Over 12,000 feet below the surface of the Pacific Ocean, in a region called the Clarion-Clipperton Zone (CCZ), ancient rocks cover the deep sea floor. Although these rocks may appear lifeless, they are actually home to a variety of tiny marine animals and microbes, many of which are specially adapted to survive in the dark.
These deep-sea rocks, known as polymetallic nodules, not only provide a habitat for numerous marine organisms, but surprisingly also produce oxygen on the seafloor.
This discovery, made by a team of scientists including experts from Boston University, challenges the conventional wisdom that oxygen production requires sunlight and typically occurs near the ocean surface, where phytoplankton photosynthesize.
Dark oxygen on the sea floor
The presence of oxygen at such depths, in total darkness, was so unexpected that researchers initially suspected an error.
“It was really strange because no one had seen anything like it before,” said Jeffrey Marlow, assistant professor of biology in Boston University’s College of Arts & Sciences and co-author of the study.
Marlow, who specializes in studying microbes in extreme environments such as solidified lava and deep-sea hydrothermal vents, initially thought that microbial activity might be responsible for oxygen production.
To conduct the study, the research team used deep-sea chambers that land on the seafloor and encapsulate seawater, sediment, polymetallic nodules and living organisms.
Confirmation of the presence of oxygen
The team then measured the oxygen levels in these chambers over a 48-hour period. As organisms use oxygen, levels usually drop depending on the activity in the chamber. In this case, however, oxygen levels increased.
“We did a lot of troubleshooting and found that the oxygen level increased several times after that first measurement,” Marlow explained. “So we are now convinced that this is a real signal.”
Mysterious oxygen production
Marlow and his colleagues conducted their research aboard a ship dedicated to studying the ecology of the CCZ. This 1.7 million square mile area between Hawaii and Mexico was the focus of an environmental study sponsored by The Metals Company, a deep-sea mining company interested in extracting these rocks for their valuable metals.
After conducting various experiments, the team led by Andrew Sweetman of the Scottish Association for Marine Science concluded that despite the presence of many microbes on and in the rocks, oxygen production was not primarily due to microbial activity.
Seawater electrolysis and dark oxygen
Polymetallic nodules are rich in rare metals such as copper, nickel, cobalt, iron and manganese, making them attractive for mining. The study suggests that the high concentration of these metals likely triggers a process called “seawater electrolysis.”
In this process, metal ions are distributed unevenly in the rock layers, causing a separation of electrical charges similar to a battery. The energy is enough to split water molecules into oxygen and hydrogen. The researchers call this process “dark oxygen” because it takes place without sunlight.
However, the exact mechanism remains unclear, as do questions about whether oxygen levels vary within the CCZ and what role this oxygen plays in maintaining local ecosystems.
The debate about deep-sea mining
The Metals Company describes polymetallic nodules as a “battery in a rock” and claims on its website that mining these nodules could accelerate the transition to battery-powered electric vehicles, potentially reducing the need for land-based mining.
Currently, mining in the CCZ is still in the exploration phase, but the United Nations International Seabed Authority, which oversees the area, could begin making decisions on mining as early as next year.
The Metals Company is working with Pacific nations such as Nauru, Tonga and Kiribati to obtain mining licenses. However, several other South Pacific countries, including Palau, Fiji and Tuvalu, have expressed strong opposition to this, calling for a moratorium or cessation of mining activities.
Environmental groups such as Greenpeace and Ocean Conservancy are calling for a permanent ban because they fear mining could cause irreversible damage to the seabed.
Disturbance of an unexplored ecosystem
Meanwhile, scientists are beginning to study the potential impacts of disturbing this largely unexplored ecosystem. The current study provides valuable baseline data on conditions in the region before large-scale mining begins.
“We don’t know the full impact, but to me this finding suggests that we should think carefully about what the consequences of changing these systems would be for wildlife,” Marlow said, stressing that all animals need oxygen to survive.
The CCZ also provides a unique environment for studying the smallest organisms on our planet, such as bacteria and archaea (single-celled organisms) found in sediments and on the nodules.
Marlow and his co-author Peter Schroedl, a doctoral student in Boston University’s Ecology, Behavior and Evolution program, are particularly interested in using microbes from extreme environments like the CCZ as models for discovering single-cell life on other planets and moons. This field, known as astrobiology, seeks to improve 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 specific evolutionary constraints and limitations,” Schroedl said. He added that the conditions in the CCZ – depth, pressure and aquatic environment – are similar to those we have measured or expect to discover on icy moons.
For example, Jupiter’s moon Enceladus and Saturn’s moon Europa are covered by thick layers of ice, so that no sunlight penetrates to the water underneath.
Effects of dark oxygen
“Who knows – if these rocks produce oxygen under the ice, it could allow a more productive biosphere to exist,” Marlow said.
He pointed out that if photosynthesis was not required to produce oxygen, other planets with oceans and metal-rich rocks like these nodules might have a more developed biosphere than we thought possible.
Marlow acknowledged that many questions remain to be answered regarding the implications of this discovery of dark oxygen for both 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,” Marlow said.
“It helps us see the deep sea as a manufacturing site, similar to what we’ve seen 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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The study was published in the journal Natural Geosciences.
