Symbiotic chemoautotrophic bacteria in marine invertebrates from sulphide-rich habitats

The primary base of the food chain of the dense animal populations found clustered around deep-sea hydrothermal vents1,2 appears to be chemoautotrophic bacteria, whose energy source is geothermally reduced hydrogen sulphide emitted from the vents3,4. Recently, a symbiotic association has been postulated5 between chemoautotrophic sulphur-oxidizing bacteria and the vent tubeworm, Riftia pachyptila Jones (phylum Pogonophora) on the basis of histological5 and enzymatic6 evidence. Hydrothermal vents thus appear to be a spectacular example of the role of reduced inorganic elements in animal nutrition. Marine muds and salt marsh sediments also produce a continuous supply of reduced sulphur compounds7,8, so the possibility arises that they support similar symbiotic associations5,9. I now present microscopic, enzymatic, and physiological evidence for the occurrence of intracellular sulphur-oxidizing chemoautotrophic bacteria in a bivalve, Solemya velum Say (phylum Mollusca), found in reducing muds of eelgrass beds. Bacterial symbionts were also observed in other animals from a variety of sulphide-rich habitats. It thus seems that symbiotic relationships with bacteria are widespread among sulphide-habitat marine invertebrates, and may have a significant role in their nutrition.