Scientists have uncovered a remarkable phenomenon in the deep ocean: “dark oxygen” being produced by metallic nodules on the seafloor.

This discovery challenges our previous understanding of oxygen production, which was thought to occur only through photosynthesis by marine plants in the presence of sunlight.

Oxygen production in the ocean is crucial, as it generates about half of the oxygen we breathe. However, at depths of 5 kilometres (3 miles) where sunlight cannot reach, scientists have now found that oxygen is being produced by metallic nodules that naturally occur on the seafloor. These nodules have the ability to split seawater (H2O) into its components, hydrogen and oxygen, without the need for sunlight.

The research, led by Prof. Andrew Sweetman from the Scottish Association for Marine Science, was conducted in a deep-sea area between Hawaii and Mexico. This region, known as the Clarion-Clipperton Zone, is covered with these metal-rich nodules. The nodules form over millions of years as dissolved metals in seawater accumulate on fragments of shell or other debris.

“I first saw this in 2013 – an enormous amount of oxygen being produced at the seafloor in complete darkness,” explains Prof Sweetman. “I just ignored it, because I’d been taught – you only get oxygen through photosynthesis.

“Eventually, I realised that for years I’d been ignoring this potentially huge discovery,” he said.

These nodules are highly sought after by mining companies because they contain valuable metals like lithium, cobalt, and copper, which are essential for manufacturing batteries. However, Prof. Sweetman and his team have raised concerns that deep-sea mining activities could disrupt the newly discovered oxygen production process and harm the marine life that depends on it.

The researchers discovered that the nodules function like natural batteries.

“If you put a battery into seawater, it starts fizzing,” explained Prof Sweetman. “That’s because the electric current is actually splitting seawater into oxygen and hydrogen [which are the bubbles]. We think that’s happening with these nodules in their natural state.”

“It’s like a battery in a torch,” he added. “You put one battery in, it doesn’t light up. You put two in and you’ve got enough voltage to light up the torch. So when the nodules are sitting at the seafloor in contact with one another, they’re working in unison – like multiple batteries.”

In laboratory experiments, they measured the voltage on the surface of these potato-sized nodules and found it comparable to that of an AA battery. This voltage is sufficient to generate electric currents that can electrolyzs seawater, splitting water molecules into hydrogen and oxygen.

This discovery has broader implications, suggesting that similar battery-powered oxygen production could occur on other planets and moons, potentially creating oxygen-rich environments where extraterrestrial life could exist.

The Clarion-Clipperton Zone, where this discovery was made, is already under exploration by several seabed mining companies. These companies are developing technology to harvest the nodules and bring them to the surface.

The US National Oceanic and Atmospheric Administration (NOAA) has warned that this seabed mining could “result in the destruction of life and the seabed habitat in the mined areas”.

The environmental risks associated with deep-sea mining have prompted more than 800 marine scientists from 44 countries to sign a petition calling for a moratorium on mining activities in these fragile ecosystems. New species are continually being discovered in the deep ocean, emphasising how little we know about these environments. The discovery of dark oxygen production suggests that these metallic nodules may be essential to sustaining life in the deep sea.

Prof Murray Roberts, a marine biologist from the Univerisity of Edinburgh is one of the scientists who signed the seabed mining petition. “There’s already overwhelming evidence that strip mining deep-sea nodule fields will destroy ecosystems we barely understand,” he told BBC News.

“Because these fields cover such huge areas of our planet it would be crazy to press ahead with deep-sea mining knowing they may be a significant source of oxygen production.”

Prof Sweetman added: “I don’t see this study as something that will put an end to mining.

“[But] we need to explore it in greater detail and we need to use this information and the data we gather in future if we are going to go into the deep ocean and mine it in the most environmentally friendly way possible.”

The potential consequences of disrupting these deep-sea ecosystems are still largely unknown, making caution and further research essential.

 

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