TWO months ago, a research team made a surprising claim. The research says in the total darkness of the ocean depths, naturally occurring metal nodules on the seabed may act like batteries, splitting seawater and producing oxygen.
If true, this “dark oxygen” would be a monumental discovery, comparable to the discovery decades ago that hydrogen emitted from hydrothermal vents supported abundant life in the depths of the ocean, far from sunlight. This would mean that Earth has a new source of oxygen that is completely independent of photosynthesis and can also support life.
The news also strengthens environmental activists’ arguments that seabed ecosystems could be destroyed by deep-sea mining companies looking to extract the nodules for valuable metals used in electric vehicles, such as manganese and cobalt.
Now, independent scientists and deep sea mining companies are voicing their doubts about the study, published in Nature Geoscience. “There is a high probability that this paper is wrong,” said Kentaro Nakamura, a geochemist at the University of Tokyo who noted that there was no sign of increased oxygen in the waters above the nodule region.
The strongest opposition came from Metals Company, a deep-sea mining company that sponsored several of the team’s research voyages to the Clarion-Clipperton Zone, a nodule-rich region east of Hawaii. In a formal critique posted as a preview today, the company said that Andrew Sweetman, a marine ecologist with the Scottish Association for Marine Science, and his colleagues did not provide a complete picture of the evidence.
Instead, the company argued the oxygen could be explained by trapped air bubbles or electrical leaks in the deep-sea equipment used by the researchers.
“None of the evidence is [Sweetman] present can withstand testing,” said Michael Clarke, a marine biologist and environmental manager at Metals Company.
Sweetman said his team is preparing a response to the criticism. “We have nothing to hide,” he said.
To explore these nodules, which lie several kilometers beneath the sea, Sweetman’s team lowered a seafloor lander with three experimental chambers that could be partially inserted into the seafloor sediment, enclosing the sample.
They know these waters have a constant baseline level of oxygen, which is renewed by currents from Antarctica and slowly consumed by life on the sea floor. But in several voyages over the past decade, he and his colleagues found that when trapped sediments contain nodules, they sometimes produce bursts of oxygen that last a day or two.
Marta Cecchetto, a marine ecologist from Heriot-Watt University who had previously helped Sweetman collect samples, recalled her astonishment. “We just thought, this isn’t working.”
But finally, during one voyage, they used a second method to measure oxygen, and the results also showed a plume. They do not believe that the oxygen was created or captured by the lander during its descent.
Microbial production couldn’t be completely ruled out, but it seemed unlikely when they saw oxygen increase even after adding toxins to the samples. Then, last year, Sweetman had a new idea: The nodules, sought for batteries, might act like batteries themselves, if metal ions within the nodule layers created a slight charge.
The researchers tested several of the extracted nodules in the laboratory, and although none produced the 1.23 volts needed to split water into hydrogen and oxygen, they saw signs that pointed to its possibility.
Because Sweetman was working with the Metals Company at the time, company scientists were able to examine copies of the team’s records. They show Sweetman at one point placing the lander on a nodule-rich surface with its chambers closed, filled only with seawater, as a control experiment.
Over two days, oxygen rose in two of the three chambers, in a pattern similar to that found in sediment containing nodules, although in lower amounts. Oxygen levels also increase when the fans in each room are on.
The company says this means the chambers may be collecting oxygen bubbles as they descend through the water column and not releasing them properly, or that an electrical leak from the fan is causing electrolysis by accident. “[Sweetman] did exercise control,” said Clarke. “He just didn’t report it because it didn’t support his hypothesis.”
A closed third room refutes this point, Sweetman said. The team injected cold surface seawater into two other chambers, but the injection failed in the third chamber, which contained only abyssal water.
In that third chamber, oxygen levels did not rise, indicating surface water injection was the cause of the increase in oxygen in the other two chambers. If air bubbles or electrical leaks from the chamber fan regularly introduced bursts of oxygen, he said, it would be difficult to explain the few instances in which researchers measured no oxygen production at all.
Sweetman also had a response to another anomaly that Metals Company cited. One figure in the paper states oxygen production in another nodule region but does not reveal that when that measurement was taken, there were no nodules in the chamber. Sweetman admits this. However, metallic manganese granules present in sediments can have a similar effect to nodules, he said.
A separate critique from mining company Adepth, released as a preview last month, focused on voltage measurements. Although the paper claims the voltage in the nodules reached 0.95 volts, the figure only shows a brief spike measured in one nodule.
No other readings from the other 11 nodules analyzed came close to the levels needed to break the water, said Lars-Kristian Trellevik, head of sustainability and operations at Adepth. Sweetman responded that electrolysis may be intermittent. “We’re just saying it might happen.”
Testing Sweetman’s claim that nodules can generate voltage should be easy, said Amy Gartman, who leads the Global Marine Minerals project at the US Geological Survey. “I have a lot of nodules in the refrigerator. I have a potentiostat. I will see what I can get.”
Another group may soon publish its biggest challenge to this claim. Several years ago, researchers from the European Mining-Impact2 project carried out several dozen similar spatial readings in nodule fields.
They saw no sign of oxygen production, although they have not published the data, said Matthias Haeckel, a marine biogeochemist at the GEOMAR Helmholtz Center for Ocean Research Kiel, who led the project.
