Cornell researchers have discovered a way for ammonia oxidizing archaea (AOA), one of the most abundant types of microorganisms on Earth, to produce nitrous oxide, a potent and long-lasting greenhouse gas.
AOA are everywhere - in soil, in water, in extreme environments, on human skin - and uncovering their chemical process is an important step toward understanding the chemistry of climate change, said Kyle Lancaster, professor of chemistry and chemical biology in the College of Arts and Sciences (A&S).
"This chemistry seems to be the cost of doing business for AOA," he said. "Humans eat sugar and exhale carbon dioxide - these organisms are blowing out nitrous oxide and it's something we must be mindful of as we add more and more nitrogen to ecosystems."
Nitrous oxide makes up about 6% of greenhouse gas emissions from human activities, while carbon dioxide makes up about 80%, according to the Environmental Protection Agency. Although there's nothing to be done about the naturally occurring nitrous oxide produced by AOA, Lancaster said, knowing how it's produced fills in an important piece of overall climate chemistry and sets researchers up to learn even more.
"This is globally proliferated biochemistry that is critical to achieving the delicate balance we rely on for a viable planet," Lancaster said.
Lancaster is the corresponding author of "Nitrous Oxide Production via Enzymatic Nitroxyl from the Nitrifying Archaeon Nitrosopumilus maritimus," which published Jan. 17 in Proceedings of the National Academy of Science. Doctoral candidate Robert Voland is the first author; Héctor Abruña, the Emile M. Chamot Professor of Chemistry and Chemical Biology (A&S) and Hongsen Wang, senior research associate in the Abruña lab, contributed.
Nitrifiers - including AOA and ammonia oxidizing bacteria (AOB) - live by oxidizing ammonia as their cellular fuel, the researchers wrote. By doing so, they drive key transformations in the nitrogen cycle and therefore play an essential role in ecosystems, even though many products and byproducts of their metabolisms are environmental and atmospheric pollutants of concern, including nitrous oxide.
In experiments led by Voland studying proteins from the AOA species Nitrosopumilus maritimus, the researchers identified an enzyme that is structurally and genetically unique to AOA: Nmar_1354, a type of multicopper oxidase.
"Finding this enzyme was interesting because up until we found it, we didn't know of any copper-containing enzyme that oxidized hydroxylamine," Voland said.
He found that hydroxylamine, an AOA metabolite, is changed by this enzyme into nitroxyl, an extremely reactive molecule, which then forms nitrous oxide, the greenhouse gas, and nitrogen gas, the most abundant chemical in Earth's atmosphere. This process explains how AOA produces nitrous oxide, the researchers said, and identifies the mechanism for formation of the enigmatic nitroxyl.
Nitroxyl is of great interest, Lancaster said, because of its reactivity (which also makes it hard to study). It even reacts with itself, which is how nitrous oxide is ultimately generated by this protein.
The reactivity of nitroxyl, although of great interest, is only one of the challenges in this line of study, Lancaster said. Another is the nature of AOA, which grow slowly, don't achieve a high cell density, and are difficult to grow in the lab.
Abruña, along with Wang, collaborated with special instruments for detecting small molecules by mass.
"Mass spectrometry is an important tool in chemistry but it's sometimes hard to detect very low-mass molecules," Lancaster said. "They have a set-up specifically to look at products of ammonia oxidation - fortuitously for us."
There's been debate about the origin of nitrous oxide coming from AOA and, in fact, from AOB, Lancaster said. It's controversial because in the microbial ecology community, which studies how microorganisms interact with each other and the environment, some say these reactions happen abiotically, with no enzyme involved.
"We can argue about that because the enzyme is there, the substrates are there, the reaction happens - there's got to be a contribution," Lancaster said. "That's the attitude we're taking here, too, especially since these proteins are some of the most abundant proteins expressed by these organisms, common to all of them."
The study received support from the National Institutes of Health, the Department of Energy and the National Science Foundation.
Kate Blackwood is a writer for the College of Arts and Sciences.
