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In the Deep Ocean, Dark Oxygen May Mysteriously Emanate from the Bottom
Key Take-Aways on Dark Oxygen: Dark oxygen is produced on the seafloor by a process other than photosynthesis.A new study published in Nature Geoscience in July 2024 suggests it’s unclear what causes dark oxygen, but a possible source could be polymetallic nodules.This discovery could rewrite our understanding of how aerobic (oxygen-respiring) life evolved on Earth. And it could even point to new possibilities for extraterrestrial life, perhaps on Saturn’s watery moons.The traditional story of Earth’s oxygen is simple: There was none until the first photosynthesizers evolved 3.4 billion years ago. Then, bacteria started converting sunlight into sugar for their own growth, with algae and plants later following suit, and they all pumped oxygen into the atmosphere as a happy byproduct. There is no other natural source for the gas that we — and most life on this planet — need for survival.That’s what Andrew Sweetman, a seafloor ecologist and biogeochemist at the Scottish Association for Marine Science, learned in his studies. However, when he measured elevated oxygen levels at the bottom of the Pacific Ocean in 2013, he was understandably baffled. His lander had returned from 13,000 feet beneath the surface, a gloomy realm untouched by sunlight, and there were bubbles in its payload of sediment.He and his colleagues dismissed it as a fluke, but then in 2015, it happened again. “The evidence just stacked up and up and up,” Sweetman recalls. “Eventually, we had to say, there’s something going on here.” They knew the claim would be controversial, but they couldn’t ignore their own data: Oxygen was being produced on the seafloor by some process other than photosynthesis.What Is Dark Oxygen?In July 2024, the team finally published their findings in Nature Geoscience, calling the phenomenon dark oxygen. It’s unclear what’s causing it — Sweetman is preparing to investigate further next year — but the geology of the ocean bottom itself suggests one possible source: polymetallic nodules. These potato-sized lumps, composed of various metals, litter the seafloor where the researchers detected oxygen in the Clarion-Clipperton zone east of Hawaii. They contain layers of manganese, cobalt, lithium, nickel, and copper. These are all common battery components, and they may behave like underwater batteries, generating enough electric current to split water molecules into hydrogen and oxygen atoms.That, at least, is Sweetman’s hypothesis. If true, it could rewrite our understanding of how aerobic (oxygen-respiring) life evolved on Earth. And it could even point to new possibilities for extraterrestrial life, perhaps on Saturn’s watery moons. “If the process is electrochemical in nature, and it happens at the bottom of our oceans,” Sweetman says, “what’s to stop it happening at the bottom of an ocean world like Titan or Enceladus?” Other Theories on Dark OxygenAlternatively, it could be the other way around, with bacteria generating oxygen and that oxygen in turn promoting the growth of polymetallic nodules. In March 2025, researchers in China posted a preprint of a study (which has not yet been peer-reviewed) in which they report that two strains of deep-sea bacteria can reduce nitrate to ammonia, a process that seems to produce dark oxygen.Still another alternative, some scientists say, is that Sweetman’s findings are simply wrong. The Nature Geoscience paper drew criticism from several groups, most notably The Metals Company, a deep-sea mining company that partly funded Sweetman’s research and hopes to mine polymetallic nodules for use in batteries.In September 2024, five scientists employed by The Metals Company posted a preprint rebuttal, which also has not yet been peer-reviewed. The paper — led by Michael Clarke, a marine biologist and environmental manager for the company — argues that the oxygen likely came from air bubbles trapped in Sweetman’s equipment. That would mean it originated in the atmosphere after all, not on the seafloor. Thus, they wrote, “the hypothesis that polymetallic nodules produce oxygen can be wholly rejected.” In response to the critiques, Sweetman says, “I don’t think people realize just how much we tried to refute the data.” Sweetman argues that his lander’s benthic chamber — a device used to measure oxygen exchange in seafloor sediment — has enough fail-safe features to ensure the experiments weren’t fouled by stowaway air bubbles. And he expects the next round of research with improved equipment, tentatively planned for early 2026, will validate his team’s original findings.Supporting Deep-Sea EcosystemsThere’s more riding on this than it may seem. If dark oxygen is, in fact, being produced, it could support deep-sea ecosystems that we know little to nothing about. Conservation groups, which had already opposed the extraction of polymetallic nodules, quickly latched onto Sweetman’s work as justification for a moratorium on mining projects. The day the study came out, Lisa Levin, a distinguished professor at the Scripps Institution of Oceanography, told the Deep Sea Conservation Coalition that dark oxygen represents “a new ecosystem function that needs to be considered when assessing the impact of deep-sea mining.” But Sweetman remains agnostic. For now, we don’t know enough to say whether dark oxygen production is happening naturally since it's possible the polymetallic nodules only generate electric current when a lander swoops down and blows the sediment off them. And even if the process is natural, it’s unclear how ecologically important it may be (not to mention how mining might affect it). “Those questions are answered on page 942,” Sweetman says. “We’re only on page 3.”Article SourcesOur writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:Nature Geoscience. Evidence of dark oxygen production at the abyssal seafloorCold Spring Harbor Laboratory. Nitrate-driven dark oxygen production by diverse deep-sea microorganismsDeep Sea Conservation Coalition. Groundbreaking Discovery in the Deep Ocean – Oxygen Production by Polymetallic Nodules Targeted by the Deep-Sea Mining IndustryCody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.
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