[Example] The deep sea is home to “dark oxygen”

THE VAST majority of Earth’s oxygen is made as a by-product of photosynthesis, the use of light to convert water and carbon dioxide into sugars. Any oxygen found in regions where photosynthesis is impossible—such as the abyssal seafloor, a pitch-black realm up to 6,000 metres deep—was thought to be surface gas on the move.

In a paper published in Nature Geoscience, however, Andrew Sweetman at the Scottish Association for Marine Science and his colleagues reveal that some is, in fact, “dark oxygen” created without sunlight.

The first indications that this gas was being produced on the abyssal seafloor came in 2013, when researchers noticed that oxygen levels were higher near polymetallic nodules, lumpy agglomerations of metal and minerals found across the ocean floor. The researchers believe that the electric field these nodules exert could strip electrons from the water, liberating oxygen from its molecular prison.

This discovery implies the possibility that aerobic (oxygen-dependent) life arose before photosynthesis evolved. That is controversial. But dark oxygen could certainly help aerobic microbes that survive in the deep today—as well as the larger organisms that eat them. ■