The availability of fixed inorganic nitrogen (nitrate, nitrite and ammonium) limits primary productivity in many oceanic regions. The conversion of nitrate to N2 by heterotrophic bacteria (denitrification) is believed to be the only important sink for fixed inorganic nitrogen in the ocean. Here we provide evidence for bacteria that anaerobically oxidize ammonium with nitrite to N2 in the world's largest anoxic basin, the Black Sea. Phylogenetic analysis of 16S ribosomal RNA gene sequences shows that these bacteria are related to members of the order Planctomycetales performing the anammox (anaerobic ammonium oxidation) process in ammonium-removing bioreactors. Nutrient profiles, fluorescently labelled RNA probes, 15N tracer experiments and the distribution of specific 'ladderane' membrane lipids indicate that ammonium diffusing upwards from the anoxic deep water is consumed by anammox bacteria below the oxic zone. This is the first time that anammox bacteria have been identified and directly linked to the removal of fixed inorganic nitrogen in the environment. The widespread occurrence of ammonium consumption in suboxic marine settings indicates that anammox might be important in the oceanic nitrogen cycle.

Many countries strive to reduce the emissions of nitrogen compounds (ammonia, nitrate, NOx) to the surface waters and the atmosphere. Since mainstream domestic wastewater treatment systems are usually already overloaded with ammonia, a dedicated nitrogen removal from concentrated secondary or industrial wastewaters is often more cost-effective than the disposal of such wastes to domestic wastewater treatment. The cost-effectiveness of separate treatment has increased dramatically in the past few years, since several processes for the biological removal of ammonia from concentrated waste streams have become available. Here, we review those processes that make use of new concepts in microbiology: partial nitrification, nitrifier denitrification and anaerobic ammonia oxidation (the anammox process). These processes target the removal of ammonia from gases, and ammonium-bicarbonate from concentrated wastewaters (i.e. sludge liquor and landfill leachate). The review addresses the microbiology, its consequences for their application, the current status regarding application, and the future developments.

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Anoxic ammonium oxidation (Anammox) and Completely Autotrophic Nitrogen removal Over Nitrite (CANON) are new and promising microbial processes to remove ammonia from wastewaters characterized by a low content of organic materials. These two processes were investigated on their feasibility and performance in a gas-lift reactor. The Anammox as well as the CANON process could be maintained easily in a gas-lift reactor, and very high N-conversion rates were achieved. An N-removal rate of 8.9 kg N (m(3) reactor)(-1) day(-1) was achieved for the Anammox process in a gas-lift reactor. N-removal rates of up to 1.5 kg N (m(3) reactor)(-1) day(-1) were achieved when the CANON process was operated. This removal rate was 20 times higher compared to the removal rates achieved in the laboratory previously. Fluorescence in situ hybridization showed that the biomass consisted of bacteria reacting to NEU, a 16S rRNA targeted probe specific for halotolerant and halophilic Nitrosomonads, and of bacteria reacting to Amx820, specific for planctomycetes capable of Anammox.