Use of dual-labeled microcapsules to discern the physiological fates of assimilated carbohydrate, protein carbon, and protein nitrogen in suspension-feeding organisms

Abstract

A mixture of either 15SN-labeled protein and 14C-labeled carbohydrate (type 1) or dual-labeled 14C-15N protein and unlabeled carbohydrate (type 2) was microencapsulated and fed to mussels (Mytilus edulis). Defecation, excretion, respiration, and incorporation of both isotopes were quantified to compare the relative utilization of ingested carbohydrate (14C in type 1), protein C (14C in type 2), and protein N (15N in types 1 and 2). Assimilation efficiencies were in the order protein 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 conserved (i.e. retained in tissues) relative to both carbohydrate and protein C. As much as 6% of absorbed protein C was excreted as dissolved organic matter, whereas only 1.7% was respired. These findings suggest that most dietary protein was completely broken down to satisfy the mussels' anabolic demand for amino N rather than catabolized for energy or retained as whole amino acids for anabolism (i.e. essential amino acids). Hence, the mussels appeared nutritionally limited by amino N rather than by energy or protein per se.