A new study suggests that metals on the deep sea floor, 4,900 meters deep, produce “dark oxygen.”

The discovery refutes the long-held belief that oxygen on Earth is produced exclusively by photosynthetic organisms – those that use sunlight, water and carbon dioxide to produce fuel and oxygen – such as plants and algae.

It might even raise the question of how life on Earth began.

The study suggests that oxygen can be produced even on the seafloor, where no light can reach, to support oxygen-breathing (aerobic) marine life in total darkness.

Andrew Sweetman of the Scottish Association for Marine Science (SAMS) made the discovery of “dark oxygen” during field research from a ship in the Pacific Ocean.

He said: “For aerobic life to arise on the planet, there had to be oxygen, and according to our current understanding, the Earth’s oxygenation began with photosynthetic organisms.

“But we now know that oxygen is produced in the deep sea, where there is no light. So I think we need to revisit questions like this: Where could aerobic life have started?”

Prof. Sweetman made the discovery while taking seafloor samples from the Clarion-Clipperton Zone, a mountainous underwater ridge along the seafloor that stretches for nearly 7,200 kilometers along the northeast quadrant of the Pacific Ocean.

When they first detected oxygen, he assumed the device must be defective.

“When we first received this data, we thought the sensors were faulty because all studies ever conducted in the deep sea have only shown oxygen consumption, not production,” said Prof. Sweetman.

“We came home and recalibrated the sensors, but over the course of ten years these strange oxygen levels kept appearing.”

The scientists then decided to try a different method, which, however, led to the same result.

Prof. Sweetman said: “When both methods produced the same result, we knew we had stumbled upon something groundbreaking and unimaginable.”

The discovery focuses on natural mineral deposits, so-called polymetallic nodules, that form on the seabed.

They are a mixture of different minerals and their size can range from tiny particles to that of an average potato.

Co-author Franz Geiger of Northwestern University in the US said: “The polymetallic nodules that produce this oxygen contain metals such as cobalt, nickel, copper, lithium and manganese – all important elements used in batteries.

“Several major mining companies are currently targeting these valuable elements from the seafloor at depths of 10,000 to 20,000 feet below the surface.

“We need to rethink our methods of mining these materials so that we do not deprive life in the deep sea of ​​oxygen.”

In previous studies, Prof. Geiger found that rust in combination with salt water can generate electricity.

The researchers investigated whether the nodules in the deep sea generate enough electricity to produce oxygen – just 1.5 volts – the same voltage as a typical AA battery – is enough to split seawater.

According to the study, voltages of up to 0.95 volts were measured on the surface of individual nodules. If several nodules are located together, the voltage can be even significantly higher – for example, if batteries are connected in series.

Prof. Geiger said: “It seems we have discovered a natural ‘geobattery’.

“These geobatteries are the basis for a possible explanation of dark oxygen production in the ocean.”

The results were published in the journal Nature Geoscience.