Discovering life in the Mariana Trench:
Scientists discovered life at great water depth (> 5 km depth) in the Mariana Trench, where living forms are rare because the sunlight cannot penetrate such water depths.
The Sun is required for most life, because most plants are photosynthetic and most animals thrive on such plants. Therefore, most sea life is restricted to the uppermost few hundred meters of the ocean surface, where sunlight can penetrate. It is thus not surprising that life is rare in the ultra-deep Mariana Trench (> 5000 m depth). But in September 2010, using the manned submersible Shinkai 6500, Japanese, American, and French scientists, exploring about halfway down the Mariana Trench (5861 to 5550 m depth), at ~ 80 km northeast from the Challenger Deep (the deepest place on Earth's solid surface), discovered dense communities of clams (Fig. 1; Fig 2A) [Ohara et al., 2012]. The clams are a type of vesicomyids, known as Calyptogena. These bivalvia molluscs are symbiotic with bacteria that thrive on nutrients (probably methane) released from a low-temperature vent, named the Shinkai Seep Field (SSF). Fluids, sustaining SSF, can derive from the subducting Pacific plate or from seawater circulating through rocks of the upper mantle (i.e. serpentinites; Fig 2B). This is the first description of vesicomyid clams in the Mariana forearc. While vesicomyid clams are ones of the dominant chemosynthesis-based invertebrates, generally found in methane cold seeps from sediment diagenesis _ such as in the Nankai Trough southwest of Japan_ and from high-temperature hydrothermal vents _such as in the Galapagos Rift_, the SSF vesicomyid clam community is the first one associated to a low-temperature serpentinite-hosted hydrothermal system either at subduction zones or at mid-ocean ridges. Origin of the low-temperature fluid is still investigated, and the scientific team is already planning to return to the area to continue studying this ecosystem.
Fig. 2: Location of the Shinkai Seep Field (SSF), denoted as a red star. A) Bathymetric map of the Mariana Trench and location of the Challenger Deep and SSF in the southernmost Marianas. B) Sketch illustrating the serpentinite-hosted low temperature fluids sustaining the clam community. The Pacific plate is hydrated by alteration and hydrothermalism since its formation at mid-ocean ridge (i.e. the east Pacific Rise). As the hydrated Pacific plate subductes, it releases its hydrous fluids into the cold, upper mantle of the forearc, resulting in serpentinized mantle. Erosion of the Mariana forearc exposes the upper mantle near SSF. The fluids sustaining the low-temperature hydrothermal ecosystem can derive from the subducting Pacific plate or from seawater circulating through the serpenitinites (i.e. serpentinized mantle); and they are expelled from the serpentinites by faulting.