Rocks at the bottom of the ocean are producing a unique form of oxygen in an area under investigation for deep-sea mining.
Deep beneath the ocean, over 12,000 feet down in a region of the Pacific called the Clarion-Clipperton Zone (CCZ), ancient rocks stretch across the seafloor. While these rocks may appear barren, they provide a habitat for tiny sea life and microbes, many of which are specially adapted to thrive in complete darkness.
The deep-sea formation known as polymetallic nodules not only serves as a home to various marine organisms but has also been found by a team of scientists that includes researchers from Boston University to produce oxygen right on the ocean floor.
This surprising finding challenges the conventional belief that oxygen is primarily generated by plants and sunlight, as it is normally associated with surface organisms. Around half of the oxygen we breathe is produced near the ocean’s surface by phytoplankton, which utilize sunlight to carry out photosynthesis. The realization that oxygen is being produced in the dark depths of the ocean, without sunlight, is indeed revolutionary; the researchers involved initially suspected a mistake.
“This was truly unusual, because no one has observed this phenomenon before,” remarks Jeffrey Marlow, an assistant professor of biology at BU’s College of Arts & Sciences and coauthor of the study published in Nature Geoscience.
As someone who studies microbes inhabiting some of Earth’s most extreme environments, such as volcanic lava and deep-sea hydrothermal vents, Marlow initially deduced that microbial activity might be responsible for the oxygen production. The research team deployed deep-sea chambers to the seafloor containing seawater, sediment, polymetallic nodules, and living organisms. They measured the changes in oxygen levels within these chambers over a period of 48 hours. Typically, if there are numerous organisms consuming oxygen, levels would drop based on their activity. However, they found an increase in oxygen instead.
“After extensive investigation, we discovered that oxygen levels rose significantly more after that initial measurement,” states Marlow. “We are now convinced this is a genuine phenomenon.”
The research team was working from a vessel dedicated to exploring the ecology of the CCZ, which encompasses 1.7 million square miles off the coasts of Hawaii and Mexico. This environmental assessment, sponsored by The Metals Company, a firm looking to extract metals from these rocks, allowed Marlow and his colleagues, led by Andrew Sweetman from the Scottish Association for Marine Science, to conclude that microbial activity was not the main driver of oxygen production, despite the variety of microbes found in and on the rocks.
Polymetallic nodules contain valuable metals like copper, nickel, cobalt, iron, and manganese, which is why there is interest in mining them. The study reveals that the dense concentration of these metals may be facilitating a process known as “seawater electrolysis.” This occurs when metal ions within the rock layers are unevenly distributed, creating a separation of electrical charges akin to a battery. This process generates enough energy to break down water molecules into oxygen and hydrogen. They refer to this process as “dark oxygen,” as it is generated without sunlight. However, questions remain about the specific mechanism behind this process, whether oxygen levels vary across the CCZ, and the role of this oxygen in supporting local ecosystems.
The Metals Company describes polymetallic nodules as a “battery in a rock” on its website, suggesting that mining these nodules could help expedite the shift to electric vehicles and reduce the need for terrestrial mining. Currently, exploration is underway in the CCZ, but the United Nations International Seabed Authority, which oversees the area, may begin to make decisions about mining as early as next year. The Metals Company is seeking mining licenses in partnership with Pacific nations like Nauru, Tonga, and Kiribati, while other South Pacific countries, including Palau, Fiji, and Tuvalu, are advocating for a moratorium on mining. Environmental advocacy organizations such as Greenpeace and Ocean Conservancy are calling for a permanent ban on such activities, fearing that they could inflict irreversible harm to the seafloor.
Meanwhile, scientists have started examining the potential consequences of disturbing this largely uncharted ecosystem. This paper in Nature Geoscience offers valuable information regarding the CCZ’s baseline conditions prior to any large-scale mining operations.
“We are not aware of all the implications, but this discovery indicates that we should seriously consider the effects of altering these systems on the local animal populations,” notes Marlow, since all animal life relies on oxygen for survival.
The CCZ also provides an ideal setting to observe the planet’s tiniest organisms, such as bacteria and archaea found in sediments and on nodules. Marlow and his coauthor Peter Schroedl (GRS’25), who is pursuing a PhD in BU’s ecology, behavior, and evolution program, are particularly interested in studying microbes from extreme environments like the CCZ as models for locating single-celled life on other planets and moons. This endeavor, known as astrobiology, seeks to enhance the search for extraterrestrial life by examining Earth’s environments.
“Studying life in environments like the CCZ allows us to explore ecosystems that have evolved under unique evolutionary pressures and constraints,” explains Schroedl, who is part of Marlow’s research team. The extreme conditions—depth, pressure, and aquatic habitat—are similar to those anticipated on icy moons.
For instance, Jupiter’s moon, Enceladus, and Saturn’s moon, Europa, are encased in ice, cutting off sunlight from the water beneath. “If these types of rocks exist beneath the ice and are producing oxygen, it could lead to a more vibrant biosphere,” predicts Marlow. “If oxygen can be generated without photosynthesis, then other celestial bodies with oceans and metal-rich rocks like these nodules might support more developed life forms than we previously imagined.”
Marlow emphasizes that numerous questions remain regarding the implications of this dark oxygen discovery for both extraterrestrial oceans and our own.
“Generally, we perceive the deep sea as a realm where decaying materials descend and creatures feast on the remnants. But this discovery is shifting that perspective,” he observes. “It encourages us to view the deep ocean as a place of production, akin to what we’ve found with methane seeps and hydrothermal vents that create rich habitats for marine life and microbes. I find it a fascinating shift in how we commonly regard deep-sea environments.”
