A sample of deep-sea sediment was obtained from the Japan Trench at a depth of 6,292 m using a pressure-retaining sediment sampler. Microorganisms in the sediment sample were cultivated in marine broth 2216 at ambient pressure (65 MPa) without decompression, and at atmospheric pressure (0.1 MPa) as a control experiment. The results of phylogenetic analysis based on 16S rDNA sequences indicated that: I) microbial diversity in the original sediment samples showed a wide distribution of types in the domain bacteria, whereas II) in the mixed cultures incubated at 65 MPa without decompression, bacterial strains belonging to the Shewanella barophiles branch and the genus Moritella existed together at the beginning of cultivation, and Moritella strains became dominant towards the end of the cultivation period, and III) in the mixed cultures incubated at atmospheric pressure, strains belonging to the genus Pseudomonas were dominant at all times.
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