gen. nov., sp. nov. and sp. nov., two acidophilic ammonia oxidising archaea isolated from acidic soil, and proposal of the new order ord. nov. within the class of the phylum .

Two obligately acidophilic, mesophilic and aerobic soil ammonia-oxidising archaea were isolated from a pH 4.5 arable sandy loam (UK) and pH 4.7 acidic sulphate paddy soil (PR China) and designated strains Nd1 and Nd2, respectively. The strains shared more than 99 % 16S rRNA gene sequence identity and their genomes were both less than 2 Mb in length, sharing 79 % average nucleotide identity, 81 % average amino acid identity and a DNA G+C content of approximately 37 mol%. Both strains were chemolithotrophs that fixed carbon dioxide and gained energy by oxidising ammonia to nitrite, with no evidence of mixotrophic growth. Neither strain was capable of using urea as a source of ammonia. Both strains were non-motile in culture, although Nd1 does possess genes encoding flagella components and therefore may be motile under certain conditions. Cells of Nd1 were small angular rods 0.5-1 µm in length and grew at pH 4.2-5.6 and at 20-30 °C. Cells of Nd1 were small angular rods 0.5-1 µm in length and grew at pH 4.0-6.1 and at 20-42 °C. Nd1 and Nd2 are distinct with respect to genomic and physiological features and are assigned as the type strains for the species sp. nov. (type strain, Nd1=NCIMB 15248=DSM 110862) and sp. nov. (type strain, Nd2=NCIMB 15249=DSM 110863), respectively, within the genus gen. nov. The family fam. nov. and order ord. nov. are also proposed officially.