SULFIDE AND CARBON-DIOXIDE UPTAKE BY THE HYDROTHERMAL VENT CLAM, CALYPTOGENA-MAGNIFICA, AND ITS CHEMOAUTOTROPHIC SYMBIONTS

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-symbiont preparations from live clams. The clams had the ability to concentrate sulfide from the medium, using the high-affinity 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.