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.

Siderite-based anaerobic iron cycle driven by autotrophic thermophilic microbial consortium

Using a sample from a terrestrial hot spring (pH 6.8, 60 °C), we enriched a thermophilic microbial consortium performing anaerobic autotrophic oxidation of hydrothermal siderite (FeCO3), with CO2/bicarbonate as the electron acceptor and the only carbon source, producing green rust and acetate. In order to reproduce Proterozoic environmental conditions during the deposition of banded iron formation (BIF), we incubated the microbial consortium in a bioreactor that contained an unmixed anoxic layer of siderite, perfectly mixed N2/CO2-saturated liquid medium and microoxic (2% O2) headspace. Long-term incubation (56 days) led to the formation of magnetite (Fe3O4) instead of green rust as the main product of Fe(II) oxidation, the precipitation of newly formed metabolically induced siderite in the anoxic zone, and the deposition of hematite (Fe2O3) on bioreactor walls over the oxycline boundary. Acetate was the only metabolic product of CO2/bicarbonate reduction. Thus, we have demonstrated the ability of autotrophic thermophilic microbial consortium to perform a short cycle of iron minerals transformation: siderite–magnetite–siderite, accompanied by magnetite and hematite accumulation. This cycle is believed to have driven the evolution of the early biosphere, leading to primary biomass production and deposition of the main iron mineral association of BIF.

Draft Genome Sequence of a Divergent Anaerobic Member of the Chloroflexi Class Ardenticatenia from a Sulfidic Hot Spring

Here, we present a draft genome sequence of Nak82, the second genome sequence available for the Chloroflexi class Ardenticatenia and the first from a sulfidic terrestrial hot spring. Nak82 is genetically and metabolically distinct from Ardenticatena maritima and likely represents a new genus- or family-level lineage lacking high-potential respiratory pathways.