Researchers are returning to the Clarion-Clipperton Zone in the Pacific Ocean to investigate how metallic nodules produce oxygen without sunlight, a phenomenon dubbed 'dark oxygen' that could sustain deep-sea life. This discovery has sparked debate over the environmental risks of deep-sea mining for critical metals. The team aims to confirm the process and address criticisms from mining interests.
In 2024, scientists discovered that potato-sized metallic nodules on the ocean floor in the Pacific and Indian oceans generate oxygen through an unexpected mechanism, challenging the long-held view that large-scale oxygen production requires photosynthesis and sunlight. These nodules, found in areas like the Clarion-Clipperton Zone—a prime target for deep-sea mining—may support diverse ecosystems at depths exceeding 4,000 meters, including microbes, sea cucumbers, and carnivorous anemones.
The finding has intensified scrutiny of plans to extract nodules for valuable metals such as cobalt, nickel, and manganese, essential for renewable energy technologies. Deep-sea mining companies, including The Metals Company, have contested the results, arguing in a published paper that the observed oxygen likely originated from surface air trapped in the researchers' equipment and that the nodules lack sufficient energy for electrolysis of seawater.
Leading the new expedition is Andrew Sweetman of the Scottish Association for Marine Science. "Where’s the oxygen coming from for these diverse animal communities to thrive?" Sweetman asked at a press briefing. "This may be a pretty significant process, and that’s what we’re trying to figure out."
The team hypothesizes that metal layers within the nodules create an electric current—measuring up to 0.95 volts, akin to an AA battery—that splits seawater into hydrogen and oxygen. Though this voltage falls short of the typical 1.23 volts needed, clustered nodules might amplify it. To test this, researchers will deploy instrument-laden landers to depths of 10,000 meters, monitoring oxygen levels, pH changes, and acidity, which could indicate electrolysis.
Samples of sediments and nodules will undergo lab analysis, including DNA and RNA sequencing to study the up to 100 million microbes per nodule. "The vast diversity of the microbes remains a moving target. We’re always discovering new species," noted Jeff Marlow of Boston University. "Are they active? Are they shaping their environment in interesting and important ways?"
Further experiments will simulate deep-sea pressures—around 400 atmospheres, comparable to conditions that imploded the Titan submersible—in a high-pressure reactor, as explained by Franz Geiger of Northwestern University. The ultimate aim includes observing the reaction under an electron microscope with live microbes.
Sweetman counters critics by noting that in 65 experiments across the zone, 10 percent showed oxygen consumption while the rest indicated production, unlike deployments in other regions like the Arctic seabed. No surface oxygen anomalies were detected elsewhere. A rebuttal with this data is under peer review at Nature Geoscience.
"In terms of the commercial interest, there’s definitely an interest to try to silence this area of work," Sweetman said. Marlow added, "Regardless of the source and motivation of the comments, they need to be addressed."
As the United Nations' International Seabed Authority deliberates on mining regulations in international waters, U.S. President Donald Trump has advocated for starting extraction, and The Metals Company seeks a U.S. permit.