Thermococcus coalescens sp. nov., a cell-fusing hyperthermophilic archaeon from Suiyo Seamount

A cell-fusing hyperthermophilic archaeon was isolated from hydrothermal fluid obtained from Suiyo Seamount of the Izu-Bonin Arc. The isolate, TS1^T, is an irregular coccus, usually 0.5-2 μm in diameter and motile with a polar tuft of flagella. Cells in the exponential phase of growth fused at room temperature in the presence of DNA-intercalating dye to become as large as 5 μm in diameter. Fused cells showed dark spots that moved along in the cytoplasm. Large cells with a similar appearance were also observed upon culture at 87 °C, suggesting the occurrence of similar cell fusions during growth. Transmission electron microscopy revealed that cells in the exponential phase possessed a thin and electron-lucent cell envelope that could be lost subsequently during culture. The fragile cell envelope must be related to cell fusion. The cells grew at 57-90 °C, pH 5.2-8.7 and at NaCl concentrations of 1.5-4.5%, with the optima being 87 °C, pH 6.5 and 2.5% NaCl. The isolate 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 53.9 mol%. Phylogenetic analysis based on 16S rRNA gene sequencing indicated that the isolate was closely related to Thermococcus species. However, no significant DNA-DNA hybridization was observed between genomic DNA of strain TS1^T and phylogenetically related Thermococcus species. We propose that isolate TS1^T represents a novel species, Thermococcus coalescens sp. nov., with the name reflecting the cell fusion activity observed in the strain. The type strain is TS1^T (= JCM 12540^T = DSM 16538^T).