Assimilation and partitioning of prey nitrogen within two anthozoans and their endosymbiotic zooxanthellae

The movement of nitrogen from zooplankton prey into the temperate scleractinian coral Oculina arbuscula and the anemone Aiptasia pallida was measured using ¹⁵N-labeled brine shrimp. The efficiency with which prey nitrogen was incorporated into cnidarian tissues was species-specific. O. arbuscula with a full complement of zooxanthellae had an assimilation efficiency of nearly 100%, compared to only 46% for corals containing few zooxanthellae. In A. pallida, symbiont density had no effect, and nitrogen assimilation was 23 to 29%. In both species, the host retained the bulk of the ingested label. Complete digestion was rapid (<4 h), as was the partitioning of the label between host amino acids and macromolecules. The label was primarily in the low-molecular weight-amino acid pool in O. arbuscula, where it remained for 30 h. A maximum of ca. 20% of the ¹⁵N appeared in the zooxanthellae, where it was rapidly converted into macromolecules. Individual amino acids in A. pallida tissues were highly labeled with ¹⁵N within 4 h and showed no subsequent enrichment with time; however, zooxanthellae amino acids became increasingly enriched over 30 h. Differences in ¹⁵N enrichment among amino acids were consistent with known synthesis and transformation pathways, but it was not possible to discriminate between host feeding and de novo synthesis.