Dodsworth, Jeremy A.

AbstractTrace metals have been an important ingredient for life throughout Earth’s history. Here, we describe the genome-guided cultivation of a member of the elusive archaeal lineage Caldarchaeales (syn. Aigarchaeota), Wolframiiraptor gerlachensis, and its growth dependence on tungsten. A metagenome-assembled genome (MAG) of W. gerlachensis encodes putative tungsten membrane transport systems, as well as pathways for anaerobic oxidation of sugars probably mediated by tungsten-dependent ferredoxin oxidoreductases that are expressed during growth. Catalyzed reporter deposition-fluorescence in-situ hybridization (CARD-FISH) and nanoscale secondary ion mass spectrometry (nanoSIMS) show that W. gerlachensis preferentially assimilates xylose. Phylogenetic analyses of 78 high-quality Wolframiiraptoraceae MAGs from terrestrial and marine hydrothermal systems suggest that tungsten-associated enzymes were present in the last common ancestor of extant Wolframiiraptoraceae. Our observations imply a crucial role for tungsten-dependent metabolism in the origin and evolution of this lineage, and hint at a relic metabolic dependence on this trace metal in early anaerobic thermophiles.

A thermophilic, filamentous, heterotrophic bacterium, designated strain JAD2T, a member of an as-yet uncultivated lineage that is present and sometimes abundant in some hot springs worldwide, was isolated from sediment of Great Boiling Spring in Nevada, USA. Cells had a mean diameter of 0.3 µm and length of 4.0 µm, and formed filaments that typically ranged in length from 20 to 200 µm. Filaments were negative for the Gram stain reaction, spores were not formed and motility was not observed. The optimum temperature for growth was 72.5–75 °C, with a range of 67.5–75 °C, and the optimum pH for growth was 6.75, with a range of pH 6.5–7.75. Peptone, tryptone or yeast extract were able to support growth when supplemented with vitamins, but no growth was observed using a variety of defined organic substrates. Strain JAD2T was microaerophilic and facultatively anaerobic, with optimal growth at 1 % (v/v) O2 and an upper limit of 8 % O2. The major cellular fatty acids (>5 %) were C16 : 0, C19 : 0, C18 : 0, C20 : 0 and C19 : 1. The genomic DNA G+C content was 69.3 mol%. Phylogenetic and phylogenomic analyses using sequences of the 16S rRNA gene and other conserved genes placed JAD2T within the phylum Chloroflexi , but not within any existing class in this phylum. These results indicate that strain JAD2T is the first cultivated representative of a novel lineage within the phylum Chloroflexi , for which we propose the name Thermoflexus hugenholtzii gen. nov., sp. nov., within Thermoflexia classis nov., Thermoflexales ord. nov. and Thermoflexaceae fam. nov. The type strain of Thermoflexus hugenholtzii is JAD2T ( = JCM 19131T = CCTCC AB-2014030T).