Over 12,000 feet below the ocean’s surface, in a region of the Pacific Ocean known as the Clarion-Clipperton Zone (CCZ), millions of years old rocks cover the seafloor. These rocks may appear lifeless, but in the nooks and crannies of their surface, tiny marine creatures and microbes have made their home, many of which are uniquely adapted to life in the dark.

These deep-sea rocks, known as polymetallic nodules, are not only home to a surprising amount of marine life. A team of researchers, including experts from Boston University, has discovered that they also produce oxygen on the sea floor.

The discovery is surprising considering that oxygen is normally produced by plants and organisms using the sun – not rocks on the sea floor. About half of the oxygen we breathe is produced near the ocean surface by phytoplankton, which photosynthesize just like land plants. Since the sun is needed for photosynthesis, the discovery of oxygen production at the sea floor, where there is no light, turns conventional wisdom on its head. It was so unexpected that the scientists involved in the study initially thought it was a mistake.

“It was really strange because no one had ever seen anything like it before,” says Jeffrey Marlow, assistant professor of biology in the BU College of Arts & Sciences and co-author of the study, published in Nature Geoscience.

An expert on microbes that live in Earth’s most extreme habitats – such as solidified lava and deep-sea hydrothermal vents – Marlow initially suspected that microbial activity might be responsible for the oxygen production. The research team used deep-sea chambers that land on the seafloor and enclose seawater, sediment, polymetallic nodules and living organisms. They then measured how oxygen levels in the chambers changed over 48 hours.

If there are many organisms breathing oxygen, the oxygen level would normally decrease depending on how active the animals are in the chamber. However, in this case, the oxygen level increased.

“We did a lot of troubleshooting and found that the oxygen level increased many times after that first measurement,” says Marlow. “So we are now convinced that this is a real signal.”

He and his colleagues were aboard a research vessel tasked with learning more about the ecology of the CCZ, which stretches across 4.7 million square kilometers between Hawaii and Mexico. The mission of the investigation was to conduct an environmental study sponsored by the Metals Company, a deep-sea mining company interested in extracting metals from the rocks on a massive scale.

After conducting experiments on board the ship, Marlow and the team, led by Andrew Sweetman of the Scottish Association for Marine Science, concluded that the phenomenon is not primarily caused by microbial activity, although there are a variety of different microbial species both on and in the rocks.

Polymetallic nodules are made up of rare metals such as copper, nickel, cobalt, iron and manganese, which is why companies are interested in mining them. According to the study, it turns out that these densely packed metals are likely to trigger “seawater electrolysis.” This means that metal ions are distributed unevenly throughout the rock layers, creating a separation of electrical charges – just like what happens inside a battery.

This phenomenon generates enough energy to split water molecules into oxygen and hydrogen. This process is called “dark oxygen” because it occurs without sunlight. It is still unclear how exactly this happens, whether oxygen levels vary in the CCZ, and whether oxygen plays an important role in maintaining the local ecosystem.

The Metals Company calls polymetallic nodules a “battery in a rock” and says on its website that mining them could accelerate the transition to battery-powered electric vehicles. It also claims that onshore mining would eventually no longer be necessary. So far, mining in the CCZ is experimental in nature, but the United Nations International Seabed Authority, which manages the area, could begin making decisions on mining as early as next year.

The Metals Company is working with the Pacific nations of Nauru, Tonga and Kiribati to obtain mining licenses. However, many other countries in the South Pacific, including Palau, Fiji and Tuvalu, have been vocal in their support for a moratorium or suspension of mining plans. Environmental groups such as Greenpeace and Ocean Conservancy are calling for a permanent ban. Opponents of the operation fear that it could cause irreversible damage to the seabed.

Meanwhile, scientists have begun to study the potential impacts of disturbing a largely unexplored ecosystem. This article in Nature Geoscience provides insights into the baseline conditions of the area before large-scale mining begins.

“We don’t yet know the exact impacts, but I think this finding suggests that we should think carefully about what the consequences of changing these systems would be for wildlife,” says Marlow, since all animals need oxygen to survive.

The CCZ is also the perfect environment to study the planet’s smallest organisms, such as bacteria and archaea (single-celled organisms) found in sediments and on the nodules. Marlow and co-author Peter Schroedl (GRS’25), a doctoral student in BU’s Ecology, Behavior and Evolution program, are particularly focused on using microbes found in extreme environments like the CCZ as templates for searching for single-celled life on other planets and moons—because deserts, volcanoes, and seafloor vents are the places most similar to Mars and Saturn’s many moons. This is called astrobiology, a field that aims to aid the search for extraterrestrial life by studying Earth systems.

“Living in environments like the CCZ offers the opportunity to study ecosystems that have evolved under certain evolutionary constraints and limitations,” says Schroedl, who works in Marlow’s lab. These conditions – depth, pressure and water environment – are “analogous to the conditions we have measured or expect to discover on icy moons,” he says.

For example, Jupiter’s moon Enceladus and Saturn’s moon Europa are covered with layers of ice that prevent sunlight from reaching the water beneath. “Who knows – if these types of rock produce oxygen beneath the ice, it could enable a more productive biosphere,” says Marlow. “If photosynthesis is not needed to produce oxygen, other planets with oceans and metal-rich rocks like these nodules could have a more developed biosphere than we previously thought possible.”

There are still many unanswered questions, Marlow says, about what the discovery of dark oxygen means for alien oceans and our own.

“We mostly think of the deep sea as a place where decaying material falls down and animals eat the remains. But this discovery changes that dynamic,” he says. “It helps us see the deep sea as a place of production, similar to what we’ve found with methane seeps and hydrothermal vents that create oases for marine animals and microbes. I think it’s a fun reversal of how we normally think about the deep sea.”

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Evidence of dark oxygen production at the deep sea floor, Nature Geoscience (Open Access)

Astrobiology,