Bo Barker Jørgensen, an expert in marine biogeochemistry who was not involved in the research but reviewed the study as a peer reviewer, said in an interview that it was a “very unusual finding.”
The findings could have implications for the deep-sea mining industry, which is seeking the right to explore the depths of the ocean and recover minerals such as those that make up polymetallic nodules. Such minerals are considered crucial for the energy transition. Environmental activists and many scientists consider deep-sea mining to be dangerous because it can destabilize ecosystems in unpredictable ways and affect the ocean’s ability to help mitigate climate change. The study was funded by companies active in deep-sea mining.
When Andrew Sweetman, the study’s lead author, first recorded unusual oxygen levels from the bottom of the Pacific Ocean in 2013, he thought his research equipment was malfunctioning.
“I basically told my students to just put the sensors back in the box. We’ll send them back to the manufacturer and have them tested because they’re just giving us rubbish,” Sweetman, head of the seafloor ecology and biogeochemistry research group at the Scottish Association for Marine Science, told CNN. “And every time the manufacturer would say, ‘They work. They’re calibrated.'”
In 2021 and 2022, Sweetman and his team returned to the Clarion-Clipperton Zone, an area beneath the central Pacific known for its large amounts of polymetallic nodules. Convinced that their sensors were working, they lowered a device more than 4,000 meters below the surface that placed small boxes in the sediment. The boxes remained in place for 47 hours, conducting experiments and measuring the oxygen consumption of the microorganisms living there.
Instead of decreasing, oxygen levels increased, suggesting that more oxygen was being produced than was being consumed.
The researchers suspected that the electrochemical activity of the different metals that make up polymetallic nodules are responsible for the oxygen production measured by the sensors – like in a battery, in which electrons flow from one electrode to the other, creating an electric current, said Tobias Hahn, one of the study’s co-authors, in an interview.
This hypothesis would expand our understanding of how organisms originated under the sea, said Hahn, who focused specifically on the sensors used in the study experiments. “We thought life on Earth began when photosynthesis began, because photosynthesis brought oxygen to Earth. It could be that this process of electrochemically splitting water into oxygen and hydrogen actually provided oxygen to the ocean,” he said.
“This could be some kind of turning point in the story of the beginning of life,” he added.
A press release about the study said the results “challenge long-held assumptions that only photosynthetic organisms such as plants and algae produce oxygen on Earth.”
However, if this finding is confirmed, “we need to rethink our methods of mining” materials such as cobalt, nickel, copper, lithium and manganese underwater “so that we do not deprive life in the deep sea of oxygen,” said Franz Geiger, a chemistry professor at Northwestern University and one of the study’s co-authors, in the press release.
Undersea mining in the 1980s serves as a cautionary tale, Geiger said. When marine biologists visited such sites decades later, “they found that even bacteria had not recovered.” In areas where no mining took place, “marine life flourished.”
“Why such ‘dead zones’ persist for decades is still unknown,” he said. But the fact that they exist suggests that seafloor mining could be particularly damaging in areas with lots of polymetallic nodules because those areas tend to have greater biodiversity than “the most diverse tropical rainforests,” he said.
Although the study has revealed an interesting new way to sustain life deep beneath the ocean, many questions remain, Hahn said. “We simply don’t know” how much “dark oxygen” can be created by this process, how it affects the polymetallic nodules, or what amounts of nodules are needed to enable oxygen production, he said.
Although the study’s methodology is sound, “there is a lack of understanding of what is going on and what kind of process it is,” says Barker Jørgensen.