Most animals, including humans, wouldn’t exist without photosynthesis and the oxygen it produces … or so we thought. A recently published paper in Nature Geoscience identifies a new source of oxygen that doesn’t rely on sunlight and isn’t produced by plants or microbes. Instead, rock-like “polymetallic nodules” produce dark oxygen which supports entire ecosystems in the deepest parts of the ocean.
No sunlight, no oxygen?
Nearly every animal needs oxygen, even creatures that live in the deepest parts of the ocean. Until recently, scientists believed that all of the naturally occurring oxygen on Earth was produced by photosynthetic organisms. Plants, algae, and cyanobacteria undergo photosynthesis to convert carbon dioxide and water into glucose for energy. This process releases oxygen as a by-product and it is what made aerobic life on Earth possible. Photosynthesis is dependent on photolysis, a process that uses energy from sunlight to split water molecules into hydrogen and oxygen (“photo” meaning “light” and “lysis” meaning “to decompose”).
Access to sunlight generally isn’t a problem on land but, in the ocean, photosynthetic organisms must live near the water’s surface where they can access the sun’s energy. As you dive deeper, the ocean gets darker and without enough sunlight to support photosynthesis, the concentration of oxygen decreases. Photosynthetic organisms on land can also contribute to oxygen in the ocean. The oxygen they release into the air can also dissolve into seawater, a process that is facilitated by the motion of wind and waves mixing the air and water at the ocean’s surface, but this too drops off the deeper you go down into the ocean.
However, there IS oxygen in the deepest, darkest parts of the ocean. As you get past 1000m, well beyond the depth of sunlight and churning waves, oxygen levels start to increase, albeit slightly. This is because cold water, which is denser than warm water, sinks and takes dissolved oxygen with it. In polar regions, oxygen dissolves into surface water and then sinks down, where ocean currents carry the water around the world. In fact, oxygen that has made its way into the deep ocean can stay there for thousands of years until it is finally consumed by bottom-dwelling organisms. But regardless, oxygen in the ocean was always thought to have originally been produced photosynthesis.
That is until 2013, when Dr. Andrew K. Sweetman, a professor of the Seafloor Ecology and Biogeochemistry research group at the Scottish Association for Marine Science, was studying the respiration of seafloor organisms that live at depths of 4000- 5,500m. Surprisingly, his measurements showed a huge increase in oxygen concentrations in areas of the CCZ seafloor, sometimes even higher than surface level waters that were teeming with photosynthetic organisms. His data seemed to indicate that oxygen was being produced faster than it was being consumed.
When he first made this observation in 2013, he thought it couldn’t be true and ignored his findings, chalking it up to faulty equipment. After all, every scientific study to date that measured oxygen in the deep sea recorded a decrease in oxygen levels as the organisms on the sea floor consumed oxygen for respiration. But then in 2021, he went back to the area and used a completely different measurement method and saw the same result. At that point he realized he had stumbled on a huge discovery.
Metal “rocks” that act as batteries
Dr. Sweetman and his team were studying an area of the seafloor called the Clarion-Clipperton Zone (CCZ) between Hawaii and Mexico. This area is littered with potato-sized lumps known as polymetallic nodules. These nodules were made over millions of years as dissolved metals, like copper, nickel, cobalt and other rare-earth metals, accumulated on shell fragments or other debris. The nodules have become of great interest to deep sea mining companies because these metals are used to make modern electronics like touch screens and rechargeable batteries.
In fact, this is exactly why Dr. Sweetman and his team were seeing a spike in oxygen levels on the seafloor; the polymetallic nodules were acting like batteries. When you run an electric current through salt water, the molecules split into hydrogen and oxygen, a process called electrolysis. (Similar to photolysis, electrolysis is also the action of splitting water molecules but using a different energy source.)
When Dr. Sweetman’s team tested the electric current on the surface of each nodule, they found a single nodule could produce up to 0.95 V. However, electrolysis requires about 1.6 V to split water into H2 and O2. In an interview with the BBC, Dr. Sweetman explained that multiple nodules sitting close together could potentially work together to generate enough electricity to electrolyze seawater.
“It's like a battery in a torch. You put one battery in, it doesn't light up. You put two in and you've got enough voltage to light up the torch. So when the nodules are sitting at the seafloor in contact with one another, they’re working in unison - like multiple batteries.”
Dr. Sweetman and his colleagues called the oxygen being produced by this method “dark oxygen” — the first naturally occurring type of oxygen known to be produced in the absence of sunlight.
This is a great modern-day example of how scientific theories are constantly evolving as new discoveries are made. For hundreds of years, any scientist would have said that photosynthesis is the only natural way of producing oxygen. But now, new scientific evidence has emerged to show there may actually be multiple sources of naturally occurring oxygen, prompting scientists around the world to rethink some of the most fundamental facts we thought we knew. For example, Sweetman said “For aerobic life to begin on the planet, there has to be oxygen and our understanding has been that Earth's oxygen supply began with photosynthetic organisms. But we now know that there is oxygen produced in the deep sea, where there is no light. I think we therefore need to revisit questions like: where could aerobic life have begun?"
Deep sea mining and the ocean ecosystem
Dr. Sweetman and other marine scientists are excited by this discovery but the deep sea mining industry… maybe not so much. The CCZ is a site of great interest to seabed mining companies, including The Metals Company, which sponsored Dr. Sweetman’s study. Many companies are already developing technologies to collect these nodules and extract the metals within them.
However, marine conservationist warn that removing the nodules from the ocean floor could have devastating effects on deep-sea ecosystems that depend on dark oxygen or colonize the nodules. Over 820 marine science and policy experts from over 44 countries have signed a petition to pause all deep sea mining. They write that “deep-sea ecosystems are currently under stress from a number of anthropogenic stressors including climate change, bottom trawling and pollution. Deep-sea mining would add to these stressors, resulting in the loss of biodiversity and ecosystem functioning that would be irreversible on multi-generational timescales.”
Dr. Sweetman said he doesn’t think his study will put an end to deep-sea mining efforts but instead, he hopes it will encourage us to figure out how to mine these metals in the “most environmentally friendly way possible.”
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