Zooxanthellae release glucose in the tissue of a giant clam, Tridacna crocea

The giant clam Tridacna crocea harbors in the mantle tissue symbiotic micro algae commonly called zooxanthellae. Isolated zooxanthellae release glycero l into the medium in the presence of mantle tissue homogenate (MH), but it is not clear whether the cells do so in situ. In order to determine the pho tosynthetic products released by zooxanthellae in the mantle of the giant c lam we traced photosynthetic fixation products from C-13- and C-14-bicarbon ate both in the clam and in isolated zooxanthellae (IZ) in the presence or absence of MH. After 15 min incubation in the absence of MH the IZ released less than 0.6% of the fixed labeled carbon, mainly as glucose. The major i ntracellular photosynthates were neutral lipids, which constituted 20 to 40 % of the total extractable C-14. In the presence of MH, the IZ released up to 5.6% of the total fixed C-14, mostly as glycerol, and the major intracel lular photosynthate was glucose. In an intact clam incubated in sea water c ontaining C-14-bicarbonate, 46 to 80% of the fixed C-14 was translocated fr om the zooxanthellae to the host tissues. Most of the C-14 in the hemolymph , in the isolated zooxanthellae and in intact mantle tissue (containing zoo xanthellae) was recovered as glucose. No C-14-glycerol was detected in the mantle after 1 to 30 min incubation, and, even after 60 min, far less C-14- glycerol was synthesized than by IZ in the presence of MH. The possibility that in clam tissue glycerol is converted to glucose was examined by tracin g the labeled carbon from C-14-glycerol injected into the adductor muscle. After 5 min incubation, no labeled glucose was found in the hemolymph, but after 60 min, some 20% was found as glucose. Thin slices containing zooxant hellae, cut from the surface of the mantle, fixed inorganic carbon supplied as (NaHCO3)-C-14 in the medium and mainly released C-14-glucose. The addit ion of MH to the surrounding medium did not affect the release rate or form of release product. When the slices were cut into smaller pieces, however, the ratio of glycerol to glucose in the release product increased. These r esults indicate that in the presence of MH the metabolism of isolated zooxa nthellae was different from that of zooxanthellae in the mantle. In the pre sence of MH, isolated zooxanthellae release mostly glycerol, whereas in the mantle they release glucose.