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October 3, 2011

Making Dinitrogen

Environment: Pathway involves microbial oxidation of ammonia via nitric oxide and hydrazine

Jyllian Kemsley

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Kartal and coworkers studied K. stuttgartiensis grown in a bioreactor. Courtesy of Boran Kartal
Kartal and coworkers studied K. stuttgartiensis grown in a bioreactor.

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 (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, the Kartal group 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, anaerobic oxidation of ammonium came to light as a second route. Now researchers believe that as much as half of atmospheric N2 comes from the anammox 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.

While 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.

For his part, Arp is particularly excited to learn more about the hydrazine synthase complex, which 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.

Chemical & Engineering News
ISSN 0009-2347
Copyright © 2011 American Chemical Society
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