"Dark oxygen" in Earth's deep seas may hold clues about alien life

Over 12,000 feet beneath the Pacific Ocean’s surface, in a region called the Clarion-Clipperton Zone (CCZ), ancient rocks blanket the deep seafloor. While these rocks might seem devoid of life, they actually host a variety of tiny sea creatures 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 also, surprisingly, generate oxygen on the ocean floor.

This discovery, made by a team of scientists including experts from Boston University, challenges the conventional understanding that oxygen production requires sunlight and typically occurs near the ocean’s surface, where phytoplankton photosynthesize. 

Dark oxygen on the seafloor

The presence of oxygen at such depths, in complete darkness, was so unexpected that the researchers initially suspected an error.

“This was really weird, because no one had ever seen it before,” said Jeffrey Marlow, an assistant professor of biology at Boston University’s College of Arts & Sciences and co-author of the study.

Marlow, who specializes in studying microbes in extreme environments like hardened lava and deep-sea hydrothermal vents, initially thought that microbial activity might be responsible for the oxygen production. 

To investigate, the research team used deep-sea chambers that land on the seafloor, encapsulating seawater, sediment, polymetallic nodules, and living organisms. 

Confirming the presence of oxygen 

The team then measured oxygen levels within these chambers over a 48-hour period. Typically, if organisms are consuming oxygen, the levels would drop based on the amount of activity in the chamber. However, in this case, oxygen levels were rising.

“We did a lot of troubleshooting and found that the oxygen levels increased many more times following that initial measurement,” Marlow explained. “So we’re now convinced it’s a real signal.”

Mysterious oxygen production 

Marlow and his colleagues conducted their research aboard a vessel dedicated to exploring the CCZ’s ecology. This area, spanning 1.7 million square miles between Hawaii and Mexico, was the focus of an environmental survey sponsored by The Metals Company, a deep-sea mining firm interested in harvesting these rocks for their valuable metals. 

After conducting various experiments, the team, led by Andrew Sweetman from the Scottish Association for Marine Science, concluded that the oxygen production was not primarily driven by microbial activity, despite the presence of many microbes on and inside the rocks.

Seawater electrolysis and dark oxygen 

Polymetallic nodules are rich in rare metals such as copper, nickel, cobalt, iron, and manganese, which makes them attractive for mining. The study suggests that the dense concentration of these metals likely triggers a process called “seawater electrolysis.” 

This process involves the uneven distribution of metal ions within the rock layers, creating a separation of electrical charges similar to a battery. The energy is sufficient to split water molecules into oxygen and hydrogen, a process the researchers have termed “dark oxygen” because it occurs without sunlight. 

However, the exact mechanism remains unclear, as do questions about whether oxygen levels vary across the CCZ and the role this oxygen plays in sustaining local ecosystems.

The debate over 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 speed up the transition to battery-powered electric vehicles, potentially reducing the need for land-based mining. 

Currently, mining in the CCZ is still in the exploratory phase, but the United Nations International Seabed Authority, which oversees this area, may start making decisions about mining as early as next year. 

The Metals Company is collaborating with Pacific nations like Nauru, Tonga, and Kiribati to secure mining licenses, though several other South Pacific countries, including Palau, Fiji, and Tuvalu, have expressed strong opposition, advocating for a moratorium or a pause on mining activities. 

Environmental groups like Greenpeace and Ocean Conservancy are calling for a permanent ban, fearing that mining could cause irreversible damage to the seafloor.

Disrupting an unexplored ecosystem

Meanwhile, scientists are beginning to study the potential impacts of disrupting this largely unexplored ecosystem. The current study provides valuable baseline data on the area’s conditions before any large-scale mining begins.

“We don’t know the full implications, but to me this finding suggests that we should deeply consider what altering these systems would do to the animal community,” Marlow said, stressing that all animals need oxygen to survive.

The CCZ also offers a unique environment for studying the planet’s smallest organisms, such as bacteria and archaea (single-celled organisms) found in sediments and on the nodules. 

The search for extraterrestrial life 

Marlow and his co-author Peter Schroedl, a PhD 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-celled life on other planets and moons. This field, known as astrobiology, seeks to enhance the search for extraterrestrial life by studying Earth’s systems.

“Life in environments like the CCZ provides an opportunity to study ecosystems that developed under distinct evolutionary pressures and constraints,” said Schroedl. He added that the conditions in the CCZ – depth, pressure, and aquatic environment – are analogous to conditions 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, with no sunlight reaching the water beneath. 

Implications of dark oxygen 

“Who knows – if these types of rocks are under the ice making oxygen, that could allow a more productive biosphere to exist,” said Marlow.

He noted that if photosynthesis isn’t required to make oxygen, then other planets with oceans and metal-rich rocks like these nodules could sustain a more evolved biosphere than what we thought was possible.

Marlow acknowledged that there are still many questions to explore regarding the implications of this dark oxygen discovery for both extraterrestrial oceans and our own.

“For the most part, we think of the deep sea as a place where decaying material falls down and animals eat the remnants. But this finding is recalibrating that dynamic,” said Marlow.

“It helps us to see the deep ocean as a place of production, similar to what we have found with methane seeps and hydrothermal vents that create oases for marine animals and microbes. I think it’s a fun inversion of how we tend to think about the deep sea.”

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The study is published in the journal Nature Geoscience.