Anammox process greatly contributes to nitrogen loss occurring in oceanic oxygen minimum zones (OMZs), where the availability of NH 4 + is scarce as compared with NO 2 - . Remineralization of organic nitrogen compounds including urea and cyanate (OCN - ) into NH 4 + has been believed as an NH 4 + source of the anammox process in oxygen minimum zones. However, urea- or OCN - - dependent anammox has not been well examined due to the lack of marine anammox bacterial culture. In the present study, urea and OCN - degradation in a marine anammox bacterial consortium were investigated based on 15 N-tracer experiments and metagenomic analysis. Although a marine anammox bacterium, Candidatus Scalindua sp., itself was incapable of urea and OCN - degradation, urea was anoxically decomposed to NH 4 + by the coexisting ureolytic bacteria ( Rhizobiaceae , Nitrosomonadaceae , and/or Thalassopiraceae bacteria), whereas OCN - was abiotically degraded to NH 4 + . The produced NH 4 + was subsequently utilized in the anammox process. The activity of the urea degradation increased under microaerobic condition ( ca. 32-42 μM dissolved O 2 , DO), and the contribution of the anammox process to the total nitrogen loss also increased up to 33.3% at 32 μM DO. Urea-dependent anammox activities were further examined in a fluid thioglycolate media with a vertical gradient of O 2 concentration, and the active collaborative metabolism of the urea degradation and anammox was detected at the lower oxycline (21 μM DO).