Nitrospira

For the following description, Nitrospira moscoviensis will be representative of the Nitrospira genus. Nitrospira is a gram-negative nitrite-oxidizing organism with a helical to vibroid morphology (0.9–2.2 × 0.2–0.4 micrometres in size).[13] They are non-planktonic organisms that reside as clumps, known as aggregates, in biofilms.[1] Visualization using Transmission electron microscopy (TEM) confirms star-like protrusions on the outer membrane (6-8 nm thick). The periplasmic space is exceptionally wide (34-41 nm thick),[5] which provides space to accommodate electron-rich molecules.[14] Electron-deprived structures are located in the cytosol and are believed to be glycogen storage vesicles; polyhydroxybutyrate and polyphosphate granules are also identified in the cytoplasm.[13] DNA analysis determined 56.9 +/- 0.4 mol% of the DNA to be guanine and cytosine base pairs.[13]

Nitrospira are capable of aerobic hydrogen oxidation[15] and nitrite oxidation[6] to obtain electrons, but high concentrations of nitrite have shown to inhibit their growth.[1] The optimal temperature for nitrite oxidation and growth in Nitrospira moscoviensis is 39 °C (can range from 33-44 °C) at a pH range of 7.6-8.0[13] Despite being commonly classified as obligate chemolithotrophs,[5] some are capable of mixotrophy.[6] For instance, under different environments, Nitrospira can choose to assimilate carbon by carbon fixation[6] or by consuming organic molecules (glycerol, pyruvate, or formate[16]). New studies also show that Nitrospira can use urea as a source of nutrient. Urease encoded within their genome can break urea down to CO
2 and ammonia. The CO
2 can be assimilated by anabolism while the ammonia and organic by-product released by Nitrospira allow ammonium oxidizers[6] and other microbes to co-exist in the same microenvironment.[1]

All members of this genus have the nitrite oxidoreductase genes, and thus are all thought to be nitrite-oxidizers.[9] Ever since nitrifying bacteria were discovered it was accepted that nitrification occurred in two steps, although it would be energetically favourable for one organism to do both steps.[17] Recently Nitrospira members with the abilities to perform complete nitrification (comammox bacteria) have also been discovered[8][11][18] and cultivated as in the case of Nitrospira inopinata.[12] The discovery of commamox organisms within Nitrospira redefine the way bacteria contribute to the Nitrogen cycle and thus a lot of future studies will be dedicated to it.[8]

With these new findings there's now a possibility to mainly use complete nitrification instead of partial nitrification in engineered systems like wastewater treatment plants because complete nitrification results in lower emissions of the greenhouse gases: nitrous oxide and nitric oxide, into the atmosphere.[19]

After sequencing and analyzing the DNA of Nitrospira members researchers discovered both species had genes encoding ammonia monooxygenase (Amo) and hydroxlyamine dehydrogenase (hao), enzymes that ammonia-oxidizing bacteria (AOB), use to convert ammonia into nitrite.[8][11][18] The bacteria possess all necessary sub-units for both enzymes as well as the necessary cell membrane associated proteins and transporters to carry out the first step of nitrification.[8] Origins of the Amo gene are debatable as one study found that it is similar to other AOB[3], while another study found the Amo gene to be genetically distinct from other lineages.[11] Current findings indicate that the hao gene is phylogenetically distinct from the hao gene present in other AOB, meaning that they acquired them long ago, likely by horizontal gene transfer.[8]

Nitrospira also carry the genes encoding for all the sub-units of nitrite oxidoreductase (nxr), the enzyme that catalyzes the second step of nitrification.[8]

• Nitrogen cycle

• MicrobeWiki -- Nitrospira