Oxygen May Be Produced Without Sunlight as Deep as Dark Parts of Ocean, Researchers Say

Oxygen has been historically linked to sunlight. But a new study has suggested that oxygen may also be produced without any sunlight at all.

Scientists at the University of Southern Denmark recently published a study, Oxygen and nitrogen production by an ammonia-oxidizing archaeon, suggesting that one particular microbe is defying previously believed science regarding the correlation between sunlight and oxygen production.

The study, published January 6 in Science, documents the microbe Nitrosopumilus maritimus and its cousins, known as ammonia-oxidizing archaea. The scientists stated that contrary to popular belief and discoveries made regarding other microbes in particular habitats, these specific microbes are able to produce oxygen without sunlight, potentially in some of the darkest depths of our environment.

"These guys are really abundant in the oceans, where they play an important role in the nitrogen cycle," said study lead author, microbiologist and University of Southern Denmark professor Beate Kraft. "For this they need oxygen, so it has been a long-standing puzzle why they are also very abundant in waters where there is no oxygen."

Oxygen is the spark that keeps life constant on Earth, benefiting plants, algae and other bacteria. Sunlight remains an integral part of photosynthesis. Innumerous species inhabit the earth's water bodies, though those that live in dark environments and are able to make oxygen are often discovered in limited capacities.

Kraft told Newsweek that she and the others wanted to study how ammonia-oxidizing archaea react to oxygen depletion under controlled laboratory conditions.

"Researchers had frequently found them in environments without oxygen," Kraft said. "We wanted to find out how they can survive there because for the oxidation of ammonia, with which they sustain themselves, they need oxygen.

"We did not expect to see any oxygen production at the beginning of the project."

Nitrospumilus maritimus is the first and best-studied isolate of this group of archaea and was isolated by Martin Könneke, a professor and one of the co-authors.

Kraft said the study showed oxygen production in the archaeon Nitrosopumilus maritimus, which is a marine ammonia-oxidizing archaeon. It has now been in culture for 15 years.

All experiments were conducted in the dark, she added, with no comparison between sunlight and darkness. Researchers wanted to visualize how abundant these ammonia-oxidizing archaea are, as they compose up to 20 percent of the microbial community.

While already knowing that the ammonia-oxidizing archaea are microorganisms that continue the global nitrogen cycle, researchers were unaware of the full extent of their capabilities.

It was discovered that Nitrosopumilus maritimus couples the oxygen production to the production of gaseous nitrogen, in turn removing bio-available nitrogen from the environment.

While the microbe did not make enough oxygen to have a heavy-handed influence on overall Earth levels, enough oxygen was created to keep the organisms alive in dark environments.

"We saw how they used up all the oxygen in the water, and then to our surprise, within minutes, oxygen levels started increasing again," said study co-author and university biology professor Donald Canfield. "That was very exciting."

Kraft said that since almost all oxygen on Earth is produced by photosynthesis, the production of oxygen by ammonia-oxidizing archaea is probably only locally important.

"Neighboring microbes may benefit," she said. "That these microbes are so abundant in ocean waters is what makes this finding so exciting."

It remains unclear how widespread and important the discovered pathway is in different environments and water bodies, she said.

Her next intentions involve investigating organisms in other ocean-based locations worldwide, including near Mexico and Costa Rica, as well as the Mariager Fjord in Denmark.

"We are particularly interested in marine environments in which oxygen is depleted," Kraft said. "For ammonia-oxidizing archaea, it becomes especially beneficial to produce oxygen in environments where oxygen is absent. In the water bodies off the coast of Mexico and Costa Rica, so-called oxygen minimum zones naturally occur. Here, oxygen saturation in the water column is at its lowest. These zones are marine hotspots of microbial nitrogen cycling."

Update 1/13/22, 9:49 AM ET: with additional comment from Beate Kraft to Newsweek.