Can tiny ocean organisms offer the key to better climate modeling?

In the cobalt waters off San Diego, the key to tracking a powerful greenhouse gas drifts just below the surface.

Tiny ocean microbes living in oxygen-starved waters turn everyday nutrients into nitrous oxide (N₂O)—a compound better known as laughing gas, but far less funny for the planet.

“The gas traps roughly 300 times more heat than carbon dioxide (CO₂) and also eats away at Earth’s ozone layer,” says Xin Sun, assistant professor of biology at Penn’s School of Arts & Sciences. “Having better information on where and how N₂Os are made can help scientists forecast global emissions more accurately as the climate changes.”

Sun and her collaborators spent six weeks at sea studying the chemistry and ecology behind this process, sampling water from 40 to 120 meters deep in the Eastern Tropical North Pacific Ocean, one of the largest oxygen-depleted regions on Earth.

Sampling bottles sinks into the waters of the Eastern Tropical North Pacific, one of the ocean’s largest oxygen-starved zones. Researchers aboard the vessel used this device to collect seawater at precise depths, capturing microbes that turn common nutrients into nitrous oxide—a greenhouse gas roughly 300 times more potent than carbon dioxide.

Their work, published in Nature Communications, shows how microbial competition—not just raw chemistry—drives the production of N₂O and how even subtle shifts in oxygen or nutrients can cause sudden, dramatic jumps in greenhouse gas output.

“There’s a multistep pathway that starts with nitrate (NO₃⁻) and turns it into nitrite (NO₂⁻) before finally producing N₂O,” Sun explains. “And there’s another that skips straight from nitrite to N₂O. You’d expect the shorter one to win, but it doesn’t.”

Sun likens the microbe populations to two neighboring delis that both make bagels but start with different ingredients.

The first group, starting with nitrate, is like a full-service bakery that begins with flour—mixing, fermenting, and baking everything in-house. The second group, starting from nitrite, is more like a specialty shop that depends on finding premade dough drifting through the water.

Because flour (nitrate) is far more abundant than ready-made dough (nitrite), the longer, multistep pathway turns out to be more efficient.

Low-oxygen conditions generally favor N₂O production, but the team found that adding more oxygen doesn’t dampen production smoothly. Instead, oxygen shakes up which microbial “shops” dominate. “Oxygen doesn’t act like a dimmer switch,” Sun says. “It changes who’s in charge.”

While feeding microbes more nutrients might seem like a good way to boost production, the teams also found it can actually push the main N₂O-makers out of the picture, cutting gas release to nearly zero.

By letting microbial groups compete and collaborate inside a new model, the team captured these sharp ecological fluctuations that older, chemistry-only models smoothed over.

Their findings could refine climate models that predict sea-level changes, extreme weather, and changing ocean chemistry—and help identify which regions contribute the most greenhouse gas output.

Xin Sun prepares samples collected from the Eastern Tropical North Pacific aboard a research vessel. By adding stable isotope tracers to these vials, Sun and her team can track how different microbial groups convert nitrogen compounds into nitrous oxide, revealing how subtle shifts in oxygen and organic matter change the ocean’s chemistry.

Xin Sun is an assistant professor in the Department of Biology at the University of Pennsylvania’s School of Arts & Sciences.

Other authors include Daniel McCoy and Emily J. Zakem of Carnegie Institution for Science; Bess B. Ward of Princeton University; Claudia Frey, Moritz A. Lehmann, and Matthias B. A. Spieler of the University of Basel; Emilio Garcia-Robledo of the University of Cadiz; Ashley E. Maloney of the University of Colorado-Boulder; and Colette L. Kelly of Woods Hole Oceanographic Institution.

This work received support from the Simons Foundation (LS-FMME-00871981), the National Science Foundation (OCE-1657663, OCE-1657868, and DGE-1656518, and 2125142), and the Spanish Agency for Research.