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Abstract

Oceanographers noticed already many years ago that far less ammonium accumulated in anoxic fjords and basins, than would be expected from the stoichiometry of heterotrophic denitrification. It was suggested that this ‘missing’ ammonium was oxidized with nitrate to freeN2. Since then several otherworkers have argued based on chemical profiles that ammonium is oxidized anaerobically in oxygen deficient marine sediments and waters with either nitrate or manganese oxides as electron acceptor. While there is as yet no direct evidence for the anaerobic ammonium oxidation with manganese oxides in either sediments or anoxic water columns, more and more evidence is being provided for anaerobic ammonium oxidation with nitrite/nitrate. The first direct evidence for the anaerobic oxidation of ammonium was provided in a waste water bioreactor, where so-called ‘anammox’ bacteria belonging to the Order Planctomycetales directly oxidize ammonium to N2 with nitrite as the electron acceptor. Although the anammox process was generally seen as a promising process for waste water treatment, it was believed to be insigni.cant in the natural environment due to the extremely slow generation times (more than 2 weeks) of the anammox organisms. However, recent studies provide direct evidence for anaerobic oxidation of ammonium by nitrate and/or nitrite in marine sediments, oxygen minimum zones, anoxic fjords and basins as well as Arctic sea ice. Phylogenetic analysis of 16S ribosomal RNA sequences show that the bacteria involved are closely related to anammox bacteria from waste water bioreactors. The combined biogeochemical and microbiological data available indicates that anammox may contribute significantly to the loss of reactive nitrogen in the ocean.