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Thermococcus celericrescens sp. nov., a fast-growing and cell-fusing hyperthermophilic archaeon from a deep-sea hydrothermal vent

Tomohiko Kuwabara¹, Masaomi Minaba^2,, Noriko Ogi¹ and Masahiro Kamekura^3,

¹ Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba 305-8572, Japan
² Master's Program in Biosystem Studies, University of Tsukuba, Tsukuba 305-8572, Japan
³ Noda Institute for Scientific Research, Noda 278-0037, Japan

Correspondence
Tomohiko Kuwabara
kuwabara{at}biol.tsukuba.ac.jp

A fast-growing and cell-fusing hyperthermophilic archaeon was isolated from a hydrothermal vent at Suiyo Seamount, Izu-Bonin Arc, Western Pacific Ocean. Strain TS2^T is an irregular, motile coccus that is generally 0.7–1.5 µm in diameter and possesses a polar tuft of flagella. In the mid-exponential phase of growth, cells that appeared black under phase-contrast microscopy fused at room temperature in the presence of a DNA-intercalating dye, as previously observed in Thermococcus coalescens. Cell fusion was not observed in later growth phases. Transmission electron microscopy revealed that the cells in the mid-exponential phase had a 5 nm-thick, electron-dense cell envelope that appeared to associate loosely with the cytoplasmic membrane. As the growth stage progressed, a surface layer developed on the membrane under the envelope and the envelope eventually peeled off. These observations suggest that the surface layer prevents the fusion of cells. Cells of strain TS2^T grew at 50–85 °C, pH 5.6–8.3 and at NaCl concentrations of 1.0 to 4.5 %, with optimal growth occurring at 80 °C, pH 7.0 and 3.0 % NaCl. Under optimal growth conditions, strain TS2^T grew very fast with an apparent doubling time of 20 min. It is suggested that the biosynthesis of the surface layer cannot catch up with cell multiplication in the mid-exponential phase and thus cells without the surface layer are generated. Strain TS2^T was an anaerobic chemo-organotroph that grew on either yeast extract or tryptone as the sole growth substrate. The genomic DNA G+C content was 54.6 mol%. Phylogenetic analysis based on 16S rRNA gene sequencing indicated that the isolate belongs to the genus Thermococcus. However, no significant DNA–DNA hybridization was observed between the genomic DNA of strain TS2^T and phylogenetically related Thermococcus species. On the basis of this evidence, strain TS2^T is proposed to represent a novel species, Thermococcus celericrescens sp. nov., a name chosen to reflect the fast growth of the strain. The type strain is TS2^T (=NBRC 101555^T=JCM 13640^T=DSM 17994^T).

The GenBank/EMBL/DDBJ accession number for the 16S rRNA gene sequence of Thermococcus celericrescens sp. nov. TS2^T is AB107768.

Phase-contrast microscope images of cells of strain TS2^T, a movie showing cell fusion, graphs showing the effects of temperature, pH and NaCl concentration on the growth of strain TS2^T and a growth curve are available as supplementary material in IJSEM Online. A supplementary table detailing DNA–DNA hybridization between genomic DNA of strain TS2^T and phylogenetically related Thermococcus species is also available.

Present address: Department of Developmental Biology, Division of Insect and Animal Sciences, National Institute of Agrobiological Sciences, Tsukuba 305-8634, Japan.

Present address: Halophiles Research Institute, 677-1 Shimizu, Noda 278-0043, Japan.