Conexivisphaera calida gen. nov., sp. nov., a thermophilic sulfur- and iron-reducing archaeon, and proposal of Conexivisphaeraceae fam. nov., Conexivisphaerales ord. nov., and Conexivisphaeria class. nov. in the phylum Thaumarchaeota.

A thermoacidophilic, anaerobic, and iron- and sulfur-reducing archaeon, strain NAS-02T, was isolated from a terrestrial hot spring in Japan, as previously reported. This organism is the first non-ammonia-oxidizing isolate in the phylum Thaumarchaeota. Here, we propose Conexivisphaera calida gen. nov., sp. nov. to accommodate this strain. The type strain of the type species is NAS-02T (=JCM 31663T=DSM 105898T). The values of 16S rRNA gene similarity and average amino acid identity between NAS-02T and its closest relatives are <86 and <42 %, respectively. Based on the phylogeny and physiology, we propose the family Conexivisphaeraceae fam. nov., the order Conexivisphaerales ord. nov. and the class Conexivisphaeria class. nov. to accommodate the novel genus.

Isolation and characterization of a thermophilic sulfur- and iron-reducing thaumarchaeote from a terrestrial acidic hot spring.

A deep-branching clade of Thaumarchaeota, conventionally called Terrestrial hot spring creanarchaeotic group (THSCG), is a missing link between thaumarchaeotic ammonia oxidizers and the deeper-branching non-ammonia oxidizers, such as Crenarchaeota and Candidatus Korarchaeota. Here, we report isolation of the first cultivated representative from the THSCG, named as NAS-02. Physiological characterization demonstrated that the isolate was a thermoacidophilic, sulfur- and iron-reducing organoheterotroph, which was supported by gene contents encoded in its complete genome. There was no evidence for ammonia oxidation by the isolate. Members in THSCG are likely thermophiles, and may play roles in degrading cell debris as a scavenger and in biogeochemical cycling of sulfur and iron in the hot environments, as suggested by the physiological characteristics of the isolate and the geographical distribution of the 16S rRNA gene sequences of THSCG in terrestrial hot springs and marine hydrothermal fields. Phylogenetic analysis suggests that the THSCG lineage represented by NAS-02 has gained the ability of sulfur reduction via horizontal gene transfer. Based on the phylogeny and physiology, we propose the name Conexivisphaera calidus gen. nov., sp. nov. to accommodate the isolate.