In a normal marine ecosystem, the organisms found at the bottom of the food chain are seagrass and phytoplankton, which produce energy through photosynthesis (a photosynthetic ecosystem). In environments rich in methane and sulfides, one type of food chain relies on chemosynthetic bacteria as basic producers. These include sulfur-oxidizing bacteria, methane-oxidizing bacteria, and sulfide-reducing bacteria. Such an environment is referred to as a chemosynthetic ecosystem.
Of the benthic fauna belonging specifically to chemosynthetic ecosystems, some have evolved to obtain necessary nutrients by forming symbiotic relationships with chemosynthetic bacteria inside their bodies. Examples include the Vestimentifera (tubeworm), the bivalve Calyptogena (giant white clam), Bathymodiolus (mussel), Solemyidae, Thyasiridae, Lucinidae, the gastropod Alviniconcha hessleri, and the sponge Cladorhiza.
A special characteristic of a chemosynthetic ecosystem is its large biomass, despite its location in the deep sea. In a cold-seep community in Sagami Bay (depth of 1,200 meters), Calyptogena soyoae and Calyptogena okutanii form a dense community, with a maximum biomass exceeding 20 kg/m3. The organisms that form different communities in different regions of the world are similar at the low taxonomic level (family, genus). We are analyzing biogeographic relationships between the communities through genealogy, through early life-cycle stages, and through a reconstruction of plate motions.
 A cold-seep community found at a depth of 1,200 meters off shore Hatsushima in Sagami Bay -a high-density Calyptogena community. |
Chemosynthetic communities occur in environments rich in methane and sulfides, and the scale of deep-sea communities is considerable. There are two types of chemosynthetic communities: hydrothermal vent communities formed around hot vents, and cold-seep communities formed around cold vents. The latter exist in temperatures close to that of the surrounding seawater.
Hydrothermal vent communities are distributed across plate formation regions and at submarine volcanoes where volcanic activity is high. In subduction zones where plates collide, cold interstitial water seeps out due to compression. Cold-seep communities are found in such locations. Chemosynthetic communities are also found around whale carcasses and grain carriers that have sunk to the deep-sea floor, subsisting on the methane and sulfides produced by the decay of fat and grain.
 |
