Earth's atmosphere owes its oxygen to life. With an energy boost from sunlight, photosynthesis combines CO₂ and water, yielding sugar and oxygen. But turn off the lights, and making oxygen gets tricky. Only a small handful of microbes are known to do it. But dark oxygen production might be far more common — and important — than previously thought. 

Ammonia-oxidizing archaea (AOA) are widespread microbes found everywhere from the seafloor to Mt. Everest. They convert ammonia into nitrite for energy in an oxygen-dependent process called nitrification. Despite this, AOA somehow thrive in oxygen-minimum zones (OMZs), regions in the ocean where oxygen concentrations plummet.

Researchers at the University of Southern Denmark recently announced in a pre-print (a completed study which has not yet passed peer-review) that an AOA called Nitrosopumilus maritimus may have let them in on the secret to its success in OMZs. Sealed up in airtight containers, N. maritimus grew in the lab under the watch of super-sensitive oxygen sensors. As expected, the cells quickly consumed all available oxygen, using it for nitrification. But then something strange happened. Right after oxygen concentrations hit zero, they rose again. After two years of experiments it was clear that instead of dying out or hibernating after running out of oxygen, N. maritimus made its own oxygen from nitrite, producing dinitrogen (N₂) as a by-product. 

Additional tests confirmed that N. maritimus wasn't using any of the three previously known ways of making oxygen in the dark — its trick was all its own, and not only a novel method of light-independent oxygen production but also a completely new chemical pathway for recycling biological nitrogen into N₂.

This new metabolism can't replace photosynthesis—oxygen in the N. maritimus cultures peaked at levels about 1000x lower than would have been expected from photosynthesis. But because AOA are both incredibly common and critical for the nitrogen cycle, dark oxygen production might be far more important and widespread than previously thought.