Growth and metabolic responses of the giant clam-zooxanthellae symbiosis in a reef-fertilisation experiment

Abstract

To evaluate the impact of elevated nutrients on reef organisms symbiotic with zooxanthellae, giant clams Tridacna maxima were exposed daily to increased ammonia and phosphate (N, P, N+P) in their natural reef environment for 3 to 6 mo. The results strongly corroborate the major responses of the syn~biotic association to nutrient enr~chlnent previously observed (wlth T gigas) under controlled outdoor conditions. Exposure of the clams to elevated N (10 p M ) increased zooxanthellae density, reduced zooxanthellae size, down-regulated N uptake by zooxanthellae freshly isolated from their hosts, and reduced glutamate in the clam haernolymph, with increased pools of some free amino acids (methionine, tyrosine) In the zooxanthellae These results confirm that the zooxanthellae in giant clams are N limited jn situ and have free access to inorganic N from the sea water. There is also corroborating evidence that the zooxanthellae are P limited jn situ a s well, possibly due to host interference While the N-P ratios of the animal host reflected ambient N and P concentrations in the sea water, those of the zooxanthellae d ~d not. Regardless of P exposure (2 FM P) of the clams, zooxanthellae N:P ratios were cons~stent'ly high (>30:1) and phosphate concentrations in the clam haemolymph bathing the zooxanthellae tube system consistently low (<0.1 PM). These field findings, consistent with previous 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 organismlevel responses of a potential bio-indicator of the early onset of eutrophication in reef waters.