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A surprising discovery published in July that metallic rocks appear to be producing oxygen on the seafloor of the Pacific Ocean, where light cannot penetrate, was a scientific bombshell.
Initial research suggests that metal-rich, potato-sized nodules, found predominantly 4,000 meters (13,100 feet) below the surface in the Clarion-Clipperton Zone, release an electrical charge and split seawater into oxygen and hydrogen through electrolysis. The unprecedented natural phenomenon challenges the idea that oxygen can only be produced from sunlight through photosynthesis.
Andrew Sweetman, a professor at Britain’s Scottish Association for Marine Science who was behind the find, is launching a three-year project to further study the production of “dark” oxygen. Sweetman and his team use custom-built drilling rigs equipped with sensors that can operate to depths of 11,000 meters (36,089 feet). The Nippon Foundation is funding the research project to the tune of $2.7 million (£2.2 million), announced on Friday.
The Nippon Foundation and the Scottish Association for Marine Science announce the launch of a research project at a press conference at Scotland House. Pictured are (from left) Nick Owens and Andrew Sweetman of SAMS and Yohei Sasakawa, Chairman of the Nippon Foundation. -Alex Rumford/SAMS/The Nippon Foundation
The discovery of dark oxygen highlighted how little is known about the deep ocean, and in particular the Clarion-Clipperton Zone (CCZ). The region is being explored for 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 environmentally friendly technologies.
“Our discovery of dark oxygen was a paradigm shift in our understanding of the deep sea and potentially life on Earth, but it raised more questions than answers,” Sweetman, the head of his institution’s seafloor ecology and biogeochemistry group, said in a message release. “This new research will allow us to explore some of these scientific questions.”
Sweetman said the original goal of the new project was to determine whether dark oxygen production was replicated in other areas of the CCZ where the nodules are located, and then to decipher exactly how the oxygen was produced.
A better understanding of the phenomenon could also help space researchers find life beyond Earth, he added.
Oxygen in unexpected places
Without the continuous energy provided by sunlight, it is difficult to produce oxygen, but other scientists have also encountered unexpected oxygen molecules in remote, light-starved places. Sweetman said dark oxygen production may be a broader phenomenon that has been overlooked.
Emil Ruff, a microbiologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, discovered oxygen in freshwater samples in Alberta, tens to hundreds of meters below the Canadian prairies, a finding he and co-authors from the University of Calgary and the Woods Hole Oceanographic Institution reported in a study published in June 2023. In some cases, dark oxygen has been isolated from the surface atmosphere for more than 40,000 years.
If oxygen is not continuously supplied to an environment (e.g. through trees and plants), it would eventually disappear.
“After 40,000 or 30,000 years (surface processes aside), there is no reason to think that oxygen should still be present. Because oxygen is such a tasty electron acceptor, it typically oxidizes either chemically or microbially,” Ruff said. “So what was it doing there?”
Similar to Sweetman, Ruff said he initially believed that atmospheric oxygen had contaminated his samples taken from 14 aquifers. Due to the age of the samples, the oxygen may have reacted with other substances and disappeared a long time ago.
After patient work in the lab and on site, Ruff finally discovered that microbes in water produce oxygen. The microbes had apparently developed an obscure but clever 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 up of one nitrogen and two oxygen atoms, to create 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.
Additionally, Ruff found that the amount of oxygen produced was sufficient to sustain other oxygen-dependent microbial life in the groundwater.
“Nature always surprises us,” he said. “There are so many things where people have said, ‘Oh, that’s impossible,’ and later it turns out that’s not the case.”
To further study dark oxygen, Ruff and his team traveled to a three-kilometer-deep mine in South Africa in August to sample water that had been trapped in the rock for 1.2 billion years.
A team of scientists hunts for microbes at a study site in a 3-kilometer-deep mine in South Africa. The researchers examine brines full of life that have been isolated in rocks for 1.2 billion years and investigate how oxygen is produced in this ancient ecosystem. -Taro KIDO
Scientists already knew that the water in the mine contained oxygen molecules, but it is unclear how they were formed. Ruff and his colleagues are still studying the samples they took, but they have two hypotheses about how oxygen molecules might form, he said.
The site is mined for gold and uranium, a radioactive metal. Radiolysis, the splitting of water molecules through radioactivity, is one way to produce oxygen without sunlight. Alternatively, the production of oxygen could involve microbes similar to those found in ruff in Canada’s groundwater.
Sweetman said Friday the new project will also examine whether microbial reactions play a role in the production of dark oxygen on the seafloor. In particular, the project will examine how hydrogen is released during oxygen production by the metal nodules and whether hydrogen was used as an energy source for microbial communities discovered in parts of the deep sea.
“I don’t think we have a complete handle on the mechanism yet and it will take us a lot of time to figure it out,” he said.
Ruff said he hopes to work with Sweetman and other scientists involved in dark oxygen research to understand how the chemical signature of oxygen produced by seawater electrolysis differs from that produced by microbes or radiolysis.
Dark oxygen and the search for extraterrestrial life
NASA officials are interested in research into dark oxygen production because it could contribute to scientific knowledge about how life could be sustained on other planets without direct sunlight, Sweetman said.
The space agency wants to conduct experiments to understand how much energy is required to potentially produce oxygen at higher pressures found on Enceladus and Europa, the icy moons of Saturn and Jupiter, respectively, he added. These moons are among the targets for exploring the possibility of life.
Deep-sea mining companies want to extract the cobalt, nickel, copper, lithium and manganese contained in the nodules for use in solar panels, electric car batteries and other environmentally friendly technologies. Some companies have voiced criticism of Sweetman’s research.
Critics say deep-sea mining could irreversibly damage pristine marine life and disrupt the way carbon is stored in the ocean, contributing to the climate crisis.
The Metals Co. said it submitted a rebuttal to Nature Geoscience, the journal that published the original research. The company said the submission is currently being peer-reviewed but has not yet been published.
Sweetman said he was aware of the critical response and would respond “through peer-reviewed channels.”
“We are completely convinced that this is an actual seafloor process,” he said.
Sweetman also said it would be wise to wait to exploit seafloor resources until the ecosystem is better understood.
Amy Gartman, research oceanographer and global marine minerals project manager at the US Geological Survey’s Pacific Coastal and Marine Science Center in Santa Cruz, California, said the USGS has not yet observed any electrical phenomena in ferromanganese nodules studied. She was not involved in either Sweetman’s or Ruff’s research.
“Researchers are currently attempting to reproduce the phenomena reported by Sweetman and others,” she said. “Scientific research is a process and it may take some time to produce a conclusive answer.”
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