Bright, M. H. Keckeis, and C.R. Fisher. 2000. An autoradiographic examination of carbon fixation, transfer, and utilization in the Riftia pachyptila symbiosis. Marine Biology 136: 621-632.
Tube worms of the genus Riftia do not possess a gut, but they do have bacteria which live inside them and perform some necessary functions for the worms. This is one of the reasons it is thought that much of the worms' nutrition is derived from their bacteria. Is the carbon fixed by the bacteria used as a nutrient by the worm? Are the bacteria themselves ingested as a further source of nutrition?
The genus Riftia includes some fast-growing invertebrates, a trait which requires substantial amounts of organic substrate. Although it is widely believed to occur, the transfer of fixed carbon from bacteria to worm has never actually been proven. Likewise, it has been hypothesized that the hosts digest the symbionts as an extra source of nutrition.
Riftia worms were collected by submersible, and held in chilled, pressurized seawater. The animals were fixed, and ultrathin sections were prepared. Autoradiography and microscopy were performed on the sections. The autoradiographs revealed that initially, cells containing the symbionts were densely labeled and that cells without symbionts were labeled following a period of incubation. Light microscopy revealed that the symbionts were in various stages of degradation in the worm.
The results suggest that not only do the worms' symbionts fix carbon, but that this carbon is subsequently released by the bacteria and then taken up by the host. Also, the results suggest that the bacteria may serve as n extra nutritive source for the worms, in that they are digested by the worm.
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