The flat, black seafloor of the Pacific Ocean’s Clarion-Clipperton Zone (CCZ) is littered with what appear to be chunks of coal. These inconspicuous mineral deposits, called polymetallic nodules, are home to a unique deep-sea ecosystem, much of which scientists have yet to catalog. The deposits are also a prime target for companies looking to deep-sea drill because they contain minerals like manganese and cobalt, which are used in batteries.
Now researchers have discovered that these valuable tubers do something remarkable: They produce oxygen, and they do it without sunlight. “This is a completely new and unexpected discovery,” said Lisa Levin, a professor emeritus of biological oceanography at the Scripps Institution of Oceanography, who was not involved in the study. The oxygen on Earth is typically thought to come from organisms that convert sunlight, carbon dioxide, and water into oxygen and sugar. The idea that some of the gas could come from these dead minerals and be produced in complete darkness “is completely at odds with what we traditionally think about oxygen production,” says Jeffrey Marlow, a microbiologist at Boston University and co-author of the study, which was published Monday in the journal Nature. Natural Sciences and Geology.
The story of the discovery dates back to 2013, when deep-sea ecologist Andrew Sweatman encountered a frustrating problem. He was part of a research team trying to measure how much oxygen was being consumed by organisms on the seafloor of the CCZ. The researchers sent landers about 4,000 meters up to create sealed chambers on the seafloor that could track how oxygen levels in the water decreased over time.
But the oxygen levels didn’t drop; on the contrary, they rose significantly. Sweatman thought the sensors were broken and sent the instruments back to the manufacturer to be recalibrated. “This happened four or five times” over the course of five years, says Sweatman, who studies seafloor ecology and biogeochemistry at the Scottish Association for Marine Science. “I literally told my students, ‘Throw these sensors in the bin. They just don’t work.’”
In 2021, he was able to return to the CCZ on a research expedition sponsored by deep-sea mining company Metals Company. Once again, his team used deep-sea landers to create sealed chambers on the seafloor. The chambers contained sediments, nodules, organisms, and seawater, and controlled oxygen levels. Sweatman and his team used a different technique to measure oxygen this time, but they saw the same strange results: oxygen levels rose dramatically.
“Suddenly I realized I was neglecting this very important process and I felt like kicking myself,” Sweetman says. “My mindset completely changed and I focused on the case.”
“My first thought was microbiology, because I’m a microbiologist,” Marlow says. It wasn’t a far-fetched idea: Scientists had recently discovered ways that microbes like bacteria and archaea can generate “dark oxygen” in the absence of sunlight. In lab tests that mimicked seafloor conditions in the new study, the researchers poisoned seawater with mercury chloride to kill the microbes. But oxygen levels still rose.
The researchers reasoned that if the dark oxygen didn’t come from a biological process, it must come from a geological process. They ran tests and ruled out a number of possible hypotheses, such as that radioactivity in the nodules was separating the oxygen from the seawater or that some other environmental factor was separating the oxygen gas from the manganese oxide in the nodules.
One day in 2022, Sweetman was watching a video about deep-sea mining when he heard nodules referred to as “batteries in a rock”—a phrase that Gerard Barron, CEO of Metals Company, likes to use. That prompted Sweetman to wonder: “Could the minerals in these nodules somehow act as natural geobatteries?” If so, they could split seawater into hydrogen and oxygen through a process called electrolysis. (You can try this at home by dropping a small battery into salt water and watching the hydrogen and oxygen gas bubble up.)
The phrase “batteries in a rock” was just a metaphor, as far as the scientists knew. Just because nodules contain minerals used to make batteries doesn’t mean they are electrically charged themselves. To create a charge, positive and negative ions must be somewhat separated within the nodules, creating a difference in electrical potential. To see if that happened, Sweetman flew to Illinois to test the nodules’ electrical charge with Franz Geiger, a physical chemist at Northwestern University.
“It’s amazing that there was almost a volt on the surface of these nodules,” Sweetman says. For comparison, a AA battery has a voltage of about 1.5 volts. The researchers’ main theory is that the charge splits seawater to create oxygen, though they haven’t yet tested whether turning off the electrical charge of the nodules stops oxygen production. The scientists plan to test that in the future.
Geiger believes that polymetallic nodules become charged as they grow, and that different minerals are deposited irregularly over time. The nodules form around a small object, such as a shark tooth. If you open one up, “they look like cross-sections of tree rings” or like layers of an onion, Geiger says. These mineral layers grow by just millimeters per million years, and the types of minerals deposited change over time, potentially creating a charge gradient between each layer that creates electrical potentials. That doesn’t explain why the nodules have different charges on their surface, but Geiger thinks their porosity exposes some of their inner layers.
Until now, Geiger said, rocks had never been known to carry a charge in this way. “This is one of the coolest things my lab and I have ever worked on,” he said.
It’s still unclear whether the nodules naturally produce oxygen on the seafloor (and to what extent). In most experiments, the oxygen concentration in the chambers remained stagnant after two days. That could suggest that the lander changed something in the environment—for example, by kicking up sediment—that led to oxygen production. It’s also possible, says Marlow, that oxygen production eventually stopped because of a “bottle effect” in a closed chamber. “The products build up, the reactants leave, and then the reaction stops more or less. But in an open system… the process might be more consistent.”
The results are “very strange” and raise many questions, says Bo Parker Jorgensen, a marine biochemist at the Max Planck Institute for Marine Microbiology in Bremen, Germany. (Jorgensen was not involved in the research, but was a peer reviewer of the paper.) Natural Sciences and Geology.) He doubts that these nodules produce oxygen if left undisturbed on the seafloor. “It seems that the electrolytic reaction on the surface of the manganese nodules does indeed produce oxygen,” he adds. That in itself is a very interesting observation, and, to my knowledge, has not been observed before.
Researchers don’t yet have any idea what role this oxygen produced by the nodules might play in the CCZ’s seafloor ecosystems. Environmental studies have shown that the nodules and the sediments surrounding them are home to deep-sea life: everything from single-celled microbes to “megafauna”—animals visible to the naked eye, such as fish, starfish and worms. About half of the megafauna catalogued in the 2013 survey were found in the nodules alone.
Like most of the deep ocean, the seafloor in the CCZ is a “poorly understood ecosystem,” Levin says. “We haven’t even discovered, let alone studied, most of the species in the deep sea.”
There is talk of deep-sea mining projects in the CCZ that would extract nodules from vast areas of the seabed. The International Seabed Authority (ISA), which administers the seabed in international waters, is currently discussing rules and regulations for nodule mining and other deep-sea targets. Twenty-seven countries, including 26 members of the Asia-Pacific Alliance, have called for a moratorium, precautionary moratorium or ban on deep-sea mining.
“I don’t think this research is a nail in the coffin for deep-sea mining, that was never the intention,” says Sweetman. “It’s just another thing we have to consider now when we have to decide, ‘Are we going to mine the deep ocean or not?’ For me, that decision has to be based on sound scientific advice and input.”
