Lynn Margulis (1938–2011)

of the nitrogen cycling has major consequences for other biogeochemical processes and ecosystem functions and services, Voss says. Recent research has uncovered an apparent imbalance in the nitrogen budget of the oceans, which isn’t completely understood yet and may point to additional processes that remain to be discovered. The traditional view that bacterial denitrification is the only significant process removing oxidised nitrogen species from the oceans had to be revised after the discovery of anammox (anaerobic ammonia oxidation) and of denitrifying eukaryotes in tropical waters. Similarly, recent discoveries have also broadened the range of organisms involved in nitrogen fixation in the oceans. Climate change is closely linked to the marine nitrogen balance, as changes in water temperature and the distribution of dissolved gases are likely to perturb the natural cycles. Conversely, the oceans are a major emitter of N2O, accounting for around 30% of the global balance of this compound, which acts as a strong greenhouse gas in the troposphere. Oxygen-deficient zones in the oceans are of particular interest for the nitrogen balance, because only they can produce a net depletion of reactive nitrogen species. Stoichiometric calculations predict that complete anaerobic removal of organic matter of typical composition should lead to 7 % of nitrogen being removed by denitrification and 29% by anammox. However, several studies in such zones in the Arabian Sea have found either much smaller proportions of denitrification or none at all. “There are several alternative explanations for this apparent deviation from theory, but as yet there is no consensus on this issue,” says Voss. “There are still major uncertainties in our understanding of the oceanic cycling of nitrogen,” Voss concludes. These affect important issues such as the imbalances in nitrogen input and removal and their effects on ecosystems and biodiversity as well as the release of N2O and the mutual influences between the nitrogen cycle and climate change. In short, we are upsetting a system that we are only beginning to understand. Most of the excess nitrogen from agriculture ends up in coastal waters and has to be denitrified there. “Up to now it seems that the human load is largely removed,” says Voss. “However, when they turn anoxic — a phenomenon often observed along eutrophied coasts — this service of the system may be lost. In a consequence we would upload all the reactive nitrogen to the marine system.”