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A New Role For Nitric Oxide In Greenhouse Gas Formation

Aug 26, 2017

Fertilizer runoff from farmlands can create algae blooms that cause hypoxic dead zones in rivers, lakes, and oceans. But the runoff also can lead to the release of nitrous oxide (N2O), a greenhouse gas that can destroy the ozone layer.

After washing off farmlands, fertilizer runoff ends up in soil and wastewater. In these soils and during wastewater treatment, microbial enzymes convert fertilizer ammonia (NH3) into other nitrogen compounds through a process called nitrification. Nitric oxide (NO) is a key intermediate in the nitrification pathway rather than a byproduct, as previously thought, reported Jonathan D. Caranto, a chemistry postdoctoral researcher at Cornell University, on Thursday at the American Chemical Society national meeting in Washington, D.C.

The results could help scientists better understand the nitrogen cycle: “We are now more confident that nitric oxide is a universal intermediate for all ammonia oxidizers, aerobic and anaerobic,” comments Lisa Y. Stein of the University of Alberta. The findings should also improve modeling of nitrogen cycling in wastewater treatment and perhaps point to ways to control N2O emissions.

Caranto and his adviser, Cornell chemistry professor Kyle Lancaster, studied hydroxylamine oxidoreductase (HAO) from Nitrosomonas europaea, which is the dominant ammonia oxidizing bacteria species in wastewater treatment plants. Another enzyme converts NH3 to hydroxylamine (NH2OH). Previously, researchers thought HAO then oxidized NH2OH to NO2, which under anaerobic conditions gets reduced to N2O, yielding some NO from incomplete catalysis.

Instead, Caranto and Lancaster found that HAO converts NH2OH to NO (Proc. Natl. Acad. Sci. USA 2017, DOI: 10.1073/pnas.1704504114). Another enzyme would then take NO to NO2. But depending on cellular conditions, some NO could escape and undergo side reactions to form N2O and other nitrogen compounds.

Meanwhile, the race is on to identify the enzyme that oxidizes NO to NO2. The two primary candidates are nitrosocyanin, a member of the cupredoxin family, and cytochrome c’-beta, said Caranto, who spoke during a session organized by the Division of Inorganic Chemistry.