by Bo Barker Jørgensen, a marine biogeochemistry expert who was not involved in the research but reviewed the study, said in an interview that it was a “very unusual finding.”
These findings could have implications for the deep-sea mining industry, whose players are seeking permission to explore the ocean depths and recover minerals such as those that make up polymetallic nodules. Deep-sea mining is seen as crucial to the green energy transition. Environmental activists and many scientists say deep-sea mining is dangerous because it can destabilize ecosystems in unpredictable ways and could affect the ocean’s ability to help contain climate change. The study received funding from companies active in deep-sea mining exploration.
When Andrew Sweetman, the study’s lead author, first recorded unusual measurements of oxygen from the Pacific Ocean floor in 2013, he thought his research equipment was malfunctioning.
“I told my students to put the sensors back in their box. We’ll send them back to the manufacturer to get them tested because they’re giving us inaccurate information,” Sweetman, head of the deep-sea ecology and biogeochemistry research group at the Scottish Marine Science Association, told CNN. “And every time, the manufacturer came back and said, ‘They’re working. 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 harboring large quantities 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 levels consumed by the microorganisms living there.
Instead of decreasing, oxygen levels increased, suggesting that more oxygen was being produced than consumed.
The researchers hypothesized that the electrochemical activity of the different metals that make up the polymetallic nodules were responsible for the oxygen production measured by the sensors — like a battery in which electrons flow from one electrode to another, creating an electric current, Tobias Hahn, one of the study’s co-authors, said in an interview.
This hypothesis would provide insight into how organisms emerged under the sea, said Hahn, who focused specifically on the sensors used in the study’s experiments. “We thought that life began on Earth when photosynthesis started, with oxygen being brought to Earth through photosynthesis. It could be that, in fact, this process of electrochemically splitting water into oxygen and hydrogen provided oxygen to the ocean,” he said.
“This could be some kind of turning point in the history of how life began,” he added.
A press release about the study says its findings challenge “long-held assumptions that only photosynthetic organisms, such as plants and algae, generate Earth’s oxygen.”
But if the finding holds true, “we need to rethink how we extract” materials like cobalt, nickel, copper, lithium and manganese underwater, “so as not to deplete the oxygen source for deep-sea life,” Franz Geiger, a chemistry professor at Northwestern University and one of the study’s co-authors, said in the statement.
The underwater mining of the 1980s is a cautionary tale, Geiger said. When marine biologists visited those sites decades later, they “found that even the bacteria had not recovered.” But in areas that had not been mined, “marine life thrived.”
“It’s still unclear why these ‘dead zones’ persist for decades,” he said. But the fact that they exist suggests that seafloor mining in areas rich in polymetallic nodules could be particularly harmful, because these areas tend to have greater wildlife diversity than “the most diverse tropical rainforests,” he added.
Although the study highlights an exciting new pathway for sustaining life in the deep ocean, many questions remain, Hahn said. “We just don’t know” how much “dark oxygen” can be created by this process, how it affects polymetallic nodules or what quantities of nodules are needed to enable oxygen production, he said.
Although the study’s methodology is sound, “what’s missing is an understanding of what’s going on, what kind of process it is,” Barker Jørgensen said.
