The hunt for ‘dark’ oxygen and why it might be more common than first thought

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A startling discovery made public in July that metallic rocks were apparently producing oxygen on the Pacific Ocean’s seabed, where no light can penetrate, was a scientific bombshell.

Initial research suggested potato-size nodules rich in metals, predominantly found 4,000 meters (13,100 feet) below the surface in the Clarion-Clipperton Zone, released an electrical charge, splitting seawater into oxygen and hydrogen through electrolysis. The unprecedented natural phenomenon challenges the idea that oxygen can only be made from sunlight via photosynthesis.

Andrew Sweetman, a professor at the UK’s Scottish Association for Marine Science who was behind the find, is embarking on a three-year project to investigate the production of “dark” oxygen further. Sweetman and his team are using custom-made rigs equipped with sensors that can be deployed to depths of 11,000 meters (36,089 feet). The Nippon Foundation is funding the $2.7 million (2.2 million-pound) research project, which was announced Friday.

Uncovering dark oxygen revealed just how little is known about the deep ocean, and the Clarion-Clipperton Zone, or CCZ, in particular. The region is being explored for the deep-sea mining of rare metals contained in the rock nodules. The latter are formed over millions of years, and the metals play a key role in new and green technologies.

“Our discovery of dark oxygen was a paradigm shift in our understanding of the deep sea and potentially life on Earth, but it threw up more questions than answers,” Sweetman, the leader of his institution’s seafloor ecology and biogeochemistry group, said in a news release. “This new research will enable us to probe some of these scientific questions.”

Sweetman said the initial goal of the new project was to determine whether dark oxygen production was replicated in other areas of the CCZ where the nodules can be found and then untangle exactly how the oxygen was being produced.

Understanding the phenomenon better could also help space scientists find life beyond Earth, he added.

Oxygen in unexpected places

Oxygen is hard to produce without the continuous energy that comes from sunlight, but other scientists have also encountered unexpected oxygen molecules in remote, light-deprived places. Sweetman said that dark oxygen production may be a wider phenomenon that has been overlooked.

Emil Ruff, a microbiologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, detected oxygen in freshwater samples in Alberta tens to hundreds of meters beneath the Canadian prairie, a finding he and coauthors from the University of Calgary and the Woods Hole Oceanographic Institution reported in a study published in June 2023. In some cases, the dark oxygen had been isolated from the atmosphere aboveground for more than 40,000 years.

If oxygen is not continuously being added to an environment (by trees and plants, for example), it would eventually disappear.

“After 40,000 years or 30,000 years (separated from surface processes), there’s no reason really to think that there should be any oxygen left. Because oxygen is such a yummy electron acceptor, it usually either chemically oxidizes or microbially oxidizes,” Ruff said. “So what was it doing there?”

Similar to Sweetman, Ruff said he first thought atmospheric oxygen had contaminated his samples, which were drawn from 14 groundwater aquifers. Given the age of the samples, any oxygen would have reacted with other substances long ago and disappeared.

After patiently working in the lab and field, Ruff ultimately discovered that microbes in the water were producing oxygen. The microbes had apparently evolved an obscure but neat trick that allowed them to produce molecules in the absence of light.

Through a series of chemical reactions, the microbes were able to break down soluble compounds called nitrites, molecules made of one nitrogen and two oxygen atoms, to produce molecular oxygen in a process known as dismutation. The microbes also had the ability to use the oxygen to consume methane in the water for energy.

What’s more, Ruff found that the quantity of oxygen produced was enough to sustain other oxygen-dependent microbial life in the groundwater.

“Nature keeps surprising us,” he said. “There are so many things that people have said, ‘Oh, this is impossible,’ and then later it turns out it’s not.”

To investigate dark oxygen further, Ruff and his team traveled to a 3-kilometer-deep (9,500-foot-deep) mine in South Africa in August to sample water that had been trapped in the rock for 1.2 billion years.

Scientists already knew the water in the mine contained oxygen molecules, but it’s unclear how they were formed. Ruff and his colleagues are still studying the samples they took, but they have two hypotheses as to how oxygen molecules might be produced, he said.

The site is mined for gold and uranium, a radioactive metal. Radiolysis, the splitting of water molecules through radioactivity, is one of the possible ways oxygen is produced without sunlight. Alternatively, the production of oxygen could involve microbes in processes similar to those Ruff found in Canada’s groundwater.

Sweetman said on Friday the new project would also seek to understand whether any micriobial reactions played a role in dark oxygen production on the seafloor. In particular, the project will look into how hydrogen is released during the production of oxygen by the metallic nodules and whether hydrogen was used as an energy source for communities of microbes detected in parts of the deep ocean.

“We don’t have the mechanism, I think, completely wrapped up yet and we’ll need a lot of time to figure that out,” he said.

Ruff said he hoped to collaborate with Sweetman and other scientists involved in the dark oxygen research to understand how the chemical signature of the oxygen produced by seawater electrolysis differed from that produced by microbes or radiolysis.

Dark oxygen and the search for extraterrestrial life

Officials at NASA are interested in the research on dark oxygen production because it could inform scientific understanding of how life might be sustained on other planets without direct sunlight, Sweetman said.

The space agency wants to run experiments to understand the amount of energy required to potentially produce oxygen at higher pressures that occur on Enceladus and Europa, the icy moons of Saturn and Jupiter, respectively, he added. Those moons are among the targets for investigating the possibility of life.

Deep-sea mining companies are aiming to mine the cobalt, nickel, copper, lithium and manganese contained in the nodules for use in solar panels, electric car batteries and other green technology. Some companies have taken issue with Sweetman’s research.

Critics say deep-sea mining could irrevocably damage the pristine underwater environment and that it could disrupt the way carbon is stored in the ocean, contributing to the climate crisis.

The Metals Co. said it had submitted a rebuttal to Nature Geoscience, the journal that published the original research. The submission was undergoing peer review but has not been published yet, the company said.

Sweetman said he was aware of the critical reaction and would respond “through peer-reviewed channels.”

“We are completely convinced that this is an actual process going on at the seafloor,” he said.

Sweetman also said it was prudent to hold off exploiting resources on the seabed until the ecosystem was better understood.

Amy Gartman, a research oceanographer and global marine minerals project chief at the US Geological Survey’s Pacific Coastal and Marine Science Center in Santa Cruz, California, said the USGS has not observed any electrical phenomena in ferromanganese nodules examined so far. She was not involved in either Sweetman’s or Ruff’s research.

“Researchers are currently trying to replicate the phenomena reported by Sweetman and others,” she said. “Scientific research is a process and it may be some time before a conclusive answer is reached.”

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