Nitrospira

General metabolism

Nitrospira are capable of aerobic hydrogen oxidation and nitrite oxidation to obtain electrons, but high concentrations of nitrite have shown to inhibit their growth. 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 Despite being commonly classified as obligate chemolithotrophs, some are capable of mixotrophy. For instance, under different environments, Nitrospira can choose to assimilate carbon by carbon fixation or by consuming organic molecules (glycerol, pyruvate, or formate). New studies also show that Nitrospira can use urea as a source of nutrients. 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 and other microbes to co-exist in the same microenvironment.

Nitrification

All members of this genus have the nitrite oxidoreductase genes, and thus are all thought to be nitrite-oxidizers. 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. Recently Nitrospira members with the abilities to perform complete nitrification (comammox bacteria) have also been discovered and cultivated as in the case of Nitrospira inopinata. 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.

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.

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. 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. 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. 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.

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

Phylogeny

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LSPN) and National Center for Biotechnology Information (NCBI). Phylogeny is based on GTDB 09-RS220 by Genome Taxonomy Database

Species incertae sedis:

• "Ca. N. alkalitolerans" Daebeler et al. 2020
• "Ca. N. bockiana" Lebedeva et al. 2008
• "N. calida" Lebedeva et al. 2011
• N. marina Watson et al. 1986
• "Ca. N. salsa" Haaijer et al. 2013