Availability of two forms of dissolved nitrogen to the coral Pocillopora damicornis and its symbiotic zooxanthellae

The relative contribution of dissolved nitrogen (ammonium and dissolved fre e amino acids DFAAs) to the nitrogen budget of the reef-building coral Poci llopora damicornis was assessed for colonies growing on control and ammoniu m-enriched reefs at One Tree Island (southern Great Barrier Reef) during th e ENCORE (Enrichment of Nutrient on Coral Reef; 1993 to 1996) project. P. d amicornis acquired ammonium at rates of between 5.1 and 91.8 nmol N cm(-2) h(-1) which were not affected by nutrient treatment except in the case of o ne morph. In this case, uptake rates decreased from 80.5 to 42.8 nmol cm(-2 ) h(-1) (P < 0.05) on exposure to elevated ammonium over 12 mo. The presenc e or absence of light during measurement did not influence the uptake of am monium ions. Nitrogen budgets revealed that the uptake of ammonium from con centrations of 0.11 to 0.13 mu M could completely satisfy the demand of gro wing P. damicornis for new nitrogen. P. damicornis also took up DFAAs at ra tes ranging from 4.9 to 9.8 nmol N cm(-2) h(-1). These rates were higher in the dark than in the light (9.0 vs 5.1 nmol m(-2) h(-1), P < 0.001). Uptak e rates were highest for the amino acids serine, arginine and alanine, and lowest for tyrosine. DFAA concentrations within the ENCORE microatolls that received ammonium were undetectable, whereas they ranged up to 100 nM with in the control microatolls. The contribution of DFAAs to the nitrogen budge t of P. damicornis constituted only a small fraction of the nitrogen potent ially contributed by ammonium under field conditions. Even at the highest f ield concentrations measured during this study, DFAAs could contribute only similar or equal to 11.3% of the nitrogen demand of P. damicornis. This co ntribution, however, may be an important source of nitrogen when other sour ces such as ammonium are scarce or during periods when high concentrations of DFAAs become sporadically available (e.g. cell breakage during fish-graz ing).