A previously unknown biological pathway allows a large-scale type of oceanic archaeon to release oxygen and nitrogen in the dark—the first time such a phenomenon has been seen in nature in a hypoxic environment. The process, the details of which are not yet fully understood, could change scientists’ understanding of the cycle of key nutrients in the oceans.
Ammonia-oxidizing archaea are among the most abundant microbes in the world’s oceans. They obtain energy by using oxygen to oxidize ammonia to nitrite. However, they have been found in ecosystems without detectable oxygen. “Do they settle from the upper water column or is there a small or periodic introduction of oxygen that we don’t see?” asks Donald Canfield of the University of Southern Denmark. “These are the kinds of explanations people usually use.”
Canfield and his colleagues, led by Beate Kraft, have studied this problem, trying to ascertain how to name a particular species Nitrosopumilus maritimus They may respond to changes in oxygen levels as a result of global warming, for example. “Betty found that the organism would draw oxygen to near zero and then increase the oxygen again, and that was really weird,” Canfield says.
After three years of additional experiments, the researchers demonstrated that the archaea were indeed producing their own oxygen. This was previously found only in NC10 bacteria, which break down nitric oxide into nitrogen and oxygen and use the oxygen to oxidize methane. However, NC10 bacteria are not known to release oxygen. Furthermore, measurements of the onset of oxygen and nitrogen production, as well as isotope labeling experiments, indicated that Nitrosopumilus maritimus Nitrogen and oxygen are produced through a different pathway than NC10. The researchers suggest that this may include nitrous oxide as a feedstock. “I feel like we made a good argument for that, but it still has to be proven in a more rigorous way,” Canfield says.
Researchers are now studying whether other ammonia-oxidizing archaea can do the same trick to grow under persistently hypoxic conditions. If so, it could change the scientific community’s understanding of ocean fertility in a warm climate. If we had more oxygen removed from the oceans, would we have more N2 Loss?’ Canfield asks.
“This is definitely a major step,” says microbiologist Willem Martins Habina of the University of Florida. “If this primitive activity was present in the oxygen minimum regions, scientists would not be able to detect it, because no one knew what pattern to look for in isotope labeling experiments,” he says. If this process is important, new tools must be developed to characterize what is canonical denitrification, what is anaerobic ammonia oxidation and what is this primitive metabolism that contributes to nitrogen loss in the oxygen minimum regions.