Aerobic and anaerobic ammonia oxidizing bacteria--competitors or natural partners?

The biological nitrogen cycle is a complex interplay between many microorganisms catalyzing different reactions. For a long time, ammonia and nitrite oxidation by chemolithoautotrophic nitrifiers were thought to be restricted to oxic environments and the metabolic flexibility of these organisms seemed to be limited. The discovery of a novel pathway for anaerobic ammonia oxidation by Planctomyces (anammox) and the finding of an anoxic metabolism by 'classical'Nitrosomonas-like organisms showed that this is no longer valid. The aim of this review is to summarize these novel findings in nitrogen conversion and to discuss the ecological importance of these processes.