Sunlight may trigger an abiotic route for producing nitrous oxide (N₂O) in oxygenated surface water, according to a 2025 study. The proposed process, called photochemodenitrification, offers a possible addition to microbial explanations for oceanic N₂O—but its contribution to emissions at ocean or global scale has not been quantified.
What is the proposed mechanism?
Nitrous oxide is a potent greenhouse gas and an ozone-depleting agent. The new study describes a way it may form in illuminated water without microbes carrying out the reaction. Researchers call that proposed pathway photochemodenitrification.
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In experiments summarized by the Georgia Institute of Technology and Chemistry World, researchers exposed water samples from several Spanish locations and the Baltic Sea to ultraviolet light in quartz vials. N₂O production tracked the intensity of UV irradiation, and adding a biocide did not suppress the effect. These observations support a light-associated process that was not dependent on living microbes in the samples.
The team also added nitrate and nitrite labeled with nitrogen-15 and detected labeled N₂O. That result supports the proposed conversion of those nitrogen compounds into N₂O under irradiation. The isotope result and UV response are evidence for a mechanism in sampled water; they do not measure how much N₂O the process produces in the ocean as a whole.
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How does it compare with microbial N₂O production?
Microbes remain established candidates for N₂O production in different water conditions. The reported photochemical route is an additional possibility, not a replacement for microbial explanations.
| Candidate source | Conditions described in the reporting | What the evidence establishes |
|---|---|---|
| Microbial production during ammonia oxidation | Oxygenated water; associated with ammonia-oxidizing bacteria and archaea | A familiar microbial explanation for N₂O in oxic water. The reporting notes that these organisms do not favor light-exposed conditions. |
| Microbial production during nitrate reduction | Oxygen-poor water | A microbial route described in the reporting; it does not account for every possible source. |
| Photochemodenitrification | Light-exposed, oxygenated surface water | UV-associated, abiotic N₂O formation was observed in the reported water-sample experiments. Its environmental-scale contribution is not established. |
As Scott Wankel of Woods Hole Oceanographic Institution put it in Chemistry World: “For a long time we’ve known that whatever nitrous oxide was getting out into the atmosphere was made in the upper [ocean] layer, and we thought that was nitrification by ammonia-oxidising bacteria and archaea – but they don’t like light,”
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Does this explain a mysterious surge in greenhouse gas?
It may help explain some oceanic N₂O that familiar microbial mechanisms do not capture, but the available reporting does not show that photochemodenitrification caused a measured surge. Nor does it establish the pathway’s share of ocean emissions or the global atmospheric total. Researchers were still working to determine precise figures, according to Chemistry World.
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The distinction is important: demonstrating that a reaction can happen in sampled water is not the same as measuring its rate across diverse natural waters and scaling that rate to a global flux. The findings point to a source worth accounting for; they do not show that it drives all, or even a specified fraction, of oceanic N₂O emissions.
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What the finding means—and what remains open
The study adds a non-biological possibility to the picture of how N₂O can form in illuminated surface waters. Jennifer Glass, a professor of Earth and Atmospheric Sciences at Georgia Tech, said in the same report: “This just goes to show that sometimes it’s not biological.”
That conclusion is appropriately narrow. The experiments support abiotic production under the tested conditions, while the pathway’s importance in natural settings and its contribution to atmospheric N₂O remain unresolved. The finding broadens the list of mechanisms researchers need to consider; it does not yet quantify the role of this one.
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