Nitrogen puzzle in oceans

Every organism needs nitrogen to survive and grow. Many organisms do not have the ability to obtain nitrogen from molecular nitrogen (N₂), the major component in the atmosphere. They do not have the nitrogen fixation pathway -- and have to rely on supply of nitrogen that has been fixed by others. The availability of fixed nitrogen, in the form of ammonium, nitrite and nitrate, consequently often limits primary production in the environment (one of the reasons why many fertilizers are rich in fixed nitrogen).

However, there are microbial processes that convert fixed nitrogen back to N₂ (production of energy instead of growth). Scientists call this process loss of fixed nitrogen, because it removes the important fixed nitrogen from the environment, and thereby limits primary productivity (i.e. production of biomass). These nitrogen-loss processes, which are carried out by different types of microbes, include the reduction of nitrogen compounds like nitrate and nitrite, the oxidation of ammonium, and a process that combines nitrite and ammonium to form N₂, the anaerobic ammonium oxidation (anammox).

In the water column of the ocean, these nitrogen-loss processes are most prominent in water bodies, known as oxygen minimum zones (OMZs), where dissolved oxygen (O₂) is rapidly consumed almost to completion. There is major concern that such OMZs will expand in the future due to climate change, which could have a massive impact on the amount of nitrogen lost from the marine realm, affecting the primary productivity in the ocean.

For these reasons, it is important to know which microbial process is responsible for what part of the observed nitrogen-loss, and where this process happens within OMZs.

Ben Brunner, one of the three main authors, explains: "We can answer this question with the help of stable nitrogen isotopes, by looking at the ratio between the stable isotopes ¹⁵N and ¹⁴N in the different pools of fixed N and in the produced N₂, because different microbial processes leave different N isotope fingerprints; some prefer the light isotope ¹⁴N over the heavier isotope ¹⁵N, and others do the opposite." Sergio Contreras, a (paleo) biogeochemist interested in the past and future of the Nitrogen cycling, continues: "However, the prerequisite to decipher the N isotope signatures in the environment is to know the isotope fingerprint of the individual nitrogen-loss processes." Moritz Lehmann, isotope biogeochemist from the University of Basel, adds: "This is where so far, there was a gaping hole in our knowledge. The isotope effects of one major N-loss process, namely anammox, were unknown, and previous N-isotope based assessments of fixed N loss rates in the global ocean may have been severely biased."

Boran Kartal, microbiologist at Radboud University Nijmegen, explains: "We used the highly enriched cultures that are available in our laboratory to determine the nitrogen isotope effects of anammox bacteria. Our findings show that the isotope effects induced by anammox can explain isotope signatures observed in the OMZs, which are very important primary production sites in the oceans."

Marcel Kuypers, director at the Max Planck Institute, summarizes: "This missing piece of information is of utmost importance to solve the nitrogen isotope puzzle, not only because anammox is an important process in OMZs, but also because anammox simultaneously affects the nitrogen isotope composition of all nitrogen pools of interest: it converts ammonium and nitrite to N₂ and nitrate."

Through their joint effort the scientists were able to decipher the intricate isotope fingerprint of anammox. Their results, published in the Proceeding of the National Academy of Sciences of the United States of America, reconcile so far mysterious N isotope patterns from OMZ, and provide the missing piece to solve the nitrogen isotope puzzle for fixed N-loss from the environment.