“The big question is, why does Andrew see it sometimes, and we haven’t?” Haeckel also notes that nearly 20 years ago, his team, using the same lander as Sweetman, thought they detected oxygen production on the sea floor—but it turned out to be just trapped air bubbles.
One explanation, Sweetman said, may be that the MiningImpact2 chamber was placed gently by the robot, whereas his team experienced a more abrupt landing, which may have scraped sediment from the surface of the nodule, allowing a brief burst of oxygen. “We shouldn’t think that these nodules produce oxygen bubbles all the time,” he said.
Other researchers are still waiting for conclusions. “The back-and-forth discussion in this paper is just normal science,” said Adrian Glover, a deep-sea ecologist at London’s Natural History Museum.
Sweetman proposed a follow-up cruise, which this time would monitor the seabed for hydrogen, another byproduct of water splitting. He is philosophical about the skepticism he faces. “Can you imagine how radical the idea is that oxygen can be produced without sunlight?” he said. “Do you think we would like to propose something so extraordinary?” (Science/Z-3)
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Interview with Dr. Andrew Sweetman: Exploring the Controversy of “Dark Oxygen” on the Seabed
Editor: Today, we’re joined by Dr. Andrew Sweetman, marine ecologist at the Scottish Association for Marine Science and lead author of the recent study claiming that metal nodules on the ocean floor can produce “dark oxygen.” Dr. Sweetman, thank you for joining us.
Dr. Sweetman: Thank you for having me.
Editor: Your research has sparked significant debate in the scientific community, as well as among deep-sea mining companies. Can you briefly explain what “dark oxygen” is and how it was discovered?
Dr. Sweetman: “Dark oxygen” refers to the oxygen that appears to be generated in total darkness at the ocean’s depths, specifically by natural metal nodules found on the seabed. During our research, we observed unexpected bursts of oxygen in seawater samples taken from areas rich in these nodules. This phenomenon suggests that these nodules might act like batteries, providing a new source of oxygen independent of sunlight.
Editor: That sounds groundbreaking! However, there has been skepticism from others in the scientific community and mining companies. What are the main criticisms you have received regarding your research?
Dr. Sweetman: Critics argue that the oxygen levels we measured could arise from trapped air bubbles or electrical leaks from our equipment rather than the nodules themselves. Some have also suggested that our experimental controls may not have been comprehensive, but we have evidence that supports our findings—such as patterns of oxygen production that do not align with these alternative explanations.
Editor: The concerns from deep-sea mining companies can be seen as self-serving, particularly given their interests in these metal nodules. How do you respond to those criticisms, and do you believe there’s a risk to the environment with continued mining?
Dr. Sweetman: The criticism is a natural part of scientific discourse, but it’s important to recognize potential biases. My team and I are committed to transparency and are preparing a detailed response to our critics. Regarding environmental risks, any destructive action taken to extract these nodules raises significant ethical questions about the preservation of deep-sea ecosystems. Our findings add urgency to these concerns, as they imply a unique, possibly overlooked, ecological role of these nodules.
Editor: If proven, how transformative could the discovery of dark oxygen be for our understanding of ocean ecosystems and life below the surface?
Dr. Sweetman: It would be monumental. If these nodules truly can produce oxygen without sunlight, it could change the fundamental understanding of how life exists in the deep sea, potentially influencing everything from marine biology to the search for extraterrestrial life in similar environments elsewhere in the universe.
Editor: Dr. Sweetman, thank you for sharing your insights with us. We look forward to seeing the forthcoming responses to your research and any implications it may have for both science and environmental policy.
Dr. Sweetman: Thank you. I appreciate the opportunity to discuss this important topic.
Etman: Absolutely, I understand the skepticism, especially since the companies involved have a financial interest in these nodules. My team and I are committed to transparency and rigorous validation of our results. As for environmental concerns, I do believe that deep-sea mining poses a significant risk to these delicate ecosystems. Our findings highlight the potential ramifications of disrupting a newly discovered and potentially vital oxygen source that could support unique life forms. Protecting these environments should be a priority before any mining operations commence.
Editor: You mentioned that your team is preparing a response to the critiques. What steps do you plan to take moving forward to address these concerns and strengthen your research?
Dr. Sweetman: We’re currently designing follow-up experiments that will better isolate the variables at play and test for hydrogen, another key byproduct. This will help validate our findings and provide a more comprehensive picture of what’s happening at the seabed. Collaboration with independent scientists and incremental experimentation will allow us to address the skepticism openly and objectively. Ultimately, our goal is to contribute valuable knowledge to marine science and ensure we take necessary precautions for the environment.
Editor: It sounds like you are very aware of the implications of your findings. Can you share how this discovery might change our understanding of deep-sea ecosystems?
Dr. Sweetman: If our hypothesis is validated, it suggests that there are processes occurring in deep-sea environments that create biological niches far removed from sunlight. This could redefine how we view the interconnectedness of ocean life and could inspire new conservation efforts to protect these unique regions. The existence of “dark oxygen” offers a refreshing perspective on the adaptive strategies of life in the depths, potentially influencing everything from ecologies to our understanding of life’s resilience.
Editor: Thank you, Dr. Sweetman, for sharing your insights with us. We’re looking forward to seeing how your research progresses in the coming months.
Dr. Sweetman: Thank you for having me and for drawing attention to this important subject!
