Life on Earth is literally swimming in a sea of nitrogen, which makes up 80 percent of the atmosphere. But in one of nature's great ironies, the essential element is not available in an atmospheric form that plants and animals can use.
This is because the atoms in dinitrogen molecules, N2, are locked together in a triple chemical bond the toughest nut in chemistry and it takes the work of enzymes to sever nitrogen atoms and recombine with hydrogen in a form plants can use: ammonia, or NH3.
Now Utah State University biochemists are beginning to unlock the natural bacterial processes that "fix" atmospheric nitrogen into the soil, where it can be used by plants. If they succeed, scientists could replicate it to vastly improve a century-old industrial process that uses high temperatures and pressures to convert nitrogen to ammonia a principal ingredient in fertilizer.
"This is green chemistry. We are starting to illuminate for the first time how this enzyme can do it in mild conditions," said researcher Lance Seefeldt, a USU professor in the department of chemistry and biochemisty. He is a senior co-author on a paper published last month describing a method for identifying key steps in natural nitrogen reduction.
Seefeldt estimates that 2 percent of the world's fossil-fuel consumption is devoted to nitrogen reduction using the existing Haber-Bosch process. Replacing this process with one that replicates nature will mean far fewer resources are burned to make fertilizer.
"The agronomic and economic and, indeed, human significance of this process can be appreciated from the perspective that the 'fixed nitrogen' of ammonia, along with water, are generally the two limiting nutrients in crop production, and that the lives of about two-thirds of Earth's population depend on the ammonia produced by nitrogenase," said co-author Brian Hoffman, professor of chemistry at Northwestern University.
Nitrogenase are the bacterial enzymes that have evolved to break down nitrogen. Without them, life would not have prospered on Earth.
"Nitrogen is essential to our proteins, our DNA. Every living thing needs it. Where we get that nitrogen is the challenge. Nitrogen is the limiting element," Seefeldt said. For his work in nitrogenase, Seefeldt was honored last week as the recipient of the 2012 D. Wynne Thorne Career Research Award, USU's highest research prize.
"It is fair to say that professor Seefeldt is the most influential thinker in the area of nitrogenase enzymology in the world today," said Southern Methodist University provost Paul Ludden, an expert in microbial biochemistry. "Through brilliant experimental design, professor Seefeldt and his collaborators have been successful where others have failed. I have heard the word 'fearless' used to describe professor Seefeldt, and this is quite an appropriate attribution."
Before Haber-Bosch, named for the Nobel-winning chemists who invented and scaled up the process around World War I, global population had plateaued. Industrial ammonia changed everything, triggering the Green Revolution.
"It's had a huge impact. The growth in population is directly correlated with Haber-Bosch. It's feeding the world, but there are expensive inputs and pollution. More than 30 percent of the nitrogen in your body was fixed by Haber-Bosch," said co-author Dennis Dean, director of the Fralin Life Science Institute at Virginia Tech.
Figuring out how nature fixes nitrogen would be a monumental scientific achievement, and much progress has been made by the group led by Seefeldt, Dean and Hoffman.
"That makes me optimistic that we are on the cusp of unravelling the whole thing, perhaps within a decade," Dean said. The research team has refined a chemical methodology to trap and detect intermediate steps in nitrogen reduction, which Seefeldt called "black boxes." The team has been able to describe the molecular contents of three of 10 boxes involved.
Co-authors Zhi-Yong Yang, a doctoral candidate, and Brett Barney, a former USU postdoctoral fellow now at University of Minnesota, liken the effort to capturing single movie frames on a moving reel. The team now has the whole "reel"; they just need to define each frame of the "movie."
"Soon, we'll be able to see the entire picture," Yang said. "We'll be able to describe each step, at the mechanistic level, of one of the most vital processes to life on Earth."
The research, funded by the National Institutes of Health and the National Science Foundation, appears online in the Proceedings of the National Academy of Sciences. Co-authors include Northwestern's Dmitriy Lukoyanov.