Marine microorganisms capable of producing oxygen in the dark

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Many species need oxygen to live. And if our atmosphere is rich in oxygen, it is mainly thanks to the photosynthetic mechanism that plants, algae and cyanobacteria are capable of. This mechanism could not take place without sunlight. However, researchers have recently discovered a marine microbe (Nitosopumilus maritimus) – an oxidizing archaea of ​​ammonia that lives in the depths of the ocean – capable of regenerating small amounts of oxygen when it is in anoxic conditions, and this, without any contribution of light.

Ammonia oxidizing archaea (AOA) are one of the most abundant groups of microbes in the world’s oceans and are key players in the nitrogen cycle, a process in which ammonia (NH3) is oxidized to nitrites (NO2–) , then in other nitrogenous forms. . This process requires oxygen, but AOAs often live in oxygen-poor environments. Researchers at the University of Southern Denmark have found that when unavailable, these microorganisms can produce the oxygen required for the reaction from nitrites.

Other microbes are known to produce oxygen without light, but it usually only exists in limited quantities and in very specific environments. The sand in question here, Nitosopumilus maritimus, is not only one of the smallest living organisms known (around 200 nanometers), but it is particularly abundant in the oceans. “These microbes are so common that one in five cells in a bucket of seawater is part of them,” said Don Canfield, professor of ecology at the University of Southern Denmark and co-author of the study news release linking the discovery.

Small amounts but sufficient to ensure survival

These microorganisms are very common in the oceans and survive even in areas where oxygen is scarce, which has greatly puzzled scientists. Your energy metabolism requires oxygen, what could be the reason for its presence in these anoxic environments? Did they have any function? To answer these questions, the researchers took samples of these microbes to study their behavior in the laboratory, based on the surrounding oxygen level.

Placed in oxygen-rich water, the microbes naturally slowly consumed all the available oxygen. But once it was completely depleted, the oxygen level began to rise as the microbes plunged into darkness – the archaea produced their own oxygen, likely from the disproportionate nitric oxide, the researchers say. At the same time, the microbes even produced dinitrogen. “Isotopic labeling of nitrogenous species has revealed a series of reactions that transform nitrite, the expected metabolic end product, into nitric oxide, nitrous oxide, and eventually dinitrogen,” the researchers write in Science.

Although this capacity is impressive, the scientists point out that the amounts of oxygen thus produced are relatively small and could in no way influence the oxygen levels of our planet. However, it allows these microorganisms to survive and even help other organisms. “If they produce a little more oxygen than they need, other nearby organisms will quickly absorb it, so the oxygen will never leave the ocean,” says Beate Kraft, assistant professor in the Department of Biology and a co-author. of The study.

But the main point of this discovery lies not only in the fact that Nitosopumilus maritimus is capable of producing nitrogen and oxygen, but also in the fact that it contributes greatly to the removal of bioavailable nitrogen from its environment.

Part of the nitrogen cycle previously unknown

Nitrogen is a basic compound of organic matter; it enters in particular in the composition of amino acids and nucleic bases of DNA. Thus, for all living organisms, the nitrogen cycle, which takes place on land and in the ocean, is as important as photosynthesis. To begin with, nitrogen-fixing bacteria produce ammonia from atmospheric nitrogen; then other bacteria, in the presence of oxygen, transform ammonia (NH3) into nitrites (NO2–), then into nitrates (NO3–). Finally, other microorganisms are responsible for reducing these nitrites and nitrates successively to nitrogen monoxide (NO), nitrous oxide (N2O), then dinitrogen (N2), which ends up returning to the atmosphere.

Nitosopumilus maritimus plays an important role in the nitrogen cycle. But as soon as oxygen is in short supply, it can produce its own oxygen, as well as dinitrogen, from the reaction products, as illustrated above. © B. Kraft et al.

The sequence of reactions initiated by Nitosopumilus maritimus appears to be slightly different, since the production of oxygen is linked to the production of nitrogen gas. “The metabolic pathway is not fully resolved, but it likely involves nitric oxide and nitrous oxide as key intermediaries,” the researchers note. AOAs are known to maintain the global nitrogen cycle, but the researchers did not expect to find such mechanisms. And if this lifestyle is more prevalent in the oceans than you think, it would challenge your understanding of the marine nitrogen cycle.

The next step is to study the phenomenon observed in the laboratory in oxygen-depleted waters in various oceanic locations around the world. The research team has already collected samples from the Mariager Fjord in Denmark and will soon be looking at the waters off Mexico and Costa Rica.

Science, B. Kraft et al.

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