Issue Date: August 28, 2017
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 can also lead to the release of nitrous oxide (N2O), a greenhouse gas that can also destroy the ozone layer. In 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 Cornell University researchers at the ACS national meeting. Postdoc Jonathan D. Caranto and his adviser, Kyle Lancaster, studied nitrification in Nitrosomonas europaea, which is the dominant ammonia-oxidizing bacteria species in wastewater treatment plants. One enzyme converts NH3 to hydroxylamine (NH2OH). Previously, researchers thought that a second enzyme, hydroxylamine oxidoreductase (HAO), then oxidized NH2OH to NO2–. Under anaerobic conditions, NO2– 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.USA2017, DOI: 10.1073/pnas.1704504114). A third enzyme would then take NO to NO2–. Depending on conditions, some NO could escape and undergo side reactions to form N2O. Meanwhile, the race is on to identify the enzyme that oxidizes NO to NO2–.
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