The Diversity of Deep-Sea Mussels and Their Bacterial Symbioses

Abstract

Deep-sea chemosynthesis-based ecosystems are inhabited by diverse groups of metazoans. Although remote from the euphotic layer, and thus from photosynthetic primary producers, these ecosystems harbour high animal biomasses, orders of ­magnitude above biomasses usually reported in the deep-sea (Sibuet and Olu 1998; Van Dover 2000). The key to this high productivity is chemoautotrophy, a type of metabolism by which many prokaryotes fix inorganic carbon into organic molecules using the chemical energy resulting from the oxidation of reduced compounds ­present in their environment (Madigan et al. 2002). Indeed, hydrothermal vents and cold seeps are characterized by the occurrence of fluid emissions originating from the subsurface, which bring reduced compounds into mixing with bottom oxygenated seawater (reviewed in Von Damm 1995; Sibuet and Olu 1998; Van Dover 2000; Tunnicliffe et al. 2003). To summarize, hydrothermal vents occur mostly on oceanic ridges, where bottom seawater circulates into the newly formed crust. The high geothermic gradient linked with the presence of a magma chamber few kilometres below the ridge causes water to heat and to be enriched in reduced compounds ­(metals, sulphide…). Heated fluids, displaying lower densities, then reach back the seafloor along cracks and are emitted. Fluid interaction with cold seawater provokes the precipitation of dissolved metals and minerals in the form of complexes with sulphide, yielding typical (and often spectacular) chimneys. At cold seeps, which are mostly located along continental margins, fluids originate from the subsurface. The thermogenic or biogenic reduction of buried organic matter produces methane and other hydrocarbons which seep to the seafloor, and processes such as the anaerobic oxidation of methane coupled with sulphate-reduction can account for local ­enrichments in sulphide.