In the global nitrogen cycle, bacterial denitrification is recognized as the only quantitatively important process that converts fixed nitrogen to atmospheric nitrogen gas, N 2 , thereby influencing many aspects of ecosystem function and global biogeochemistry. However, we have found that a process novel to the marine nitrogen cycle, anaerobic oxidation of ammonium coupled to nitrate reduction, contributes substantially to N 2 production in marine sediments. Incubations with 15 N-labeled nitrate or ammonium demonstrated that during this process, N 2 is formed through one-to-one pairing of nitrogen from nitrate and ammonium, which clearly separates the process from denitrification. Nitrite, which accumulated transiently, was likely the oxidant for ammonium, and the process is thus similar to the anammox process known from wastewater bioreactors. Anaerobic ammonium oxidation accounted for 24 and 67% of the total N 2 production at two typical continental shelf sites, whereas it was detectable but insignificant relative to denitrification in a eutrophic coastal bay. However, rates of anaerobic ammonium oxidation were higher in the coastal sediment than at the deepest site and the variability in the relative contribution to N 2 production between sites was related to large differences in rates of denitrification. Thus, the relative importance of anaerobic ammonium oxidation and denitrification in N 2 production appears to be regulated by the availability of their reduced substrates. By shunting nitrogen directly from ammonium to N 2 , anaerobic ammonium oxidation promotes the removal of fixed nitrogen in the oceans. The process can explain ammonium deficiencies in anoxic waters and sediments, and it may contribute significantly to oceanic nitrogen budgets.

Cryptic Sulfur Cycling Aerobic bacteria and ocean circulation patterns control the formation and distribution of oxygen-minimum zones at moderate depth in the oceans. These habitats host microorganisms that thrive on other metabolic substrates in the absence of oxygen—most commonly, metabolizing thermodynamically favorable nitrogen compounds like nitrate. Off the coast of Chile, however, Canfield et al. (p. 1375 , published online 11 November; see the Perspective by Teske ) suggest that bacteria may often reduce sulfate as well. Metagenomic sequencing revealed the presence of both sulfate-reducing and sulfide-oxidizing bacteria. With the coincidence of sulfate and nitrate reduction, the sulfur and nitrogen cycles may be intimately linked; for example, sulfate reduction could provide nitrogen-rich ammonium for bacteria that ultimately transform it into nitrogen gas.

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In oxygen-depleted zones of the open ocean, and in anoxic basins and fjords, denitrification (the bacterial reduction of nitrate to give N2) is recognized as the only significant process converting fixed nitrogen to gaseous N2. Primary production in the oceans is often limited by the availability of fixed nitrogen such as ammonium or nitrate, 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 N2 production in some marine sediments. 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 N2 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. We therefore expect the anammox reaction to be a globally significant sink for oceanic nitrogen.

Active expression of putative ammonia monooxygenase gene subunit A (amoA) of marine group I Crenarchaeota has been detected in the Black Sea water column. It reached its maximum, as quantified by reverse-transcription quantitative PCR, exactly at the nitrate maximum or the nitrification zone modeled in the lower oxic zone. Crenarchaeal amoA expression could explain 74.5% of the nitrite variations in the lower oxic zone. In comparison, amoA expression by ␥-proteobacterial ammonia-oxidizing bacteria (AOB) showed two distinct maxima, one in the modeled nitrification zone and one in the suboxic zone. Neither the amoA expression by crenarchaea nor that by ␤-proteobacterial AOB was significantly elevated in this latter zone. Nitrification in the suboxic zone, most likely microaerobic in nature, was verified by 15 ammonia-oxidizing bacteria ͉ amoA gene expression ͉ marine group ͉ Crenarchaeota ͉ marine nitrogen loss N itrification, the stepwise oxidation of ammonium to nitrite and then nitrate, is a key process in marine nitrogen cycling. It is responsible for the formation of the large deep-sea nitrate reservoir. It connects the recycling of organic nitrogen to the ultimate nitrogen loss from the oceans, because its products are substrates for denitrification and anaerobic ammonium oxidation (anammox), the only two presently known nitrogen loss processes. In productive waters such as upwelling regions, high fluxes of organic matter and thus remineralization create strong subsurface oxygen minima, enabling denitrification (1-4) or anammox (5-8) to occur. Nitrogen losses from these oxygen minimum zones (OMZs) are estimated to account for 30-50% of total nitrogen loss from the oceans (9, 10). Because remineralization also releases large amounts of ammonium, high nitrification rates are often associated with these OMZs (11), implying that nitrification may play an important role in promoting marine nitrogen loss.The Black Sea is the largest marine anoxic basin in the world. A 20-to 40-m-thick suboxic transitional zone, characterized by low oxygen (Ͻ5 M) and undetectable sulfide, persists throughout the basin between the surface oxic layer and the sulfidic anoxic deep water (Ն100 m) (12, 13). The exact depth zonation varies according to the location within the basin because of circulation and gyre formation, but similar concentrations of chemical species can be traced along isopycnals or density ( t ) surfaces throughout the basin (12). Therefore, the Black Sea provides an ideal model system to study nitrogen cycling processes along oxygen gradients. Nitrification has been reported in the lower oxic zone (14) and so has nitrogen loss via anammox in the suboxic zone (15). Nevertheless, the identity and abundance of the responsible nitrifiers, or any coupling between nitrification and nitrogen losses, remain poorly documented.The first and rate-limiting step of nitrification is aerobic ammonia oxidation. It is a microbially mediated reaction. For decades, only specific groups of ␤-and ␥-proteobacteria have been found to exh...

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