Normally light is needed – oxygen is produced by photosynthetic organisms on earth. But now researchers are reporting an exciting exception: Certain microbes can produce oxygen in dark areas of the sea in order to supply themselves with the elixir of life. Since these ammonia-oxidizing representatives of the archaea are very common in the oceans, the metabolic pathway could be of importance for the marine nitrogen cycle, say the scientists. How exactly the single-cell organisms produce oxygen, however, has yet to be clarified.
The famous process of photosynthesis forms the basis of life on our planet in two ways: Plants, algae and cyanobacteria use light to generate the energy sources at the base of the food chain. The organisms that carry out photosynthesis also release oxygen, which in turn serves as an oxidizing agent for animals and many microorganisms for their energy metabolism. It was already known that some microbes can produce oxygen without sunlight, but until now these creatures were considered rare exceptions: They were only discovered in limited quantities and in very special habitats.
The researchers led by Beate Kraft from the University of Southern Denmark in Odense, on the other hand, now focused on single-cell organisms, which are very common in the oceans and which are already assigned an important role in the earth’s nitrogen cycle: ammonia-oxidizing archaea convert NH3 to nitrite (NO2) and thus start an important biological process in the breakdown of biomass – the so-called nitrification. In order to be able to convert ammonia to nitrite, however, the microorganisms need, as is well known, molecular oxygen. “That is why it was previously unclear why some are often found in bodies of water with very low oxygen concentrations,” says Kraft. “At first we thought that they would just hang around there without being active – like a kind of ghost cell,” says the scientist.
Active even when there is a lack of oxygen
In order to find out more about the connections, the scientists decided to experiment with how the microbes react to a lack of oxygen. They carried out experiments with laboratory cultures of the Archae Nitrosopumilus maritimus. The conditions corresponded to those as they can occur in the deep and dark sea areas in which these single-celled organisms occur. “We wanted to see what happens when you switch from oxygen-rich water to oxygen-poor water. Would they survive? ”Said Kraft.
To their surprise, the scientists discovered: “After the microbes had used up the last of the oxygen, the content in the water rose again within minutes. It was very exciting! ”Says senior author Don Canfield from the University of Southern Denmark. It turned out that Nitrosopumilus maritimus is able to produce oxygen in a dark environment. The researchers also found that the unicellular organisms release gaseous nitrogen (N2) in an oxygen-poor environment.
As can be seen from the analyzes, the amount of oxygen produced is apparently sufficient to operate the ammonia metabolism. How the microbes produce the gas, however, must first be investigated. Apparently this is a previously unknown concept. The scientists did not find any genes in the genome of Nitrosopumilus maritimus that are previously known for processes in the production of oxygen by microbes.
Possible importance for the marine nitrogen cycle
But what is the significance of the findings? The scientists say that at least the oxygen production by Nitrosopumilus maritimus cannot have an impact on terrestrial supplies. At most, local effects on other organisms in the area seem possible. “If these microbes produce a little more oxygen than they need themselves, it is quickly taken up by other organisms in their vicinity, so that this oxygen would never leave the ocean,” explains Kraft.
Still, the system may have far-reaching implications. Because when certain archaea can link the production of oxygen to the production of gaseous nitrogen, they remove bioavailable nitrogen from the environment. “If this way of life is widespread in the oceans, we would have to change our previous view of the marine nitrogen cycle,” said Kraft. To what extent this is the case and the details of the metabolic processes involved, she and her colleagues now want to clarify through further studies.
Source: University of Southern Denmark, University of Oldenburg, specialist article: Science, doi: 10.1126 / science.abe6733