He first tried to show that the dissolved oxygen in the samples was the result of mishandling. “It’s like being Sherlock Holmes,” Ruff said. “You try to find evidence and indications” to disprove your assumptions. However, the dissolved oxygen content seemed consistent across hundreds of samples. Mishandling couldn’t explain it.

If the dissolved oxygen did not come from contamination, where did it come from? Ruff realized that he was on the brink of something big, even though making controversial claims ran against his nature. Many of his co-authors had doubts too: The finding threatened to shatter the foundation of our understanding of subsurface ecosystems.

Making Oxygen for Everyone

In theory, the dissolved oxygen in the groundwater could have originated in plants, microbes or from geological processes. To find the answer, the researchers turned to mass spectrometry, a technique that can measure the mass of atomic isotopes. Typically, oxygen atoms from geological sources are heavier than oxygen from biological sources. The oxygen in the groundwater was light, which implied that it must have come from a living entity. The most plausible candidates were microbes.

The researchers sequenced the genomes of the entire community of microbes in the groundwater and tracked down the biochemical pathways and reactions most likely to produce oxygen. The answers kept pointing back to a discovery made over a decade ago by Marc Strous of the University of Calgary, the senior author of the new study and the head of the laboratory where Ruff was working.

While working in a lab in the Netherlands in the late 2000s, Strous noticed that a type of methane-feeding bacteria often found in lake sediments and wastewater sludges had a strange way of life. Instead of taking in oxygen from its surroundings like other aerobes, the bacteria created its own oxygen by using enzymes to break down the soluble compounds called nitrites (which contain a chemical group made of nitrogen and two oxygen atoms). The bacteria used the self-generated oxygen to split methane for energy.

When microbes break down compounds this way, it’s called dismutation. Until now, it was thought to be rare in nature as a method for generating oxygen. Recent laboratory experiments involving artificial microbe communities, however, revealed that the oxygen produced by dismutation can leak out of the cells and into the surrounding medium to the benefit of other oxygen-dependent organisms, in a kind of symbiotic process. Ruff thinks that this could be what’s enabling entire communities of aerobic microbes to thrive in the groundwater, and potentially in the surrounding soils as well.

Chemistry for Life Elsewhere

The finding fills a crucial gap in our understanding of how the huge subterranean biosphere has evolved, and how dismutation contributes to the cycle of compounds moving through the global environment. The mere possibility that oxygen is present in groundwater “changes our understanding about the past, present and future of subsurface,” said Ruff, who is now an assistant scientist at the Marine Biological Laboratory in Woods Hole, Massachusetts.

Understanding what lives in the subsurface of our planet is also “crucial for translating that knowledge elsewhere,” Sherwood Lollar said. The soil of Mars, for instance, contains perchlorate compounds that some Earth microbes can turn into chloride and oxygen. Jupiter’s moon Europa has a deep, frozen ocean; sunlight may not penetrate it, but oxygen could potentially be produced there by microbial dismutation instead of photosynthesis. Scientists have observed plumes of water vapor shooting from the surface of Enceladus, one of the moons of Saturn. The plumes likely originate from a subsurface ocean of liquid water. If we someday find life on other worlds like those, it could be using dismutation pathways to survive.

Regardless of how important dismutation turns out to be elsewhere in the universe, Lloyd is astonished by how much the new findings defy preconceived notions about life’s needs, and by the scientific cluelessness they reveal about one of the planet’s biggest biospheres. “It’s as if we have had egg on our face all along,” she said.

Editor’s note: Ruff has been awarded early career investigator funding by the Simons Foundation, which also supports Quanta as an editorially independent science news magazine. Funding decisions do not affect editorial coverage.

Correction: July 17, 2023
An earlier version of this article incorrectly described nitrites as containing three oxygen atoms rather than two.