NUTRIENT ENRICHMENT AND THE ULTRASTRUCTURE OF ZOOXANTHELLAE FROM THE GIANT CLAM TRIDACNA-MAXIMA

The separate and combined effects of ammonium (10 mu M) and phosphate (2 mu M) on the ultrastructure of zooxanthellae (Symbiodinium sp.) fro m giant clams, Tridacna maxima, were examined in the held. Nitrogen ad dition significantly changed the ultrastructure of the zooxanthellae i nhabiting the clams. After 9 mo exposure, the cross-sectional area of zooxanthellae from N-treated clams was significantly lower than that f rom other treatments [N = 39.3 mu m(2); C = 47.9 mu m(2); P = 43.2 mu m(2); N + P = 44.5 mu m(2); (P = 0.001)]. There was also a significant decrease in the size of starch bodies, especially around the pyrenoid of the zooxanthellae from N and N+P treatments [N = 1.2 mu m(2); C = 2.0 mu m(2); P = 1.8 mu m(2); N + P = 1.2 mu m(2); (P = 2.08E-11)]. Th is presumably occurs as a result of the mobilization of organic carbon stores in response to stimulated amino acid synthesis under enriched nutrient conditions. These data strongly suggest that the symbiotic zo oxanthellae of clams are limited to some extent by the availability of inorganic nitrogen, and that relatively minor changes to the nutrient loading of the water column can have substantial effects on the bioch emistry of symbioses such as that which exists between clams and zooxa nthellae.