There is more going on in the deep, dark ocean waters than you may think: Uncountable numbers of invisible microorganisms go about their daily lives in the water columns, and now researchers have discovered that some of them produce oxygen in an unexpected way. The study is published in the journal Science.

Oxygen is vital for life on Earth, and is mainly produced by plants, algae and cyanobacteria via photosynthesis. A few microbes are known to make oxygen without sunlight, but so far they have only been discovered in very limited quantities and in very specific habitats.

Enter the ocean living microbe Nitrosopumilus maritimus and its cousins, called ammonia oxidizing archaea.

"These guys are really abundant in the oceans, where they play an important role in the nitrogen cycle. 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", says biologist, adding:

"We thought; They just hang out there with no function; they must be some kind of ghost cells".

But there was something puzzling to this;

"These microbes are so common, that every 5^th cell in a bucket of sea water is one of them", adds co-author of the paper.

So the researchers became curious; could they have a function in the oxygen depleted water after all?

The authors decided to test them in the lab;

"We wanted to see what would happen if they ran out of oxygen – like they do when they move from the oxygen rich waters to oxygen depleted waters. Would they survive?" 

"We saw how they used up all the oxygen in the water, and then to our surprise, within minutes, oxygen levels started increasing again. That was very exciting!", the author recalls.

Nitrosopumilus maritimus turned out to be able to make oxygen in a dark environment. Not much – not at all so much that it would influence oxygen levels on Earth, but enough to keep itself going.

"If they produce a little more oxygen than they need themselves, it will quickly be taken by other organisms in their neighborhood, so this oxygen would never leave the ocean", the author explains.

But what effect do they have on the environment they live in, these extremely abundant oxygen-producing microbes?

Researchers already knew that the ammonia oxidizing archaea are microorganisms, that keep the global nitrogen cycle going, but they were not aware of the full extent of their capabilities.

In the newly discovered pathway, Nitrosopumilus maritimus couples the oxygen production to the production of gasous nitrogen. By doing so they remove bioavailable nitrogen from the environment.

"If this lifestyle is widespread in the oceans, it certainly forces us to rethink our current understanding of the marine nitrogen cycle", adds the author.

"My next step is to investigate the phenomenon we saw in our lab cultures in oxygen depleted waters in various ocean spots around the world", the author adds.

https://www.science.org/doi/10.1126/science.abe6733