Certain hydrothermal vent invertebrates, e.g.
Riftia pachyptila and
Calyptogena magnifica, are clearly established as harboring dense populations of chemoautotrophic sulfur bacteria in specialized tissues. By contrast, the physiological characteristics of the abundant intracellular gill symbiont of the vent mussel
Bathymodiolus thermophilus have been questioned. The low activities of enzymes diagnostic for CO
2 fixation (Calvin cycle) and for sulfur-driven energy generation, as measured by other investigators, have been attributed to bacterial contamination of the gill surface. Based on research at the Galápagos Rift hydrothermal vents in 1988 and subsequent laboratory experiments, the current study confirms that the
B. thermophilus symbiont is a psychrophile for which thiosulfate and sulfide stimulate CO
2 fixation. It strongly indicates that the symbiont is a chemoautotroph by establishing the following: (1) Sulfide and thiosulfate can stimulate CO
2 fixation by partially purified symbionts by up to 43-fold and 120-fold, respectively; (2) the ribulose-1,5-bisphosphate carboxylase/oxygenase activity of the symbiont is sufficient to account for its sulfide- or thiosulfate-stimulated CO
2 incorporation; (3) the symbiont's molar growth yield on thiosulfate, as judged by CO
2 incorporation, is indistinguishable from that of free-living chemoautotrophs. Due to the high protein-degrading activity of
B. thermophilus gill lysate, it is also suggested that host lysis of symbionts plays a more important role in the nutrition of the vent mussel than in
R. pachyptila or
C. magnifica, for which no comparable protein-degrading activity was found.
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