Although dense animal communities at hydrothermal vents and cold seeps rely on symbioses with chemoautotrophic bacteria [1, 2], knowledge of the mechanisms underlying these chemosynthetic symbioses is still fragmentary because of the difficulty in culturing the symbionts and the hosts in the laboratory. Deep-sea Calyptogena clams harbor thioautotrophic bacterial symbionts in their gill epithelial cells [1, 2]. They have vestigial digestive tracts and nutritionally depend on their symbionts [3], which are vertically transmitted via eggs [4]. To clarify the symbionts' metabolic roles in the symbiosis and adaptations to intracellular conditions, we present the complete genome sequence of the symbiont of Calyptogena okutanii. The genome is a circular chromosome of 1,022,154 bp with 31.6% guanine + cytosine (G + C) content, and is the smallest reported genome in autotrophic bacteria. It encodes 939 protein-coding genes, including those for thioautotrophy and for the syntheses of almost all amino acids and various cofactors. However, transporters for these substances to the host cell are apparently absent. Genes that are unnecessary for an intracellular lifestyle, as well as some essential genes (e.g., ftsZ for cytokinesis), appear to have been lost from the symbiont genome. Reductive evolution of the genome might be ongoing in the vertically transmitted Calyptogena symbionts.

Several barophilic and barotolerant bacteria were isolated from deep-sea mud samples of Suruga Bay (2485 m depth), the Ryukyu Trench (5110m depth), and the Japan Trench (land-side 6356 m, and sea-side 6269 m depth, respectively). The barophilic bacteria, strains DB5501, DB6101, DB6705 and DB6906, were able to grow better under high hydrostatic pressures than under atmospheric pressure (0.1 megapascals; MPa). The optimal growth pressures for the barophilic bacteria were approximately 50 MPa at 10°C. The barotolerant strains DSKl and DSS12 were determined to be psychrophilic, and had optimal growth temperatures of 10°C and 8°C respectively. The degree of barophily and barotolerance was shown to be very dependent on temperature. For example, at 4°C the barophilic strains were indistinguishable from barotolerant bacteria, whereas at 15°C the barotolerant strains behaved more like the barophilic strains. Based on sequence analysis of 16s ribosomal DNA, all of the strains included in this study belong to the gamma subgroup of the Proteobacteria. Phylogenetic relations between the isolated strains and the known gamma subgroup bacteria suggested that the isolated strains belong to a new sub-branch of this group.

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A novel marine bacterium, strain JCS350T, was isolated from marine sediment samples collected from a cold-seep area. The 16S rRNA gene sequence of the isolate showed high similarity to that of Erythrobacter luteolus SW-109T (95.9 % sequence similarity). Lower 16S rRNA gene sequence similarities were shown to other members of the genus Erythrobacter (94.6–95.4 %) and members of the genus Porphyrobacter (94.5–95.2 %). Phylogenetic analysis with all members of the family Erythrobacteraceae and several members of the family Sphingomonadaceae revealed that the isolate formed a phyletic line with [Erythrobacter] luteolus that was distinct from other members of the family Erythrobacteraceae. The dominant fatty acids of strain JCS350T were 18 : 1ω7c, 16 : 1ω7c and cyclopropane 17 : 0. The major respiratory quinone was ubiquinone 10. The DNA G+C content was 54.5 mol%. The isolate did not contain bacteriochlorophyll a. Optimal growth required the presence of 2 % (w/v) NaCl with either 0.18 % CaCl2 or 0.59 % MgCl2, at pH 6.5 and at 35 °C. On the basis of the evidence of this polyphasic taxonomic study, strain JCS350T should be classified in a novel genus and species in the family Erythrobacteraceae, for which the name Altererythrobacter epoxidivorans gen. nov., sp. nov. is proposed. The misclassified species [Erythrobacter] luteolus is transferred to the new genus as Altererythrobacter luteolus comb. nov. The type strain of Altererythrobacter epoxidivorans is JCS350T (=KCCM 42314T =JCM 13815T) and the type strain of Altererythrobacter luteolus is SW-109T (=KCTC 12311T =JCM 12599T).

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