Nitrifying bacteria

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Nitrifying bacteria are chemoautotrophic or chemolithotrophs depending on the genera (Nitrosomonas, Nitrosococcus, Nitrobacter, Nitrococcus) bacteria that grow by consuming inorganic nitrogen compounds.[1] Many species of nitrifying bacteria have complex internal membrane systems that are the location for key enzymes in nitrification: ammonia monooxygenase which oxidizes ammonia to hydroxylamine, and nitrite oxidoreductase, which oxidizes nitrite to nitrate.

Ecology[edit]

Nitrifying bacteria are a narrow species in the environment, and are found in lowest numbers where considerable amounts of ammonia are present (areas with extensive protein decomposition, and sewage treatment plants).[2] Nitrifying bacteria thrive in lakes and rivers streams with high inputs and outputs of sewage and wastewater and freshwater because of the low ammonia content.

Oxidation of ammonia to nitrate[edit]

Nitrification in nature is a two-step oxidation process of ammonium (NH4+) or ammonia (NH3) to nitrate (NO3-) catalyzed by two ubiquitous bacterial groups. The first reaction is oxidation of ammonium to nitrite by ammonium oxidizing bacteria (AOB) represented by the "Nitrosomonas" species. The second reaction is oxidation of nitrite (NO2-) to nitrate by nitrite-oxidizing bacteria (NOB), represented by the Nitrobacter species.[3][4]

First step nitrification - molecular mechanism[edit]

Figure 1. Molecular mechanism of ammonium oxidation by AOB

Ammonia oxidation in autotrophic nitrification is a complex process that requires several enzymes, proteins and presence of oxygen. The key enzymes, necessary to obtaining energy during oxidation ammonium to nitrite are ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO). First is a transmembrane copper protein which catalyzes the oxidation of ammonium to hydroxylamine (1.1) taking two electrons directly from the quinone pool. This reaction requires O2. In the second step (1.2), a trimeric multiheme c-type HAO converts hydroxylamine into nitrite in the periplasm with production of four electrons. The stream of four electron are channelled through cytochrome c554 to a membrane-bound cytochrome c552. Two of the electrons are routed back to AMO, where they are used for the oxidation of ammonia (quinol pool). Rest two electrons are used to generate a proton motive force and reduce NAD(P) through reverse electron transport.[5]

NH3 + O2NO
2
+ 3H+ + 2e (1)
NH3 + O2 + 2H+ + 2e → NH2OH + H2O (1.1)
NH2OH + H2ONO
2
+ 5H+ + 4e (1.2)

Second step nitrification - molecular mechanism[edit]

Nitrite produced in the first step of autotrophic nitrification is oxidized to nitrate by nitrite oxidoreductase (NXR)(2). It is a membrane-associated iron-sulfur molybdoprotein, and is part of an electron transfer chain which channels electrons from nitrite to molecular oxygen. The molecular mechanism of oxidation nitrite is less described than oxidation ammonium. In new research (e.g. Woźnica A. et al., 2013)[6] proposed new hypothetical model of NOB electron transport chain and NXR mechanism (Figure 2.). It should be noted that, in contrast to earlier models [7] the NXR acts on the outside of the plasma membrane, directly contributing to postulated by Spieck [8] and coworkers mechanism of proton gradient generation. Nevertheless, the molecular mechanism of nitrite oxidation is an open question.

NO
2
+ H2O → NO
3
+ 2H+ + 2e (2)

Characteristic of ammonia and nitrite oxidizing bacteria[edit]

Nitrifying bacteria that oxidize ammonia [3][9][edit]

Genus Phylogenetic group DNA (mol% GC) Habitats Characteristics
Nitrosomonas Beta 45-53 Soil, Sewage, freshwater, Marine Gram-negative short to long rods, motile (polar flagella)or nonmotile; peripheral membrane systems
Nitrosococcus Gamma 49-50 Freshwater, Marine Large cocci, motile, vesicular or peripheral membranes
Nitrosospira Beta 54 Soil Spirals, motile (peritrichous flagella); no obvious membrane system
Nitrosolobus Beta 54 Soil Pleomorphic, lobular, compartmented cells; motile (peritrichous flagella)

Nitrifying bacteria that oxidize nitrite [3][9][edit]

Genus Phylogenetic group DNA (mol% GC) Habitats Characteristics
Nitrobacter Alpha 59-62 Soil, Freshwater, Marine Short rods, reproduce by budding, occasionally motile (single subterminal flagella) or non-motile; membrane system arranged as a polar cap
Nitrospina Delta 58 Marine Long, slender rods, nonmotile, no obvious membrane system
Nitrococcus Gamma 61 Marine Large Cocci, motile (one or two subterminal flagellum) membrane system randomly arranged in tubes
Nitrospira Nitrospirae 50 Marine, Soil Helical to vibroid-shaped cells; nonmotile; no internal membranes

See also[edit]

References[edit]

  1. ^ Mancinelli RL (1996). "The nature of nitrogen: an overview". Life support & biosphere science : international journal of earth space 3 (1–2): 17–24. PMID 11539154. 
  2. ^ Belser LW (1979). "Population ecology of nitrifying bacteria". Annu. Rev. Microbiol. 33: 309–333. doi:10.1146/annurev.mi.33.100179.001521. PMID 386925. 
  3. ^ a b c Schaechter M. „Encyclopedia of Microbiology", AP, Amsterdam 2009
  4. ^ Ward BB (1996). "Nitrification and ammonification in aquatic systems". Life support & biosphere science : international journal of earth space 3 (1–2): 25–9. PMID 11539155. 
  5. ^ Byung Hong Kim, Geoffrey Michael Gadd (2008). Bacterial Physiology and Metabolism. Cambridge University Press. 
  6. ^ Woznica A. et al (2013). "Stimulatory Effect of Xenobiotics on Oxidative Electron Transport of Chemolithotrophic Nitrifying Bacteria Used as Biosensing Element". PLOS ONE 8 (1). 
  7. ^ Ferguson SJ, Nicholls DG (2002). Bioenergetic III. Academic Press. 
  8. ^ Spieck E. et al. (1998). "Isolation and immunocytochemical location of the nitrite-oxidizing system in Nitrospira moscoviensis". Arch Microbiol 169 (3): 225–230. doi:10.1007/s002030050565. PMID 9477257. 
  9. ^ a b Michael H. Gerardi (2002). Nitrification and Denitrification in the Activated Sludge Process. John Wiley & Sons,.