For centuries, it was believed that the production of oxygen relied on one component: photosynthesis (the conversion of light energy into chemical energy). Plants and algae use sunlight to convert carbon dioxide into oxygen, providing the foundation of life on earth as we know it. However, the discovery of ‘dark oxygen’ — produced in environments with little to no sunlight — challenges this assumption, proving that oxygen production is not actually limited to photosynthesis.
In 2013, Scottish marine scientist Andrew Sweetman made this discovery by accident while conducting deep sea mining research in the Clarion-Clipperton Zone. The CCZ is a vast plain in the central Pacific Ocean, located between Hawaii and Mexico. Sweetman, much to his surprise, measured elevated levels of oxygen the further he was descending into the sea. Upon this discovery, Sweetman could not help but believe that his measuring devices were incorrect and dismissed the reading entirely. The deep seabed is one of the coldest, harshest and darkest places on earth. With no light source, it seemed impossible to Sweetman that oxygen could exist there, as he previously assumed the only way to produce it was through photosynthesis, a reaction that requires light.
However, over the last 10 years, Sweetman, along with teams of scientists, has been conducting more experiments in the CCZ. These researchers carried out numerous ‘in situ’ benthic chamber lander experiments, wherein they sectioned out a part of the seabed inside a chamber and measured the oxygen levels over time. They also expected a reduction in the levels of oxygen due to its consumption from small organisms like microbes for respiration. However, oxygen levels consistently increased, proving a net production of oxygen in the dark depths of the ocean. In fact, in some experiments, oxygen levels rose to more than three times the background concentration.
Scientists think one potential explanation for increased oxygen levels is the existence of polymetallic nodules, substances which range in size from a small particle to about the dimensions of a potato. Sweetman explains that these nodules are effectively batteries in a rock which uses electric charge to split the seawater into hydrogen and oxygen. This split can occur with up to 1½ volts of electricity, and some nodules can possess up to 0.95 volts of electricity. Multiple nodules combined can produce even higher voltages, allowing this seawater electrolysis to occur. In 2024, Sweetman and his team of researchers shared and published these findings.
This discovery could revolutionize deep sea mining primarily because these polymetallic nodules contain metals such as manganese, nickel and cobalt, which are essential for everyday life. They are specifically used in lithium-ion batteries, which are implemented in consumer appliances like technological devices and power tools, as well as products like electric vehicles. Mining companies view this metal as a new, potentially pursued source. However, the production of this dark oxygen may be vital for deep sea ecosystems and disturbing it could be harmful to species relying on it. Hence, it is wise to take caution and properly weigh the costs and benefits of deep sea mining.
Explorers have seen less than 0.001% of the ocean floor and it is entirely possible that discoveries like dark oxygen are just the beginning. We can be certain that there are countless other mysteries about the ocean hidden in darkness and waiting to be discovered.
