another, been funneled through these nitrogen-fixing microbes.
A single pinch of garden soil may contain 10,000 bacterial species, but probably no more than 100 to 200 species of free-living nitrogen fixers exist worldwide. Many of these are members of an ancient group of self-sufficient organisms known as cyanobacteria, which can fix both carbon (through photosynthesis) and nitrogen. Cyanobacteria are vitally important as "primary producers" at the base of the food chain in the oceans. On land, the most common and important nitrogen-fixing bacteria are those that live symbiotically within the roots of plants, providing their hosts with nitrogen and receiving in return the products of photosynthesis (carbon-laden, energy-rich sugars).
With 99 percent of soil organisms not yet named or studied, we still have much to learn about subterranean symbioses and the nitrogen cycle. Scientists frequently turn up new nitrogen-fixing microbes, some of which form partnerships with animals rather than plants. For example, bacteria living in the intestinal glands of a wood-boring mollusk known as the shipworm provide the animal with as much as one-third of its nitrogen. And recently, researchers identified a nitrogen-fixing spirochete bacterium that lives in the gut of termites.
All of the world's nitrogen fixers, whether free-living or symbiotic, rely on the same enzyme--nitrogenase--to do the job. Nitrogenase is a giant among enzymes, both in the literal sense (it is a huge and complex molecule) and in its significance for earth's biochemistry. Those of us who tend to fret about nature's fragile balance find it a bit disconcerting to learn that our planet's entire supply of nitrogenase could fit into a single large bucket. Lose this, and life as we know it would come to a screeching halt.
Modern molecular techniques have identified more than twenty bacterial genes involved in the manufacture and control of nitrogenase, while X-ray crystallography and other techniques have revealed the enzyme's structure: long, twisted chains of atoms, arranged like spaghetti, or a ball of yarn after a cat has been playing with it. Nitrogenase actually consists of two huge proteins that physically separate and reunite eight times in the course of 1.2 seconds as they work to break the bond between the two nitrogen atoms and convert one molecule of [N.sub.2] into two molecules of ammonia. Most chemical reactions occur in millionths of a second; a duration of 1.2 seconds is almost unheard-of, and it reflects how difficult nitrogen fixation is and how much energy must be expended to accomplish it. Nitrogen-fixing bacteria that can team up with plants or animals have the advantage of using the energy-rich sugars they receive from their hosts to help defray some of these manufacturing costs.
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