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The ability of intact hydrothermal vent clams, Calyptogena magnifica,to take up sulfide was tested in pressurized aquaria. In addition, stimulation of carbon fixation by sulfur substrates was tested in vitro with gill-symbiontpreparationsfrom live clams. The clams had the ability to concentrate sulfide from the medium, using the high-afinity sulfide-binding component in their serum, by about one order of magnitude, reaching internal sulfide concentrations as high as 6.6 mM. This binding ability was shown to be reversible in vivo. These clams were also shown to accumulate thiosulfate in their blood to about 1 mM and to quickly lose it when sulfide was removed The symbionts were shown to use sulfide as a substrate for carbon fixation, and the data also suggest that they can use thiosulfate. The symbionts were shown to be quite sensitive to sulfide, with inhibition of carbon fixation occurring at low sulfide concentrations. Incubation media containing sulfide-binding substances, either Riftia pachyptila hemoglobins or C. magnifica serum sulfide-binding component, were shown to protect the symbionts from this inhibition and to stimulate carbon fixation. A general model of the functioning of the symbiosis is presented.

Dissolved H2S is a major environmental factor in hydrothermal vent ecosystems. In a study of adaptations to sulfide by alvinellid polychaetes, the sulfide‐binding capacity of body fluids was examined in Paralvinella palmiformis from northeast Pacific ridges and Alvinella species from the East Pacific Rise. Sulfide concentrations in vascular blood and coelomic fluid of freshly collected animals were notably variable. Separation of P. palmiformis body‐fluid components revealed that most sulfide (ca. 77%) was accumulated in the dissolved fraction. In P. palmiformis, both vascular blood and coelomic fluid could reversibly bind sulfide in vitro with a low affinity, saturating only at high dialysate concentrations (ca. 2 mmol L−1). No sulfide‐binding activity was observed in the vascular blood from Alvinella species. A dissolved protein component of greater than 90 kDa appears to be involved in sulfide binding in Paralvinella, probably a vascular extracellular high‐molecular‐weight hemoglobin. Some sulfide may also adsorb onto a 15.38‐kDa intracellular hemoglobin present in the coelomic erythrocyte fraction. In the absence of epibiotic bacteria, Paralvinella body fluids may function as a sulfide buffer to protect tissues from deleterious effects of sulfide exposure.