PHOSPHATE ACQUISITION IN THE GIANT CLAM-ZOOXANTHELLAE SYMBIOSIS

The effect of phosphate on the giant clam Tridacna gigas and on its sy mbiotic dinoflagellate Symbiodinium sp. was compared with that on cult ured Symbiodinium sp. originally isolated from the same clam species. Incubation of whole clams in elevated phosphate (10 mu M) reduced thei r capacity for phosphate uptake, but the uptake capacity of the clam's zooxanthellae population was not influenced. In addition, there was n o change in the zooxanthellae density and the N:P ratio of these algae . On the other hand, cultured zooxanthellae were influenced by the pho sphate regimen of their culture medium. Compared with controls (0 mu M P), addition of 10 mu M phosphate to the culture medium caused an inc rease of 100% in cell density and decreases of 50% in the N:P ratio, a nd 80% in the phosphate-uptake capacity of the zooxanthellae. Zooxanth ellae freshly isolated from the clams exhibited properties similar to those of zooxanthellae cultured in the absence of phosphate. These res ults demonstrate that the zooxanthellae population of T. gigas have li mited access to the inorganic phosphate in sea water and the phosphate reserves within the animal host.