Scientists Discover ‘Dark Oxygen’ on the Ocean Floor Generated—Surprisingly—by Lumps of Metal

Twelve thousand feet under the ocean surface is a world of eternal midnight. No sunlight can penetrate to this depth to promote photosynthesis, so no plants are producing oxygen there. Yet, the life-supporting gas is abundant in this darkness-cloaked region, thanks to an unlikely oxygen factory: potato-sized, “battery rocks” on the seafloor.

New research published this week in the journal Nature Geoscience reveals that nature has devised a way to produce oxygen without the involvement of plants. It’s “an amazing and unexpected finding,” Daniel Jones, a researcher at the National Oceanography Center in the United Kingdom who wasn’t involved in the study, tells CNN’s Katie Hunt.

Previously, scientists had understood oxygen to be the product of life itself, namely created by photosynthesizing autotrophs such as plants and algae. But the new study overturns that simplistic narrative. In fact, the discovery was so astonishing that, when lead author Andrew Sweetman first measured this “dark oxygen” in the Pacific Ocean’s Clarion-Clipperton Zone in 2013, he dismissed it outright.

“I just ignored it, because I’d been taught—you only get oxygen through photosynthesis,” Sweetman, an ecologist with the Scottish Association for Marine Science, tells Victoria Gill of BBC News. “Eventually, I realized that for years I’d been ignoring this potentially huge discovery.”

Tracing the origins of ‘dark oxygen’

Sweetman was part of a research team aiming to measure how much oxygen was being consumed by organisms at the bottom of the ocean. What he saw in the seawater scooped from the deep was a rise in oxygen levels, instead of the predicted decrease. Initially, he thought his sensors were faulty and sent them back to their manufacturer for recalibration—four to five times. 

In 2021, the opportunity came for Sweetman to return to the same spot, this time to survey the seabed for a deep-sea mining firm called the Metals Company. Using a different technique, Sweetman’s team once again measured dramatic increases in dissolved oxygen, writes Scientific American’s Allison Parshall. He finally had good grounds to take the numbers seriously—and motivation to hunt for the oxygen’s real source.

First came a process of elimination. The scientists ruled out the possibility that microbes were behind the act: In lab tests simulating the seafloor, the researchers killed off any organisms in the water with mercury chloride. Yet the oxygen levels still rose.

That region of the CCZ was dotted with rock-like lumps known as polymetallic nodules, which form when metals such as manganese and cobalt precipitate out of the water and coalesce around shell fragments or shark teeth. Scientists zeroed in on these metals as the source of the oxygen, but they still weren’t sure how they created the gas. For instance, the oxygen wasn’t due to radioactive substances in the nodules splitting water molecules or the decomposition of oxygen-containing minerals like manganese oxide.

A breakthrough came when Sweetman was watching a documentary about deep-sea mining in a hotel bar in São Paulo, Brazil. According to CNN, he heard a character calling the nodules “a battery in a rock.” An idea zinged into his brain: Could the oxygen be generated electrochemically?

If you put a standard AA battery into saltwater, you’ll observe bubbles and hear a fizz—that’s the generation of hydrogen and oxygen gases as the electricity splits water molecules, a process known as electrolysis. The researchers suspected the same thing was happening in the deep ocean, thanks to the polymetallic nodules. Indeed, voltage measurements on the nodule surface confirmed the rocks carried as much juice as 0.95 volts. It was a little short of the theoretical requisite of 1.5 volts for seawater electrolysis, but the researchers suspected that the nodules clustering together helped the reaction overcome this hurdle.

This “is one of the most fascinating things [I and my lab] have ever worked on,” Franz Geiger, a physical chemist at Northwestern University and co-author of the paper, tells Scientific American.

The naturally occurring rock-batteries force scientists to broaden the theories of life’s evolutionary trajectory. Scientists had assumed that complex life evolved after photosynthesizing cyanobacteria cooked up enough oxygen on early Earth. Perhaps life could have leveled up in pockets, feeding off oxygen in unlikely places.

“I think we therefore need to revisit questions like: Where could aerobic life have begun?” Sweetman tells Live Science’s Sascha Pare. The same process could also be playing out in other ocean worlds, such as Enceladus and Europa.

Targets for deep-sea mining

Whether they had a hand in shaping life’s evolution or not, the nodules are important to marine life today. A 2013 survey of the CCZ found that half of the documented megafauna species—those large enough for the naked eye to make out—were present only on the nodules. The deep sea remains a poorly studied region for biodiversity, with up to two thirds of its dwellers still unknown to science.

However, the CCZ is already attracting interest for its deep-sea mining potential. The very nodules that perform this oxygen-generating alchemy are also a mining target for their richness in rare earth metals. These elements, which include cobalt, nickel, copper and manganese, are important constituents in batteries, smartphones, wind turbines and solar panels.

Deep-sea mining, which is still in an experimental phase rather than a full-scale industry, has garnered immense backlash. So far, 27 countries and 52 corporations have pledged support for a moratorium on deep sea mining. More than 800 scientists and policy experts have petitioned against the practice. Mining activities in international waters, such as the CCZ, are regulated by the International Seabed Authority, which issues permits for mining exploration.

The Metals Company, the same firm that sponsored the new study, was the first to receive authorization for a mining trial in 2022, to the surprise and dismay of conservationists. To date, 16 deep-sea mining contracts have been awarded to private companies to survey approximately 400,000 square miles of the seafloor in the CCZ.

Studies like Sweetman’s are important to uncover the full scale of what’s at stake. “I don’t see this study as something that will put an end to mining,” he tells BBC News. But what researchers can do, he adds, is gather the data that can help stakeholders make informed decisions, and possibly spark new ideas about how to minimize the fallout, if—or when—the industry ever gets off the ground.

Shi En Kim | | READ MORE

Shi En Kim is a Washington, D.C.-based freelance science journalist. Her work has appeared in National Geographic, Scientific American, the Atlantic, Popular Science and others. In 2021, she interned at Smithsonian magazine as an AAAS Mass Media Fellow.