USE OF DUAL-LABELED MICROCAPSULES TO DISCERN THE PHYSIOLOGICAL FATES OF ASSIMILATED CARBOHYDRATE, PROTEIN CARBON, AND PROTEIN NITROGEN IN SUSPENSION-FEEDING ORGANISMS

A mixture of either N-15-labeled protein and C-14-labeled carbohydrate (type 1) or dual-labeled C-14-N-15 protein and unlabeled carbohydrate (type 2) was microencapsulated and fed to mussels (Mytilus edulis). D efecation, excretion, respiration, and incorporation of both isotopes were quantified to compare the relative utilization of ingested carboh ydrate (C-14 in type 1), protein C (C-14 in type 2), and protein N (N- 15 in types 1 and 2). Assimilation efficiencies were in the order prot ein N (26.6%) > carbohydrate (16.3%) > protein C (8.6%). Incorporation of protein N was 1.9 x that of carbohydrate and 3.2 x that of protein C, which indicates that the amino-N fraction of dietary protein was c onserved (i.e. retained in tissues) relative to both carbohydrate and protein C. As much as 6% of absorbed protein C was excreted as dissolv ed organic matter, whereas only 1.7% was respired. These findings sugg est that most dietary protein was completely broken down to satisfy th e mussels' anabolic demand for amino N rather than catabolized for ene rgy or retained as whole amino acids for anabolism (i.e. essential ami no acids). Hence, the mussels appeared nutritionally limited by amino N rather than by energy or protein per se.