The Enigma of Dark Oxygen

A recent study has found that metal nodules on the ocean floor release oxygen into their surroundings. Until now, scientists believed that all the oxygen found in the deep sea originated from the air and shallow waters, produced by plants and algae through photosynthesis. This new finding reveals that oxygen is also being generated directly in the ocean depths, rather than solely transported from the surface or nearby layers. While the amount of oxygen produced this way is relatively small, it provides a steady supply that may help support deep-sea ecosystems.

Tracing the Source of Oxygen

The surprising study, conducted both in the lab and in the field, used cube-shaped chambers equipped with measuring instruments. Researchers lowered these chambers to the abyssal seafloor of the Pacific Ocean, allowing seawater to flow through them. This effectively isolated the measured material from the surrounding environment, creating conditions similar to a controlled laboratory setting. Using the equipment inside the chambers, the researchers measured various factors in the deep water, such as oxygen levels, temperatures, and the presence of microorganisms.

The study's findings were recently published in Nature Geosciences. The research team demonstrated that naturally occurring metal deposits on the seafloor, which contain valuable minerals, release oxygen into their surroundings. These polymetallic nodules, also known as manganese nodules, were first identified in a region of the Pacific Ocean between Hawaii and Mexico.

Led by Professor Andrew K. Sweetman, a specialist in deep-sea ecosystem studies in the Pacific Ocean, the team explored extensive areas of the ocean floor covered with these mineral-rich polymetallic nodules. Oxygen sensors repeatedly detected an unexpected phenomenon: oxygen in the deep ocean was not only being consumed by organisms, as expected, but was also being produced and released. Initially skeptical, the researchers recalibrated the sensors and adjusted their measurement sensitivity, yet the same "anomalies" in oxygen levels persisted.

The team remained doubtful and conducted a series of experiments to rule out the possibility that the detected oxygen came from bubbles trapped in the equipment or from components of the research apparatus. After thorough testing, these possibilities were ruled out. Subsequent laboratory experiments simulating deep-ocean conditions confirmed that dissolved oxygen levels in the water increased in the presence of the nodules.

“That’s when we said ‘My god, we have another source of oxygen,'" Sweetman told ScienceNews. He further explained that oxygen production likely occurs on the nodules’ surface, as evidenced by the correlation between the rate of oxygen production and the average surface area of the nodules.

Laboratory experiments simulating deep-ocean conditions confirmed that dissolved oxygen levels increased in the presence of the nodules. Polymetallic nodules from the ocean floor in a laboratory | Franz Geiger/Northwestern University

Oxygen and Hydrogen: Together and Apart

How do metal nodules produce oxygen? Researchers found that these nodules function like tiny batteries, generating a voltage of 0.95 volts across various points on their surface—comparable to a standard AA battery. This voltage likely causes the breakdown of water molecules into hydrogen and oxygen. Indeed, manganese nodules, sometimes referred to as "a battery in a rock," contain high levels of metals used in battery production, such as copper, manganese, and cobalt.

The breakdown of water into oxygen and hydrogen is a well-understood and common process. Each water molecule consists of two hydrogen atoms covalently bonded to one oxygen atom, meaning the atoms share electrons. Water molecules can be split into hydrogen and oxygen when subjected to an electric current in a process called electrolysis. Although seawater electrolysis typically requires a voltage of at least 1.5 volts, the researchers estimate that under certain conditions, clusters of nodules can collectively generate higher voltages.

The oxygen produced in this way has been termed "dark oxygen," as it is generated without the need for light, in contrast to photosynthesis, which relies on light energy and is carried out by plants and algae.

The release of oxygen from the metal nodules may help explain why over half of the biodiversity in these ecosystems thrives on the nodules’ hard surfaces. However, further research is needed to determine the extent to which these organisms rely on the oxygen emitted by the nodules.

Mining companies are setting their sights on these polymetallic nodules. Illustration of underwater mining | Mikkel Juul Jensen / Science Photo Library

Economic Interests in the Deep Sea

Mining companies are already setting their sights on these metal nodules, which are rich in rare metals essential for various technological applications. However, deep-sea mining could have far-reaching impacts, not only on the specific areas where the nodules are harvested but also on the broader environment. Mining activities are expected to generate dust clouds that may settle over vast regions, similar to the effects observed near construction sites on land. As of now, the full extent of the potential ecological damage from such mining remains unknown.

Moreover, beyond the direct environmental impacts of the nodules on their surroundings, the study of these unique systems holds broader significance. The mechanism uncovered in this research deepens our understanding of oxygen production processes on Earth and prompts a reevaluation of the existing theories regarding the origins of life on our planet.