The uptake and release of dissolved free amino acids (DFAA) by four coral species and the sea anemone Aiptasia pallida was investigated using high pressure liquid chromatography and radioisotopic methods. All species exhibited net uptake of a mixture of eight amino acids at near-ambient (low nanomolar) levels. Further studies using A. pallida indicated that DFAA uptake is animal mediated, energy requiring, and likely coupled with influx of Na+. Under conditions of food deprivation, endosymbiotic dinoflagellates (zooxanthellae) indirectly enhanced DFAA uptake by providing the animal with a ready source of energy in the form of translocated photosynthetic products. Amino acid uptake may be particularly important for nonfeeding, early life-history stages and cnidarian species inhabiting DFAA-enriched environments.

A. pallida released minimal amounts of DFAA. However, the loss of combined amino acids from the symbiosis can be substantial.

Changes in the size and composition of the internal free amino acid (FAA) pools of cultured and freshly isolated zooxanthellae were used to assess algal nitrogen status. Since N-rich cultured algae sequestered basic amino acids (particularly Arg) for nitrogen storage, FAA ratios of Arg:Glu, Arg:Total, and Basic:Total were responsive indicators of N-sufficiency. Ratios were elevated in algae freshly isolated from A. pallida which were fed daily but declined in algae from hosts fed less frequently. Thus, in oligotrophic environments the N-status of zooxanthellae largely reflects the feeding history of their host.

Changes in host FAA pools were also monitored as a function of feeding history and inorganic nitrogen availability in both zooxanthellate and aposymbiotic anemones. Zooxanthellate animals fed daily exhibited much greater protein biomass and biomass-normalized FAA pool size than all other treatments. However, Tau:Gly ratios indicated that, under conditions of food deprivation, zooxanthellate anemones displayed greater nutritional stress than their aposymbiotic counterparts. Making inorganic N available during food deprivation decreases this stress in zooxanthellate anemones. These findings suggest that, with regard to nitrogen metabolism, the nature of the symbiotic relationship may be influenced by the availability of nitrogen.