GROWTH AND METABOLIC RESPONSES OF THE GIANT CLAM-ZOOXANTHELLAE SYMBIOSIS IN A REEF-FERTILIZATION EXPERIMENT

To evaluate the impact of elevated nutrients on reef organisms symbiot ic with zooxanthellae, giant clams Tridacna maxims were exposed daily to increased ammonia and phosphate (N, P, N+P) in their natural reef e nvironment for 3 to 6 mo. The results strongly corroborate the major r esponses of the symbiotic association to nutrient enrichment previousl y observed (with T, gigas) under controlled outdoor conditions. Exposu re of the clams to elevated N (10 mu M) increased zooxanthellae densit y, reduced zooxanthellae size, down-regulated N uptake by zooxanthella e freshly isolated from their hosts, and reduced glutamate in the clam haemolymph, with increased pools of some free amino acids (methionine , tyrosine) in the zooxanthellae. These results confirm that the zooxa nthellae in giant clams are N Limited in situ and have free access to inorganic N from the sea water. There is also corroborating evidence t hat the zooxanthellae are P limited in situ as well, possibly due to h ost interference. While the N:P ratios of the animal host reflected am bient Nand P concentrations in the sea water, those of the zooxanthell ae did pot. Regardless of P exposure (2 mu M P) of the clams, zooxanth ellae N:P ratios were consistently high (>30:1) and phosphate concentr ations in the clam haemolymph bathing the zooxanthellae tube system co nsistently low (<0.1 mu M) These field findings, consistent with previ ous laboratory observations, confirm the limiting roles of both N and P in the giant clam- zooxanthellae symbiosis. That significant changes occurred earlier and at lower nutrient loading compared to some reef organisms investigated within the same experimental framework further demonstrates organism-level responses of a potential bio-indicator of the early onset of eutrophication in reef waters.