Making Dinitrogen

The mechanics of a key biogeochemical reaction that affects atmospheric chemistry have been identified by an international research group. The research shows how one of two major routes for putting dinitrogen into the atmosphere happens via a microbiological process known as anammox, for anaerobic ammonium oxidation.

In the new work, Boran Kartal of the Netherlands’ Radboud University Nijmegen and colleagues demonstrate that microbes turn ammonium into dinitrogen anaerobically through a pathway that involves the intermediates nitric oxide and hydrazine (Nature, DOI: 10.1038/nature10453).

To pin down the reaction sequence, Kartal and colleagues worked with the bacterium Kuenenia stuttgartiensis. They grew the microbe in a bioreactor and studied which genes the microbe transcribed, which enzymes it made, and the activity of those enzymes.

Kartal and colleagues found that K. stuttgartiensis first uses a reductase enzyme to convert NO₂^– to NO. Then a three-protein hydrazine synthase complex combines NO and NH₄⁺ to form N₂H₄. Finally, a hydrazine dehydrogenase enzyme converts N₂H₄ to N₂. The electrons for the first two steps of the process come from the final oxidation of N₂H₄ to N₂.

Scientists used to think that N₂ in the atmosphere came only from denitrification, or reduction of NO₃^– to N₂. But about 15 years ago, anammox came to light, and now researchers believe that as much as half of atmospheric N₂ comes from that process, says Daniel J. Arp, a professor of botany and plant pathology at Oregon State University. Anammox is also of interest as a way to remove nitrogen from wastewater streams.

Although there was some evidence that allowed researchers to guess at the mechanics of the anammox process,“there was not a fully integrated pathway with supporting evidence for each step,” Arp says. The new work provides that complete pathway. Arp was not involved in the research.

Arp is particularly excited to learn more about the hydrazine synthase complex that Kartal and coworkers discovered. “It will be fascinating to learn about the mechanism” of hydrazine formation and what intermediates are formed, how electrons are transferred, and which metals may be involved, he says.