The anaerobic oxidation of ammonium.

From recent research it has become clear that at least two different possibilities for anaerobic ammonium oxidation exist in nature. 'Aerobic' ammonium oxidizers like Nitrosomonas eutropha were observed to reduce nitrite or nitrogen dioxide with hydroxylamine or ammonium as electron donor under anoxic conditions. The maximum rate for anaerobic ammonium oxidation was about 2 nmol NH4+ min-1 (mg protein)-1 using nitrogen dioxide as electron acceptor. This reaction, which may involve NO as an intermediate, is thought to generate energy sufficient for survival under anoxic conditions, but not for growth. A novel obligately anaerobic ammonium oxidation (Anammox) process was recently discovered in a denitrifying pilot plant reactor. From this system, a highly enriched microbial community with one dominating peculiar autotrophic organism was obtained. With nitrite as electron acceptor a maximum specific oxidation rate of 55 nmol NH4+ min-1 (mg protein)-1 was determined. Although this reaction is 25-fold faster than in Nitrosomonas, it allowed growth at a rate of only 0.003 h-1 (doubling time 11 days). 15N labeling studies showed that hydroxylamine and hydrazine were important intermediates in this new process. A novel type of hydroxylamine oxidoreductase containing an unusual P468 cytochrome has been purified from the Anammox culture. Microsensor studies have shown that at the oxic/anoxic interface of many ecosystems nitrite and ammonia occur in the absence of oxygen. In addition, the number of reports on unaccounted high nitrogen losses in wastewater treatment is gradually increasing, indicating that anaerobic ammonium oxidation may be more widespread than previously assumed. The recently developed nitrification systems in which oxidation of nitrite to nitrate is prevented form an ideal partner for the Anammox process. The combination of these partial nitrification and Anammox processes remains a challenge for future application in the removal of ammonium from wastewater with high ammonium concentrations.

Novel principles in the microbial conversion of nitrogen compounds.

Some aspects of inorganic nitrogen conversion by microorganisms like N2O emission and hydroxylamine metabolism studied by Beijerinck and Kluyver, founders of the Delft School of Microbiology, are still actual today. In the Kluyver Laboratory for Biotechnology, microbial conversion of nitrogen compounds is still a central research theme. In recent years a range of new microbial processes and process technological applications have been studied. This paper gives a review of these developments including, aerobic denitrification, anaerobic ammonium oxidation, heterotrophic nitrification, and formation of intermediates (NO2-, NO, N2O), as well as the way these processes are controlled at the genetic and enzyme level.

Missing lithotroph identified as new planctomycete.

With the increased use of chemical fertilizers in agriculture, many densely populated countries face environmental problems associated with high ammonia emissions. The process of anaerobic ammonia oxidation ('anammox') is one of the most innovative technological advances in the removal of ammonia nitrogen from waste water. This new process combines ammonia and nitrite directly into dinitrogen gas. Until now, bacteria capable of anaerobically oxidizing ammonia had never been found and were known as "lithotrophs missing from nature". Here we report the discovery of this missing lithotroph and its identification as a new, autotrophic member of the order Planctomycetales, one of the major distinct divisions of the Bacteria. The new planctomycete grows extremely slowly, dividing only once every two weeks. At present, it cannot be cultivated by conventional microbiological techniques. The identification of this bacterium as the one responsible for anaerobic oxidation of ammonia makes an important contribution to the problem of unculturability.