The Family Nitrospinaceae

The only genus within the family Nitrospinaceae Garrity et al. 2006 is Nitrospina, encompassing two isolated type strain species, N. gracilis Watson and Waterbury 1971 and N. watsonii Spieck et al. 2014. Historically, the family has been thought to belong to the Deltaproteobacteria, but recently was tentatively reclassified into the candidate phylum Nitrospinae phyl. nov., with emended classification as Nitrospinia class. nov., Nitrospinales ord. nov., and with N. gracilis Watson and Waterbury 1971 as the type species. Nitrospina species are obligate marine aerobic chemolithoautotrophic nitrite-oxidizing bacteria that grow with nitrite and carbon dioxide as sole energy and carbon source, respectively. Genomic analyses of N. gracilis strain 3/211 have shown that Nitrospina use the reverse tricarboxylic acid cycle for carbon fixation and lack classical defense mechanisms against oxidative stress, thus indicating a microaerophilic evolutionary origin and an adaptation to elevated partial oxygen pressures by yet unidentified mechanisms. This observation is in accordance with their environmental distribution, as Nitrospina-like organisms are frequently detected by molecular methods in oxic waters but still also in suboxic environments like marine oxygen minimum zones and apparently anoxic sediments. The isolated, closely related Nitrospina species represent only a small subgroup of the genus, as phylogenetic analyses of environmental 16S rRNA sequences reveal a huge undiscovered diversity forming at least four additional sequence clusters that contain no cultured representatives.