It's not clear exactly where the anammox bacteria obtain the ammonium and nitrite ions they use as nitrogen sources. With this in mind, a team from Germany, the Netherlands and Peru has investigated the nitrogen cycle in the Eastern Tropical South Pacific oxygen minimum zone (OMZ) off Peru. The study found that up to half of the nitrogen loss comes from remineralized nitrogen from organic matter, with the rest coming from deep-sea nitrate via upwelling.
"As oceanic OMZs are expanding globally, it is ever more imperative to incorporate the correct nitrogen loss mechanisms in global biogeochemical models, in order to accurately assess the current oceanic nitrogen balance and more precisely predict how the closely-linked nitrogen and carbon cycles in our future ocean will respond," Phyllis Lam of the Max Planck Institute for Marine Microbiology in Bremen, Germany told environmentalresearchweb. "Since most oceanic N-loss calculations thus far assume that all nitrogen loss comes from deep-sea nitrate, our findings of having up to half of the N-loss come from remineralized nitrogen would mean that oceanic nitrogen budgets need to be revised."
Together with colleagues at the Max Planck Institute, Radboud University Nijmegen in the Netherlands, and the Instituto del Mar del Peru, Lam looked at nitrogen isotopes and functional gene expression analysis to find out more about the nitrogen cycle.
"We found a considerably different and complex picture of nitrogen cycling in the Peruvian OMZ," said Lam. "Our results indicated that anammox was directly coupled to multiple aerobic and anaerobic nitrogen transformations."
It appeared that nitrate reduction provided anammox with nitrite and ammonium ions. "Previously unrecognised, dissimilatory nitrate reduction to ammonium (DNRA) occurred throughout the oxygen minimum zone and could sometimes supply most of the anammox-NH4+ needs," said Lam. "Meanwhile, aerobic ammonia oxidation supplied substantial amounts of NO2–, particularly in the upper oxygen minimum zone, and strongly suggested microaerobic conditions that would enable microaerobic remineralization. Our crude estimates of depth-integrated NO2– and NH4+ fluxes in the Peruvian OMZ suggested that just about half of the nitrogen loss originated from upwelled, deep-sea NO3–."
Now the researchers would like to examine nitrogen transformations in the other major oceanic oxygen minimum zones, in the Arabian Sea, eastern tropical north Pacific and off the coast of Namibia. "We would also like to examine which might be the important environmental factors regulating these interacting N-cycling processes," said Lam.
The researchers reported their work in PNAS.