A bed of manganese nodules offshore of the Cook Islands

About half the oxygen we breathe comes from the ocean.  It was long presumed that this oceanic oxygen was produced by photosynthesis, the process by which green plants use sunlight to synthesize foods by absorbing carbon dioxide and water, emitting oxygen as a byproduct. For at least a decade, however, researchers have observed significant levels of oxygen in the dark, deep ocean, far too deep for sunlight to reach and therefore too deep for photosynthesis.

New research published in the journal Nature Geoscience reveals that nature has devised a way to produce oxygen without the involvement of plants. It’s “an amazing and unexpected finding,” Daniel Jones, a researcher at the National Oceanography Center in the United Kingdom who wasn’t involved in the study, tells CNN’s Katie Hunt.

Rather than relying on sunlight and plants, the new research concluded that “dark oxygen” is being produced in the deep ocean, apparently by electrolysis involving lumps of metal on the seafloor.

The discovery was so astonishing that, when lead author Andrew Sweetman first measured this “dark oxygen” in the Pacific Ocean’s Clarion-Clipperton Zone in 2013, he dismissed it outright.

“I just ignored it, because I’d been taught—you only get oxygen through photosynthesis,” Sweetman, an ecologist with the Scottish Association for Marine Science, tells Victoria Gill of BBC News. “Eventually, I realized that for years I’d been ignoring this potentially huge discovery.”

On the deep ocean floor where no sunlight can penetrate, the oxygen appears to be produced by the electrolysis of naturally occurring metallic “nodules” which split seawater – H2O – into hydrogen and oxygen.

Several mining companies have plans to collect these nodules, which marine scientists fear could disrupt the newly discovered process – and damage any marine life that depends on the oxygen they make.

Thanks to Alaric Bond for contributing to this post.