The Mariana Trench is located in the Western Pacific, to the east of the Philippines and to the south of Japan. It is formed by the subduction of the Pacific Plate under the smaller Mariana Plate, and contains the deepest point in Earth's oceans, the Challenger Deep, at 10.91 km bellow sea level.
A diagrammatic cross section through the Mariana Trench. From Wikipedia.
The 9 February 2012 edition of the Proceedings of the National Academy of Sciences contains a paper by a team of scientists lead by Yasuhiko Ohara of the Hydrographic and Oceanographic Department of Japan and the Japan Agency for Marine-Earth Science and Technology, detailing the discovery of a hydrothermal vent community in the southern part of the Mariana Trench, about 80 km northeast of the Challenger Deep, at a depth of 5550-5861 m. The discovery was made during a study of the trench by the Japanese research ship R/V Yokosuka and was found by the diving vehicle DSV Shinkai 6500; the site has been named the Shinkai Seep Field.
Map of the Mariana Trench showing the location of the Shinkai Seep Field (SSF). WSRBF stands for TheWest Santa Rosa Bank Fault. Colours represent sea-floor depth, see key at top of map. From Ohara et al. (2012).
The research team did not have the means to collect water samples from the sight, as they were visiting it to study the geology of the area, and had not expected to find hydrothermal vent communities, but they note that the site was located on serpentinized peridotite, so by comparison with other sites conclude that the vents are probably emitting water at between 40 °C and 90 °C that is rich in hydrogen (H₂) and Methane (CH₄), but poor in metal compounds. Methane is easily oxidized to Hydrogen Sulphide (H₂S), which can provide an energy source for sulphur-bacteria, likely to form the base of the food chain at the vents.
Photographs from the Shinkai Seeps. (A) Vesycomyid clams on a serpentinized peridotite outcrop. (B) Clam community at 5622 m. (C) Serpentinized peridote outcrop, with aragonite intrusions. (D) Clam community, also showing zoanthid corals (Zo) and Beroe, a type of comb jelly. (E) Clam community, also showing zoanthid corals (Zo), an actiniarian sea anemone (Ac) and a galatheid crab (GC). From Ohara et al. (2012).
The communities were dominated by vesycomyid clams of the genus Abyssogena, which are known to host chemotropic symbiotic bacteria. In addition corals, sea anemones, comb jellies, gastropods (snails) and crabs were witnessed at the Seeps. Samples of the clams were gathered for DNA analysis, which showed them to be most closely related to vesycomyid clams from the Caribbean. This is not a total surprise, as the Caribbean Plate is know to have migrated eastwards from the Pacific (Central America is a very young landmass in geological terms), and benthic organisms in the Caribbean have been demonstrated to have close affinities with Pacific organisms before.
A vesycomyid clam from the Shinkai Seep Field. From Ohara et al. (2012).