Animal communities at the recently discovered deep Sea hydrothermal vents (depth >2000 m) exhibit biomass densities which are probably as high as any ecosystem on earth. As a matter reaches the sea floor at such depths (Suess, 1980). Instead, local production of organic carbon by chemoautotrophic bacteria is believed to form much or all of the nutritional base at these vents. At the vents some of these bacteria are free-living either suspended in hydrothermal plumes or attached to surfaces (Jannasch and Mottl, 1985; Jannasch, 1989). These offer food sources for filter feeding and surface grazing vent animals. In this permanently dark environment, however, the major portion of life-supporting interactions between bacteria and animals occurs in endosymbiotic associations (reviewed by Childress et al., 1987). In most examples studied to date the symbiont is some type of chemoautotrophic sulfur bacterium, but methane oxidizing symbionts are also known (Cavanaugh et al., 1987; Childress et al., 1986). Chemoautotrophic sulfur bacteria generate energy (ATP or a proton motive force) by the oxidation of reduced sulfur compounds (sulfide, elemental sulfur, thiosulfate, etc.) and usually fix CO2 via the Calvin- Benson cycle also employed by green plants and algae, There are two general ways a symbiont might provide nutrients to its host. The symbiont might remain intact but overproduce certain organics which leak out of the symbiont or be deposited extracellularly. Alternatively, the host might degrade a fraction of the bacteria using lytic enzymes, possibly localized in vacuoles. There is little evidence pointing to the operation of either mechanism.
