New pathways for ammonia conversion in soil and aquatic systems

Ammonia conversion processes are essential for most soil and aquatic system s. Under natural conditions, the many possible reactions are difficult to a nalyze. For example, nitrification and denitrification have long been regar ded as separate phenomena performed by different groups of bacteria in segr egated areas of soils, sediments or aquatic systems sequentially in time. I t has now been established that strict segregation in place and time of the two processes is not necessary and that both denitrifiers and nitrifiers h ave versatile metabolisms. However, the rates described for aerobic denitri fiers are very low compared to the rates observed under anoxic conditions. Also the rates of nitrifier denitrification are quite low, indicating that these conversions may not play an important role under natural conditions. In addition, these processes often result in the emission of quite large am ounts of undesirable products, NO and N2O. Heterotrophic nitrification migh t be of relevance for systems, that contain a high carbon to nitrogen ratio . Recently, a novel process (Anammox) has been discovered in which ammonium serves as the electron donor for denitrification of nitrite into dinitroge n gas. N-15 labeling studies showed that hydrazine and hydroxylamine were i mportant intermediates in this process. Enrichment cultures on ammonium, ni trite and bicarbonate resulted in the dominance of one morphotypical microo rganism. The growth rate of the cultures is extremely low (doubling time 11 days), but the affinity for ammonium and nitrite and the conversion rates (9.2 10(-4) mol kg(-1) s(-1)) are quite high. Some of the reported high nit rogen losses in soil and aquatic systems might be attributed to anaerobic a mmonium oxidation. In addition, this conversion offers new opportunities fo r nitrogen removal, when it is combined with recently developed processes f or partial nitrification.