Cultivation and genomic analysis of Candidatus Nitrosocaldus islandicus, a novel obligately thermophilic ammonia-oxidizing thaumarchaeon. Candidatus Nitrosocaldus islandicus

Ammonia-oxidizing archaea (AOA) within the phylum Thaumarchaea are the only known aerobic ammonia oxidizers in geothermal environments. Although molecular data indicate the presence of phylogenetically diverse AOA from the Nitrosocaldus clade and the group 1.1a Thaumarchaea in terrestrial high-temperature habitats, only one enrichment culture of an AOA thriving above 50°C has been functionally characterized and published. From the biofilm of an Icelandic hot spring (73 °C), we have obtained a high (~85%) enrichment of a novel thaumarchaeon from the deep-branching Nitrosocaldaceae family and determined its complete genome sequence. This AOA, which we provisionally refer to as “Candidatus Nitrosocaldus islandicus”, is an obligately thermophilic chemolithoauto trophic ammonia oxidizer, which stoichiometrically converts ammonia to nitrite.