Nitrospira (from Latin: nitro, meaning "nitrate" and Greek: spira, meaning "spiral") translate into “a nitrate spiral” is a genus of bacteria within the
monophyletic clade[1] of the
Nitrospirota phylum. The first member of this genus was described 1986 by Watson et al. isolated from the Gulf of Maine. The bacterium was named Nitrospira marina.[2] Populations were initially thought to be limited to
marine ecosystems, but it was later discovered to be well-suited for numerous habitats, including
activated sludge of
wastewater treatment systems,[3] natural biological marine settings (such as the
Seine River in France[4] and beaches in Cape Cod[5]), water circulation
biofilters in
aquarium tanks,[4] terrestrial systems,[5] fresh and salt water ecosystems, and
hot springs.[6]Nitrospira is a ubiquitous bacterium that plays a role in the nitrogen cycle[7] by performing nitrite oxidation in the second step of nitrification.[6]Nitrospira live in a wide array of environments including but not limited to, drinking water systems, waste treatment plants,
rice paddies,
forest soils, geothermal springs, and sponge tissue.[8] Despite being abundant in many natural and engineered ecosystems Nitrospira are difficult to culture, so most knowledge of them is from molecular and genomic data.[9] However, due to their difficulty to be cultivated in laboratory settings, the entire genome was only sequenced in one species, Nitrospira defluvii.[10] In addition, Nitrospira bacteria's
16S rRNA sequences are too dissimilar to use for
PCRprimers, thus some members go unnoticed.[9] In addition, members of Nitrospira with the capabilities to perform complete
nitrification (
comammox bacteria) has also been discovered[8][11] and cultivated.[12]
Morphology
For the following description, Nitrospira moscoviensis will be representative of the Nitrospira genus. Nitrospira is a
gram-negativenitrite-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]
General metabolism
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 nutrients.[17]Urease encoded within their genome can break urea down to CO2 and
ammonia. The CO2 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]
Nitrification
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.[18] Recently Nitrospira members with the abilities to perform complete
nitrification (
comammox bacteria) have also been discovered[8][11][19] 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.[20]
Genome
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][19] 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]
^
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^Stanley W. Watson, Eberhard Bock, Frederica W. Valois, John B. Waterbury, Ursula Schlosser (1986). "Nitrospira marina gen. nov. sp. nov.: a chemolithotrophic nitrite-oxidizing bacterium". Arch Microbiol. 144 (1): 1–7.
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abcdEhrich S, Behrens D, Lebedeva E, Ludwig W, Bock E (July 1995). "A new obligately chemolithoautotrophic, nitrite-oxidizing bacterium,Nitrospira moscoviensis sp. nov. and its phylogenetic relationship". Archives of Microbiology. 164 (1): 16–23.
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^Koch H, Galushko A, Albertsen M, Schintlmeister A, Gruber-Dorninger C, Lucker S, Pelletier E, Le Paslier D, Spieck E, Richter A, Nielsen PH, Wagner M, Daims H (28 August 2014). "Growth of nitrite-oxidizing bacteria by aerobic hydrogen oxidation". Science. 345 (6200): 1052–1054.
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^Costa E, Pérez J, Kreft JU (2006). "Why is metabolic labour divided in nitrification?". Trends in Microbiology. 14 (5): 213–219.
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^Rodriguez-Caballero A, Ribera A, Balcázar J, Pijuan M (2013). "Nitritation versus full nitrification of ammonium-rich wastewater: Comparison in terms of nitrous and nitric oxides emissions". Bioresource Technology. 139: 195–202.
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