Kublanov, Ilya V.

SummaryA novel moderately thermophilic, facultatively anaerobic chemoorganotrophic bacterium strainP3M‐2Twas isolated from a microbial mat developing on the wooden surface of a chute under the flow of hot water (46°C) coming out of a 2775‐m‐deep oil exploration well (Tomsk region,Russia). StrainP3M‐2Tis a moderate thermophile and facultative anaerobe growing on mono‐, di‐ or polysaccharides by aerobic respiration, fermentation or by reducing diverse electron acceptors [nitrite,Fe(III),As(V)]. Its closest cultivated relative (90.8%rRNAgene sequence identity) isIgnavibacterium album, the only chemoorganotrophic member of the phylumChlorobi. New genus and speciesMelioribacter roseusare proposed for isolateP3M‐2T. Together withI. album, the new organism represents the classIgnavibacteriaassigned to the phylumChlorobi. The revealed group includes a variety of uncultured environmental clones, the16S rRNAgene sequences of some of which have been previously attributed to the candidate divisionZB1. Phylogenetic analysis ofM. roseusandI. albumbased on their23S rRNAandRecAsequences confirmed that these two organisms could represent an even deeper, phylum‐level lineage. Hence, we propose a new phylumIgnavibacteriaewithin theBacteroidetes–Chlorobigroup with a sole classIgnavibacteria, two familiesIgnavibacteriaceaeandMelioribacteraceaeand two speciesI. albumandM. roseus. This proposal correlates with chemotaxonomic data and phenotypic differences of both organisms from other cultured representatives ofChlorobi. The most essential differences, supported by the analyses of complete genomes of both organisms, are motility, facultatively anaerobic and obligately organotrophic mode of life, the absence of chlorosomes and the apparent inability to grow phototrophically.

Two novel thermophilic and slightly acidophilic strains, Kam940Tand Kam1507b, which shared 99 % 16S rRNA gene sequence identity, were isolated from terrestrial hot springs of the Uzon caldera on the Kamchatka peninsula. Cells of both strains were non-motile, regular cocci. Growth was observed between 55 and 85 °C, with an optimum at 65–70 °C (doubling time, 6.1 h), and at pH 4.5–7.5, with optimum growth at pH 5.5–6.0. The isolates were strictly anaerobic organotrophs and grew on a narrow spectrum of energy-rich substrates, such as beef extract, gelatin, peptone, pyruvate, sucrose and yeast extract, with yields above 107cells ml−1. Sulfate, sulfite, thiosulfate and nitrate added as potential electron acceptors did not stimulate growth when tested with peptone. H2at 100 % in the gas phase inhibited growth on peptone. Glycerol dibiphytanyl glycerol tetraethers (GDGTs) with zero to four cyclopentyl rings were present in the lipid fraction of isolate Kam940T. The G+C content of the genomic DNA of strain Kam940Twas 37 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the isolates were archaea of the phylumCrenarchaeota, only distantly related to the cultured members of the classThermoprotei(no more than 89 % identity), and formed an independent lineage adjacent to the ordersDesulfurococcalesandAcidilobalesand clustering only with uncultured clones from hot springs of Yellowstone National Park and Iceland as the closest relatives. On the basis of their phylogenetic position and novel phenotypic features, isolates Kam940Tand Kam1507b are proposed to be assigned to a new genus and species,Fervidicoccus fontisgen. nov., sp. nov. The type strain ofFervidicoccus fontisis strain Kam940T(=DSM 19380T=VKM B-2539T). The phylogenetic data as well as phenotypic properties suggest that the novel crenarchaeotes form the basis of a new family,Fervidicoccaceaefam. nov., and order,Fervidicoccalesord. nov., within the classThermoprotei.