DENSE COMMUNITY OF HYPERTHERMOPHILIC SULFUR-DEPENDENT HETEROTROPHS INA GEOTHERMALLY HEATED SHALLOW SUBMARINE BIOTOPE NEAR KODAKARA-JIMA ISLAND, KAGOSHIMA, JAPAN

Microbial communities in marine hydrothermal sediments (0 to 30 cm dee p) in an inlet of Kodakara-Jima Island, Kagoshima, Japan, were studied with reference to environmental factors, especially the presence of a mino acids. The study area was shallow, and the sea floor was covered with sand through which hot volcanic gas bubbled and geothermally heat ed water seeped out. The total bacterial density increased with depth in the sediments in parallel with a rise in the ambient temperature (8 0 degrees C at the surface and 104 degrees C at a depth of 30 cm in th e sediments). As estimated by most-probable-number studies, hypertherm ophilic sulfur-dependent heterotrophs growing at 90 degrees C dominate d the microbial community (3 x 10(7) cells . g of sediment(-1) at a de pth of 30 cm in the sediments), followed in abundance by hyperthermoph ilic sulfur-dependent facultative autotrophs (3.3 x 10(2) cells . g of sediment(-1)). The cooler sandy or rocky floor surrounding the hot sp ots was covered with white bacterial mats which consisted of large Beg giatoa-like filaments. Both the total organic carbon content, most of which was particulate (75% in the surface sediments), and the amino ac id concentration in void seawater in the sediments decreased with dept h. Amino acids, both hydrolyzable and free, constituted approximately 23% of the dissolved organic carbon in the surface sediments. These re sults indicate that a lower amino acid concentration is probably due t o consumption by dense populations of hyperthermophilic sulfur-depende nt heterotrophs, which require amino acids for their growth and thus c reate a gradient of amino acid concentration in the sediments. The rol e of primary producers, which supply essential amino acids to sustain this microbial community, is also discussed.