Something pumps out large amounts of oxygen on the bottom of the Pacific Ocean, in depths in which a complete lack of sunlight makes photosynthesis impossible.

The phenomenon was discovered in a region that is covered with old, plum-sized formations called polymetal tubers that could catalyze oxygen production by probably promoting the split of water molecules. The results are published in Nature Geoscience 1

"We have another oxygen source on the planet, apart from photosynthesis," says study mitachor Andrew Sweetman, a marine floor ecologist at the Scottish Association for Marine Science in Oban - although the mechanism behind this oxygen production remains a mystery. The results could also have an impact on understanding, as life began, he says, as well as the possible effects of Tiefsebergban

The observation is "fascinating," says Donald Canfield, a biogeochimist at the University of South Denmark in Odense. "But I find it frustrating because it raises a lot of questions and does not provide very many answers."

sweetman and his employees noticed somewhat discrepanced during the field work in 2013. The researchers examined sea floor ecosystems in the Clarion-Clipperton-Zone between Hawaii and Mexico, which is larger than India and a potential goal for the mining of metal-rich bulb. During such expeditions, the team releases a module that sinks to the sea floor in order to carry out automated experiments. There the module drives cylindrical chambers down to lock small sections of the sea floor - together with a little sea water - and create "a closed microcosm of the sea floor", the authors write. The "Lander" then measures how the oxygen concentration in the locked sea water changes over time periods of up to several days.

oxygen flows

Without any photosynthetic organisms that release oxygen into the water, and with any other organism that consumes the gas, the oxygen concentrations within the chambers should slowly drop. Sweetman has observed this in studies that he did in areas of southern, Arctic and Indian oceans as well as in the Atlantic. Worldwide, sea floor ecosystems owe their existence to the oxygen, which is brought up by currents from the surface, and would quickly die if they were cut off. (Most of this oxygen comes from the North Atlantic and is transported to the deep oceans of the world by a "global conveyor belt".)

But in the Clarion Clipperton zone, the instruments showed that the locked water richer, not poorer, became oxygen. First, Sweetman attributed the readings to a sensor error. But the phenomenon occurred again and again during the following expeditions in 2021 and 2022 and was confirmed by measurements with an alternative technology. "Suddenly I realized that I had ignored this potentially amazing new process, 4,000 meters deep on the sea floor for eight years," says Sweetman.

A detailed view of a tuber on a petri dish.

The amount of oxygen produced is not low: the gas in the chambers reaches concentrations higher than that in algae -rich surface waters, says Sweetman. None of the other regions that Sweetman examined contained polymetal tubers, which indicates that these stones play an important role in the production of this "dark oxygen".

As the first test of this hypothesis, the team reproduced the conditions that they found on the sea floor in a laboratory on their ship. They monitored samples that were collected by the sea floor-including polymetal tubers-and found that the oxygen concentration increased at least temporarily. "They start to produce oxygen to a certain point. Then they stop," says Sweetman - presumably because the energy that drives water molecules is exhausted. This raises the question of where this energy comes from. If the tubers themselves acted as batteries - energy generated by a chemical reaction - they would have been exhausted long ago.

electrical potential

But the tubers could serve as catalysts that enable the splitting of water and the formation of molecular oxygen. The researchers measured tensions on the surface of tubers and found tension differences of up to 0.95 volts. This is not entirely sufficient for the 1.5 volt that are required to split a water molecule, but in principle higher voltages could be generated, similar to how battery stresses can be doubled by switching two batteries in series, says Sweetman.

Mit author Franz Geiger, a chemist at Northwestern University in Evanston, Illinois, says that it is still unclear whether the reaction also creates molecular hydrogen - which is released in industrial electrolysisur reactions thanks to a catalyst - or protons in the water while it is pushing the remaining electrons elsewhere. But the understanding could ultimately have useful applications, he says. "Perhaps there is a blueprint on the sea floor that could help us produce better catalysts."

Eva Stüeken, a biogeochimist at the University of St Andrews, Great Britain, says that the results could also have an impact on suggestions to look for possible life in the light spectrum of extrasolar planets. "The presence of O 2 gas on other planets may have to be interpreted with additional caution," she says.

sweetman says that researchers before the deep -sea mountain building begins should map the areas where oxygen is produced. Otherwise, ecosystems that have become dependent on this oxygen could collapse if the tubers are removed. "If large amounts of oxygen are produced, this may be important for the animals living there."