Wilmot, D.B. and R.D. Vetter. 1990. The bacterial symbiont from the hydrothermal vent tubeworm Riftia pachyptila is a sulfide specialist. Marine Biology 106: 273-283.
The deep-sea is a nutrient-poor environment, where heterotrophic animals like Pogonophorans normally could not survive. However, Riftia possesses a modified gut called a trophosome, housing symbiotic bacteria that reportedly oxidize sulfide to fix carbon and provide the worm with nutrients, yet this function has never been isolated from the worm itself. Is it truly symbiotic action that is carrying out these processes, or is there greater participation from the worm itself? And what fuel, exactly, is being used?
T It was found that whole worms incubated in sea -water with radioactive-sulfide took up this sulfide and accumulated radioactive sulfur-waste products in the body fluids. Isolated trophosomal tissue with bacterial cells and purified bacterial cells took up and oxidized sulfide, but not any other reactive sulfur compound tested (like thiosulfate). Both cultures also accumulated radioactive-sulfur compounds. Blood samples (which lack bacteria) incubated in the same materials did not show significant sulfide oxidation or production of sulfur-compounds like the trophosomal and pure bacterial cultures.
This study has demonstrated that oxidation of sulfide is carried out only by the bacteria, which serves a nutritive, but also protective function (decreasing toxicity). The bacteria rapidly oxidize sulfide, producing sulfur compounds that can be safely stored. Furthermore, Riftia has exploited the abundant sulfide issuing from the hydrothermal vents and uses this form only, no other. Since no other cell or tissue can significantly oxidize sulfide, and sulfide is the sole fuel source, the energy produced by the symbiotic bacteria is necessary to the animal's survival.
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