Dark oxygen produced by deep-sea ‘batteries’

Scientists have discovered “dark oxygen” produced in the deep ocean, apparently by lumps of metal on the sea floor.

About half of the oxygen we breathe comes from the ocean. But before this discovery, it was known to be made by the photosynthesis of sea plants – something that requires sunlight.

Here, at a depth of 5km, where no sunlight can penetrate, the oxygen appears to be produced by naturally occurring metal ‘nodules’ that split seawater – H2O – into hydrogen and oxygen.

Several mining companies plan to collect these nodules, which scientists fear could disrupt the newly discovered process — and harm any marine life that depends on the oxygen they produce.

“I first saw this in 2013 – huge amounts of oxygen are being produced on the seabed in complete darkness,” explains lead researcher Professor Andrew Sweetman of the Scottish Marine Science Association. “I just ignored it because I was taught – you only get oxygen through photosynthesis.”

“I finally realized that I had been ignoring this potentially huge discovery for years,” he told BBC News.

He and his colleagues conducted their research in the region of the deep sea between Hawaii and Mexico – part of a vast swath of seafloor that is covered with these metal nodules. Nodules form when dissolved metals in seawater collect on fragments of shell – or other debris. It is a process that takes millions of years.

And because these nodules contain metals like lithium, cobalt and copper – all of which are needed to make batteries – many mining companies are developing the technology to collect them and bring them to the surface.

But Professor Sweetman says the dark oxygen they produce could also support life on the sea floor. And his discovery, published in the journal Nature Geoscience, raises new concerns about the risks of proposed deep-sea mining ventures.

Scientists have discovered that the metal nodes are able to produce oxygen precisely because they function as batteries.

“If you put a battery in seawater, it starts to hiss,” Professor Sweetman explained. “This is because the electric current actually splits the seawater into oxygen and hydrogen [which are the bubbles]. We think this happens with these nodes in their natural state.’

“It’s like a battery in a flashlight,” he added. “If you put one battery in, it won’t light up.” You put two in and you have enough voltage to light a torch. So when the nodes sit on the seabed and are in contact with each other, they work in unison – like several batteries.”

Scientists tested this theory in the laboratory, collecting and studying metal nodules the size of potatoes. Their experiments measured the surface tension of each metal lump—essentially the strength of an electric current. They found that it is almost equal to the voltage in a typical AA-sized battery.

That means, they say, the nodules sitting on the seafloor could generate electrical currents large enough to split, or electrolyze, seawater molecules.

Scientists believe the same process—battery-powered oxygen production that requires no light and no biological process—could take place on other moons and planets, creating oxygen-rich environments where life could thrive.

The Clarion-Clipperton zone, where the discovery was made, is an area already being explored by a number of seabed mining companies that are developing technology to collect the nodules and transport them to a ship on the surface.

The US National Oceanographic and Atmospheric Administration has warned that this seafloor mining could “result in the destruction of seafloor life and habitat in mined areas”.

More than 800 marine scientists from 44 countries have signed a petition warning of the environmental risks and calling for a halt to mining activity.

New species are constantly being discovered in the depths of the ocean – it is often said that we know more about the surface of the moon than the depths of the sea. And the discovery suggests that the nodules themselves could provide oxygen to support life there.

Professor Murray Roberts, a marine biologist from the University of Edinburgh, is one of the scientists who signed the seabed mining petition. “There is already overwhelming evidence that strip mining will destroy ecosystems we barely understand,” he told BBC News.

“Because these fields cover such vast areas of our planet, it would be crazy to continue deep mining knowing that they can be a significant source of oxygen production.”

Professor Sweetman added: “I don’t see this study as something that will end mining.

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