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 in the United States[5]), water circulation biofilters in aquarium tanks,[4] terrestrial systems,[5] fresh and salt water ecosystems, agricultural lands[6] and hot springs.[7]Nitrospira is a ubiquitous bacterium that plays a role in the nitrogen cycle[8] by performing nitrite oxidation in the second step of nitrification.[7]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.[9] 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.[10] However, due to their difficulty to be cultivated in laboratory settings, the entire genome was only sequenced in one species, Nitrospira defluvii.[11] In addition, Nitrospira bacteria's 16S rRNA sequences are too dissimilar to use for PCRprimers, thus some members go unnoticed.[10] In addition, members of Nitrospira with the capabilities to perform complete nitrification (comammox bacteria) has also been discovered[9][12] and cultivated.[13]
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).[14] 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.[15] 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.[14] DNA analysis determined 56.9 +/- 0.4 mol% of the DNA to be guanine and cytosine base pairs.[14]
General metabolism
Nitrospira are capable of aerobic hydrogen oxidation[16] and nitrite oxidation[7] 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[14] Despite being commonly classified as obligate chemolithotrophs,[5] some are capable of mixotrophy.[7] For instance, under different environments, Nitrospira can choose to assimilate carbon by carbon fixation[7] or by consuming organic molecules (glycerol, pyruvate, or formate[17]). New studies also show that Nitrospira can use urea as a source of nutrients.[18]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[7] 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.[10] 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.[19] Recently Nitrospira members with the abilities to perform complete nitrification (comammox bacteria) have also been discovered[9][12][20] and cultivated as in the case of Nitrospira inopinata.[13] 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.[9]
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.[21]
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.[9][12][20] 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.[9] 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.[12] 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.[9]
Nitrospira also carry the genes encoding for all the sub-units of nitrite oxidoreductase (nxr), the enzyme that catalyzes the second step of nitrification.[9]
^ abcdFujitani H, Ushiki N, Tsuneda S, Aoi Y (October 2014). "Isolation of sublineage I by a novel cultivation strategy". Environmental Microbiology. 16 (10): 3030–3040. doi:10.1111/1462-2920.12248. PMID25312601.
^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. Bibcode:1986ArMic.144....1W. doi:10.1007/BF00454947. S2CID29796511.
^Wagner M, Loy A, Nogueira R, Purkhold U, Lee N, Daims H (2002). "Microbial community composition and function in wastewater treatment plants". Antonie van Leeuwenhoek. 81 (1/4): 665–680. doi:10.1023/A:1020586312170. hdl:1822/1616. PMID12448762. S2CID21315850.
^ 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. Bibcode:1995ArMic.164...16E. doi:10.1007/BF02568729. PMID7646315. S2CID2702110.
^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. Bibcode:2014Sci...345.1052K. doi:10.1126/science.1256985. hdl:2066/133107. PMID25170152. S2CID206559794.
^Costa E, Pérez J, Kreft JU (2006). "Why is metabolic labour divided in nitrification?". Trends in Microbiology. 14 (5): 213–219. doi:10.1016/j.tim.2006.03.006. PMID16621570.
^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. Bibcode:2013BiTec.139..195R. doi:10.1016/j.biortech.2013.04.021. PMID23665516.
