A new role for nitric oxide in greenhouse gas formation

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 by-product, as previously thought, reported Jonathan D. Caranto, a chemistry postdoctoral researcher at Cornell University, on last week at the ACS 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.