How ocean microbes are secretly contributing to global warming

Nitrous oxide packs about 273 times the heat- trapping power of carbon dioxide over a century, according to the IPCC.

A new study shows that ocean microbes can make this gas even when more oxygen is present than scientists expected, and that fresh organic particles can widen the zones where it forms.

Where ocean microbes live

The work focused on oxygen minimum zones, layers of seawater where dissolved oxygen is scarce and patchy. These zones sit off places like California and Mexico and they host busy microbial communities.

The work was led by Claudia Frey, a biogeochemist at the University of Basel (UB). Her research explores how ocean microbes drive nitrous oxide emissions in low-oxygen waters.

Ocean microbes and emissions

In these waters, microbes use denitrification, a stepwise pathway that turns nitrate into nitrite, nitric oxide, nitrous oxide, then nitrogen gas. Each step helps cells capture energy when oxygen is limited.

The researchers tested whether bacteria would take a shorter route when extra nitrite was available. They did not. The microbes kept running the full pathway, which shows that the shortcut does not save energy in real seawater.

Microbes defy oxygen

The team found nitrous oxide production at oxygen levels higher than many earlier lab tests suggested.

Instead of one tidy curve, the response jumped around because different microbes share and compete for resources.

Fresh organic matter, small dead algae and other edible particles for microbes, raised the oxygen tolerance of the bacteria.

When particles were plentiful, production carried on in places that would otherwise seem too oxygenated.

Climate and ozone impact

Open ocean and shelf waters together supply about 40 percent of natural nitrous oxide emissions, according to a global synthesis. That share means small shifts at the edges of low oxygen zones can echo through the climate system.

Atmospheric nitrous oxide reached record highs in 2023, according to a WMO update. Rising fertilizer use and warming seas point the trend in the wrong direction for now.

Nitrous oxide also weakens the protective ozone layer. A landmark analysis showed that it is the dominant ozone depleting emission in this century. The chemistry links climate warming with stratospheric risks in one molecule.

“The emission of this almost forgotten greenhouse gas is decisive for the global climate,” said Dr. Frey. Her team argues that these marginal zones deserve far closer attention.

From fieldwork to models

For six weeks at sea off California and Mexico, the team worked around the clock to collect hundreds of water samples from different depths.

The researchers kept samples cold and free of oxygen to preserve the original microbial state while the ship rolled through tropical waters.

They then folded the results into an ecosystem model that simulates many microbial guilds without forcing a fixed oxygen cutoff.

The model reproduced the patchy behavior seen at sea and showed that fresh particles can extend nitrous oxide production into waters with more oxygen.

New insights on ocean microbes

First, the study confirms that many denitrifying microbes do not tap external nitrite when it is added. That finding clarifies why the long pathway dominates and it helps explain measured rates in the field.

Second, the work shows that organic particle quality matters, not just quantity. Fresh phytoplankton material boosted activity more than smaller or older particles, which likely mirrors what microbes actually eat offshore.

The ocean also makes nitrous oxide through ammonia oxidizers that prefer oxygen rich water. Scientists still lack a full stoichiometric map for those pathways, so their yields remain uncertain in many places.

Physical mixing can shuffle oxygen and particles on short timescales, and simple models can miss those pulses. Coupling these microbial rules into three dimensional circulation models is the next logical step.

Ocean microbes are driving emissions

The edge between oxygenated and oxygen poor waters acts as a switching region for multiple metabolisms. When particles sink in, microbes gain fuel and can tolerate more oxygen than textbooks implied.

That edge is growing in many coastal regions as warming and nutrient runoff expand low oxygen layers. Tracking these margins will improve nitrous oxide forecasts and help set better coastal management plans.

This study links careful ship work with a model that respects ecological competition. It replaces a simple oxygen threshold with a richer picture in which food supply, microbe type, and oxygen all share control.

For climate watchers, the message is direct. If particle-rich waters widen the nitrous oxide factory floor, emissions from the sea may be higher and more variable than many budgets assume today.

The study is published in the journal Nature Communications.

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