The anammox reaction, a microbial process that was first observed in waste-water treatment plants, looks as if it may be a key player in the nitrogen cycle in certain parts of the oceans. In the oceans, denitrification is the process by which nitrate (primarily) is reduced to N_2 ? inert dinitrogen gas. It occurs when certain bacteria decompose organic matter in environments where oxygen concentrations are vanishingly low, and is how 'fixed' nitrogen is converted into a form that cannot be used by most marine plants. This was believed to be the only mechanism of N_2 production in the oceans ― and so by far the largest marine sink for fixed nitrogen. On pages 606 and 608 of this issue, however, Dalsgaard et al. and Kuypers et al. show that large-scale conversion of fixed nitrogen to N_2 is probably occurring through another route. They find that N_2 can be produced by the anaerobic oxidation of ammonia in the oceanic water column, and that this 'anammox reaction' may be common in natural marine environments. Although there was evidence that the anammox reaction occurs in marine sediments, the new findings enlarge the picture and could significantly alter our understanding of nitrogen cycling in the ocean. Denitrification is balanced by nitrogen fixation, which is carried out in surface waters by highly specialized organisms that can reduce N_2 for incorporation into organic tissue. The balance between the two processes is such that, over vast areas, plant productivity in the oceans is limited by the amount of fixed nitrogen. So the details of denitrification and nitrogen fixation are very important.
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