More than 3,000 meters below the sea surface, in a region of the Pacific Ocean known as the Clarion-Clipperton Zone (CCZ), million-year-old rocks cover the seafloor. These rocks may seem lifeless, but between the nooks and crannies of their surfaces, tiny marine creatures and microbes make their home, many of which are uniquely adapted to life in the dark.
These deep-sea rocks, called polymetallic nodules, are not only home to a surprising number of marine animals. A team of scientists, including experts from Boston University, have discovered that they also produce oxygen on the seabed.
The discovery is a surprise, as oxygen is typically produced by plants and organisms with the help of the sun – and not by rocks on the ocean floor. About half of all the oxygen we breathe is made at the surface of the ocean by phytoplankton that photosynthesize like plants on land. Because the sun is necessary for photosynthesis, finding oxygen production at the bottom of the sea, where there is no light, turns conventional wisdom on its head. It was so unexpected that scientists involved in the study at first thought it was a mistake.
“This was really weird, because no one had ever seen it before,” said Jeffrey Marlow, assistant professor of biology in the BU College of Arts & Sciences and co-author of the study, which was published in Natural Geosciences.
An expert on microbes that live in Earth’s most extreme habitats – such as hardened lava and deep-sea hydrothermal vents – Marlow initially suspected that microbial activity might be responsible for making oxygen. The research team used deep-sea chambers that landed on the seabed and encased seawater, sediment, polymetallic nodules and living organisms. They then measured how the oxygen level in the chambers changed over 48 hours. If there are abundant organisms breathing oxygen, levels will normally decrease depending on how much animal activity is present in the room. But in this case the oxygen increased.
“We solved a lot of problems and found that oxygen levels increased much more frequently after that first measurement,” says Marlow. “So we are now convinced that it is a real signal.”
He and his colleagues were aboard a research vessel tasked with learning more about the ecology of the CCZ, which covers an area of 2.7 million square kilometers between Hawaii and Mexico, for an environmental study sponsored by The Metals Company, a deep-sea mining company interested in mining the rocks en masse for metals. 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, despite its abundance to many different types of microbes both on the inside and in the ship. the rocks.
Polymetallic nodules are made of rare metals, including copper, nickel, cobalt, iron and manganese, which is why companies are interested in mining them. The research shows that these densely packed metals are likely to cause ‘seawater electrolysis’. This means that metal ions in the rock layers are unevenly distributed, causing a separation of electrical charges, just like what happens in a battery. This phenomenon creates enough energy to split water molecules into oxygen and hydrogen. They called this ‘dark oxygen’ because it is oxygen made without sunlight. What remains unclear is the exact mechanism of how this happens, whether the oxygen level in the CCZ varies, and whether the oxygen plays an important role in maintaining the local ecosystem.
The Metals Company calls polymetallic nodules a “battery in a rock” and states on its website that mining them could accelerate the transition to battery electric vehicles and claims that land-based mining would eventually no longer be necessary. So far, mining in the CCZ has been exploratory, but the United Nations International Seabed Authority, which manages the area, could start making decisions on mining as early as next year. The Metals Company is working with the Pacific states of Nauru, Tonga and Kiribati to access mining permits, but many other South Pacific countries, including Palau, Fiji and Tuvalu, have been outspoken in favor of a moratorium or pause on mining . plans. Environmentalist groups such as Greenpeace and Ocean Conservancy are calling for a permanent ban, and opponents of the operation fear it could cause irreversible damage to the seabed.
In the meantime, scientists have begun studying the potential consequences of disrupting a largely unexplored ecosystem. This Natural Geosciences paper provides insights into the basic conditions of the area before any large-scale mining begins.
“We don’t know the full implications, but to me this finding suggests that we need to think deeply about what changing these systems would do to the animal community,” says Marlow, because all animals need oxygen to survive.
The CCZ is also the perfect environment to study the world’s smallest organisms, such as bacteria and archaea (single-celled organisms) found in sediments and on nodules. Marlow and his co-author Peter Schroedl (GRS’25), a doctoral candidate in BU’s ecology, behavior and evolution program, are particularly focused on using microbes found in extreme environments such as the CCZ as templates for finding single-celled life on other planets and moons – as deserts, volcanoes and seafloor vents are the places most similar to the many moons of Mars and Saturn. This is called astrobiology, a field that aims to inform the search for extraterrestrial life by studying Earth’s systems.
“Life in environments like the CCZ provides the opportunity to study ecosystems that have evolved under different evolutionary pressures and constraints,” says Schroedl, who works in Marlow’s lab. Those conditions – the depth, the pressure and the water environment – are “analogous to the conditions we have measured or expect to detect on icy moons,” he says.
For example, Jupiter’s moon Enceladus and Saturn’s moon Europa are covered in layers of ice, without sunlight reaching the water trapped beneath. “Who knows – if these types of rocks are under the ice and produce oxygen, it could create a more productive biosphere,” says Marlow. “If photosynthesis is not needed to make oxygen, then other planets with oceans and metal-rich rocks like these nodules could support a more developed biosphere than we thought possible in the past.”
There are plenty of questions to keep asking, Marlow says, about what the discovery of dark oxygen means for alien oceans and our own.
“For the most part, we think of the deep sea as a place where decaying material falls and animals eat the remains. But this finding recalibrates that dynamic,” he says. “It helps us see the deep ocean as a place of production, similar to what we’ve discovered with methane seeps and hydrothermal vents that create oases for marine animals and microbes. I think it’s a nice reversal of how we tend to think about the deep sea.”
