Heterotrophic nitrification

As the result of a study in which NO and N2O production was measured among several organisms (e.g., Al. faecalis) as a function of the partial pressure of O2, Anderson et al. (1993) concluded that heterotrophic nitrification may well be a significant source of N2O in microaerobic soils. 

Castignetti, D., and Hollocher, T. C. (1984). Heterotrophic nitrification among denitri-fiers. Appl. Environ. Microbiol. 47, 620-623. 

Robertson, L. A., and Kuenen, J. G. (1990). Combined heterotrophic nitrification and aerobic denitrification in Thiosphaera pancotropha and other bacteria. Antonie Van Leenwenhoek 57, 139-152. 

It has been argued that heterotrophic nitrification involves enzyme systems that are quite different from those of the autotrophs (Wehrfritz et al, 1993) and that heterotrophic nitrification cannot serve as an energy generating mechanism (Castignetti, 1990), as the autotrophic process does. In the aerobic denitrifiers, which are also capable of nitrification, the initial enzyme, AMO, appears to be quite similar to the enzyme in autotrophic nitrifiers. However, HAO differs significantly and in the heterotrophs, is a smaller, simpler enzyme that performs a two electron transfer (instead of the four electron transfer of the autotrophic HAO) and... 

Because autotrophic nitrification is such a hard way to make a living, it might seem curious that heterotrophic nitrification is not more common. The amounts of N02 or formed are usually quite small compared to autotrophic nitrifica-... 

Heterotrophic nitrification has been studied in terrestrial systems, especially acid forest soils, where it has been difficult to document autotrophic nitrification. Experiments using isotopes to differentiate production of N03 from inorganic and organic substrates in a forest system found that heterotrophic nitrification accounted for less than 10% of the total nitrification rate (Barraclough and Puri, 1995). No information of this sort is available on the occurrence or significance of heterotrophic nitrification in aquatic systems. The potential for NH3 and N02 oxidation by heterotrophic bacteria in aquatic systems warrants further exploration, and the capability may be present in many strains already in culture. If heterotrophic nitrification is common in nature, then a focus on autotrophic nitrification as the... 

Nemergut, D. R., and Schmidt, S. K. (2002). Disruption of narEl, narjand moaE inhibits heterotrophic nitrification in Pseudomonas strainM19. Applied and Environmental Microbiology 68, 6462—6465. 

Wehrfritz, J. M., ReiUy, A., Spiro, S., and Richardson, D.J. (1993). Purification of hydroxylamine oxidase from Thiosphaera pantotropha Identification of electron acceptors that couple heterotrophic nitrification to aerobic denitrification. FEES Letters 335, 246—250. 

Castignetti D, Paulutsis D, Turley J (1990) An examination of protein translocation and energy conservation during heterotrophic nitrification. FEMS Microbiol Lett 66 175-182... 

Vernon LP, Mangum JH, Beck JV, Shafia FM (1960) Studies on a ferrous-iron-oxidizing bacterium II. Cytochrome composition. Arch Biochem Biophys 88 227-231 Verstraete W, Alexander M (1973) Heterotrophic nitrification in samples of natural ecosystems. Environ Sci Technol 7 39-42... 

Wehrfritz J-M, Carter JP, Soiro S, Richardson DJ (1997) Hydroxylamine oxidation in heterotro-phic nitrate-reducing soil bacteria and purification of hydroxylamine cytochrome c oxidore-ductase from Pseudomonas species. Arch Microbiol 166 421 —424 Wehrfritz J-M, Reilly A, Spiro S, Richardson DJ (1993) Purification of hydroxylamine oxidase from Thiosphaera pantotropha. Identification of electron acceptors that couple heterotrophic nitrification to aerobic denitrification. FEBS Lett 335 246-250 Wetzstein H-G, Ferguson SJ (1985) Respiration-dependent proton translocation and the mechanism of proton motive force generation in Nitrobacter winogradskyi. FEMS Microbiol Lett 30 87-92... 

Marine nitrifying bacteria, especially the N02 oxidizers are ubiquitous in the world ocean and key to the regeneration of NOs, which dominates waters below the well-illuminated, euphoric zone. However they are never very abundant and, at least for those species in culture, grow very slowly. Certain heterotrophic bacteria can also oxidize NH4" " to both N02 and N03 during metabolism of preformed organic matter. However, very little is known about the potential for heterotrophic nitrification in the sea. 

Redox potential It is usually considered that no autotrophic nitrification will occur when the redox potential is below 250 mV. Under mildly aerobic conditions, the oxidation of ammonium to nitrate is carried out joiutly by autotrophic and heterotrophic bacteria, whereas under anaerobic conditions with the redox potential below -85 mV, only heterotrophic nitrification takes place. 

This process is used primarily by a group of aerobic chemotrophs called the nitrifiers. While some low levels of heterotrophic nitrification do occur, rates are low and quantities of nitrate produced are relatively small compared to those by the chemotrophs. The oxidation of ammonia into nitrite is done by Nitrosomonas species with the second step performed by Nitrobacter species. 

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