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Electron microscopy of trophosome tissue from the vestimentiferan tubeworm Riftia pachyptila clearly indicates that the bacterial symbionts are enclosed within animal cells (bacteriocytes). The structure of this lobular tissue is complex. Each lobule consists of an outer layer of trophochrome cells (devoid of symbionts, but with numerous pigmented granules), an inner region of bacteriocytes, and a central hemolymph space. Sulfur deposits within bacteria decrease in size and number with increasing distance ofthe bacteria from the hemolymph space. Bacteria located toward the center of the lobule appear smaller than those nearer the periphery, Suggesting that metabolic and developmental gradients exist. Trophochrome cells and free bacteria were enriched from the trophosome of R. pachyptila. A procedure is described for the isolation of bacteriocytes from gill tissue of the bivalves Calyptogena magnifica and Lucina floridana. Numerous bacteria reside in vacuoles within the bacteriocyte cytoplasm, as do large (5-10 micron), heterogeneous granules. Maximum CO 2 fixation rate at 20°C for bacteriocytes from C. magnifica is 13.2 nmoles CO 2 /mg protein/h, compared to 21.6 nmoles CO 2 /mg protein/h for L. floridana bacteriocytes. Fixation by bacteriocytes from C. magnifica is inhibited by sulfide, and to a lesser extent thiosulfate, at 0.1-1.0 mM. Thiosulfate increases CO 2 fixation two-fold in L. floridana bacteriocytes. C. magnifica bacteriocytes incubated for 1 h in 0.5 mM sulfide maintain higher intracellular ATP concentrations (3.3 nmoles/million cells; 1.01 mM) than do control cells without sulfide (1.02 nmoles/million cells; 0.3 1 mM). These results and comparable observations suggest that the identities of exogenous sulfur compounds exploited for chemical energy by the symbiosis may depend on the structural integrity and organization of the experimental preparation.

TROPHOSOME ULTRASTRUCTURE AND THE CHARACTERIZATION OF ISOLATED BACTERIOCYTES FROM INVERTEBRATE-SULFUR BACTERIA SYMBIOSES