N2 production by the anammox reaction in the anoxic water column of Golfo Dulce, Costa Rica

In oxygen-depleted zones of the open ocean, and in anoxic basins and fjords, denitrification (the bacterial reduction of nitrate to give N-2) is recognized as the only significant process converting fixed nitrogen to gaseous N-2. Primary production in the oceans is often limited by the availability of fixed nitrogen such as ammonium or nitrate(1), and nitrogen-removal processes consequently affect both ecosystem function and global biogeochemical cycles. It was recently discovered that the anaerobic oxidation of ammonium with nitrite-the 'anammox' reaction, performed by bacteria-was responsible for a significant fraction of N-2 production in some marine sediments(2). Here we show that this reaction is also important in the anoxic waters of Golfo Dulce, a 200-m-deep coastal bay in Costa Rica, where it accounts for 19-35% of the total N-2 formation in the water column. The water-column chemistry in Golfo Dulce is very similar to that in oxygen-depleted zones of the oceans-in which one-half to one-third of the global nitrogen removal is believed to occur(3,4). We therefore expect the anammox reaction to be a globally significant sink for oceanic nitrogen.